rcu_dereference_key() and user_key_payload() are currently being used in
two different, incompatible ways:

 (1) As a wrapper to rcu_dereference() - when only the RCU read lock used
     to protect the key.

 (2) As a wrapper to rcu_dereference_protected() - when the key semaphor is
     used to protect the key and the may be being modified.

Fix this by splitting both of the key wrappers to produce:

 (1) RCU accessors for keys when caller has the key semaphore locked:

	dereference_key_locked()
	user_key_payload_locked()

 (2) RCU accessors for keys when caller holds the RCU read lock:

	dereference_key_rcu()
	user_key_payload_rcu()

This should fix following warning in the NFS idmapper

  ===============================
  [ INFO: suspicious RCU usage. ]
  4.10.0 #1 Tainted: G        W
  -------------------------------
  ./include/keys/user-type.h:53 suspicious rcu_dereference_protected() usage!
  other info that might help us debug this:
  rcu_scheduler_active = 2, debug_locks = 0
  1 lock held by mount.nfs/5987:
    #0:  (rcu_read_lock){......}, at: [<d000000002527abc>] nfs_idmap_get_key+0x15c/0x420 [nfsv4]
  stack backtrace:
  CPU: 1 PID: 5987 Comm: mount.nfs Tainted: G        W       4.10.0 #1
  Call Trace:
    dump_stack+0xe8/0x154 (unreliable)
    lockdep_rcu_suspicious+0x140/0x190
    nfs_idmap_get_key+0x380/0x420 [nfsv4]
    nfs_map_name_to_uid+0x2a0/0x3b0 [nfsv4]
    decode_getfattr_attrs+0xfac/0x16b0 [nfsv4]
    decode_getfattr_generic.constprop.106+0xbc/0x150 [nfsv4]
    nfs4_xdr_dec_lookup_root+0xac/0xb0 [nfsv4]
    rpcauth_unwrap_resp+0xe8/0x140 [sunrpc]
    call_decode+0x29c/0x910 [sunrpc]
    __rpc_execute+0x140/0x8f0 [sunrpc]
    rpc_run_task+0x170/0x200 [sunrpc]
    nfs4_call_sync_sequence+0x68/0xa0 [nfsv4]
    _nfs4_lookup_root.isra.44+0xd0/0xf0 [nfsv4]
    nfs4_lookup_root+0xe0/0x350 [nfsv4]
    nfs4_lookup_root_sec+0x70/0xa0 [nfsv4]
    nfs4_find_root_sec+0xc4/0x100 [nfsv4]
    nfs4_proc_get_rootfh+0x5c/0xf0 [nfsv4]
    nfs4_get_rootfh+0x6c/0x190 [nfsv4]
    nfs4_server_common_setup+0xc4/0x260 [nfsv4]
    nfs4_create_server+0x278/0x3c0 [nfsv4]
    nfs4_remote_mount+0x50/0xb0 [nfsv4]
    mount_fs+0x74/0x210
    vfs_kern_mount+0x78/0x220
    nfs_do_root_mount+0xb0/0x140 [nfsv4]
    nfs4_try_mount+0x60/0x100 [nfsv4]
    nfs_fs_mount+0x5ec/0xda0 [nfs]
    mount_fs+0x74/0x210
    vfs_kern_mount+0x78/0x220
    do_mount+0x254/0xf70
    SyS_mount+0x94/0x100
    system_call+0x38/0xe0
Reported-by: NJan Stancek <jstancek@redhat.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Tested-by: NJan Stancek <jstancek@redhat.com>
Signed-off-by: NJames Morris <james.l.morris@oracle.com>

Add a facility whereby proposed new links to be added to a keyring can be
vetted, permitting them to be rejected if necessary.  This can be used to
block public keys from which the signature cannot be verified or for which
the signature verification fails.  It could also be used to provide
blacklisting.

This affects operations like add_key(), KEYCTL_LINK and KEYCTL_INSTANTIATE.

To this end:

 (1) A function pointer is added to the key struct that, if set, points to
     the vetting function.  This is called as:

	int (*restrict_link)(struct key *keyring,
			     const struct key_type *key_type,
			     unsigned long key_flags,
			     const union key_payload *key_payload),

     where 'keyring' will be the keyring being added to, key_type and
     key_payload will describe the key being added and key_flags[*] can be
     AND'ed with KEY_FLAG_TRUSTED.

     [*] This parameter will be removed in a later patch when
     	 KEY_FLAG_TRUSTED is removed.

     The function should return 0 to allow the link to take place or an
     error (typically -ENOKEY, -ENOPKG or -EKEYREJECTED) to reject the
     link.

     The pointer should not be set directly, but rather should be set
     through keyring_alloc().

     Note that if called during add_key(), preparse is called before this
     method, but a key isn't actually allocated until after this function
     is called.

 (2) KEY_ALLOC_BYPASS_RESTRICTION is added.  This can be passed to
     key_create_or_update() or key_instantiate_and_link() to bypass the
     restriction check.

 (3) KEY_FLAG_TRUSTED_ONLY is removed.  The entire contents of a keyring
     with this restriction emplaced can be considered 'trustworthy' by
     virtue of being in the keyring when that keyring is consulted.

 (4) key_alloc() and keyring_alloc() take an extra argument that will be
     used to set restrict_link in the new key.  This ensures that the
     pointer is set before the key is published, thus preventing a window
     of unrestrictedness.  Normally this argument will be NULL.

 (5) As a temporary affair, keyring_restrict_trusted_only() is added.  It
     should be passed to keyring_alloc() as the extra argument instead of
     setting KEY_FLAG_TRUSTED_ONLY on a keyring.  This will be replaced in
     a later patch with functions that look in the appropriate places for
     authoritative keys.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Reviewed-by: NMimi Zohar <zohar@linux.vnet.ibm.com>

Changes the definition of the pointer _expiry from time_t to
time64_t. This is to handle the Y2038 problem where time_t
will overflow in the year 2038. The change is safe because
the kernel subsystems that call dns_query pass NULL.
Signed-off-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NAya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Merge the type-specific data with the payload data into one four-word chunk
as it seems pointless to keep them separate.

Use user_key_payload() for accessing the payloads of overloaded
user-defined keys.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
cc: linux-cifs@vger.kernel.org
cc: ecryptfs@vger.kernel.org
cc: linux-ext4@vger.kernel.org
cc: linux-f2fs-devel@lists.sourceforge.net
cc: linux-nfs@vger.kernel.org
cc: ceph-devel@vger.kernel.org
cc: linux-ima-devel@lists.sourceforge.net

Make the key matching functions pointed to by key_match_data::cmp return bool
rather than int.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NVivek Goyal <vgoyal@redhat.com>

A previous patch added a ->match_preparse() method to the key type.  This is
allowed to override the function called by the iteration algorithm.
Therefore, we can just set a default that simply checks for an exact match of
the key description with the original criterion data and allow match_preparse
to override it as needed.

The key_type::match op is then redundant and can be removed, as can the
user_match() function.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NVivek Goyal <vgoyal@redhat.com>

Preparse the match data.  This provides several advantages:

 (1) The preparser can reject invalid criteria up front.

 (2) The preparser can convert the criteria to binary data if necessary (the
     asymmetric key type really wants to do binary comparison of the key IDs).

 (3) The preparser can set the type of search to be performed.  This means
     that it's not then a one-off setting in the key type.

 (4) The preparser can set an appropriate comparator function.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NVivek Goyal <vgoyal@redhat.com>

Make use of key preparsing in the DNS resolver so that quota size determination
can take place prior to keyring locking when a key is being added.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NSteve Dickson <steved@redhat.com>
Acked-by: NJeff Layton <jlayton@primarydata.com>

*_result[len] is parsed as *(_result[len]) which is not at all what we
want to touch here.
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Fixes: 84a7c0b1 ("dns_resolver: assure that dns_query() result is null-terminated")
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Special kernel keys, such as those used to hold DNS results for AFS, CIFS and
NFS and those used to hold idmapper results for NFS, used to be
'invalidateable' with key_revoke().  However, since the default permissions for
keys were reduced:

	Commit: 96b5c8fe
	KEYS: Reduce initial permissions on keys

it has become impossible to do this.

Add a key flag (KEY_FLAG_ROOT_CAN_INVAL) that will permit a key to be
invalidated by root.  This should not be used for system keyrings as the
garbage collector will try and remove any invalidate key.  For system keyrings,
KEY_FLAG_ROOT_CAN_CLEAR can be used instead.

After this, from userspace, keyctl_invalidate() and "keyctl invalidate" can be
used by any possessor of CAP_SYS_ADMIN (typically root) to invalidate DNS and
idmapper keys.  Invalidated keys are immediately garbage collected and will be
immediately rerequested if needed again.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Tested-by: NSteve Dickson <steved@redhat.com>

dns_query() credulously assumes that keys are null-terminated and
returns a copy of a memory block that is off by one.
Signed-off-by: NManuel Schölling <manuel.schoelling@gmx.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

According to RFC1035 "[...] the total length of a domain name (i.e.,
label octets and label length octets) is restricted to 255 octets or
less."
Signed-off-by: NManuel Schölling <manuel.schoelling@gmx.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Several files refer to an old address for the Free Software Foundation
in the file header comment.  Resolve by replacing the address with
the URL <http://www.gnu.org/licenses/> so that we do not have to keep
updating the header comments anytime the address changes.

CC: John Fastabend <john.r.fastabend@intel.com>
CC: Alex Duyck <alexander.h.duyck@intel.com>
CC: Marcel Holtmann <marcel@holtmann.org>
CC: Gustavo Padovan <gustavo@padovan.org>
CC: Johan Hedberg <johan.hedberg@gmail.com>
CC: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

patch found using checkpatch.pl
Signed-off-by: NCosmin Stanescu <cosmin90stanescu@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Give the key type the opportunity to preparse the payload prior to the
instantiation and update routines being called.  This is done with the
provision of two new key type operations:

	int (*preparse)(struct key_preparsed_payload *prep);
	void (*free_preparse)(struct key_preparsed_payload *prep);

If the first operation is present, then it is called before key creation (in
the add/update case) or before the key semaphore is taken (in the update and
instantiate cases).  The second operation is called to clean up if the first
was called.

preparse() is given the opportunity to fill in the following structure:

	struct key_preparsed_payload {
		char		*description;
		void		*type_data[2];
		void		*payload;
		const void	*data;
		size_t		datalen;
		size_t		quotalen;
	};

Before the preparser is called, the first three fields will have been cleared,
the payload pointer and size will be stored in data and datalen and the default
quota size from the key_type struct will be stored into quotalen.

The preparser may parse the payload in any way it likes and may store data in
the type_data[] and payload fields for use by the instantiate() and update()
ops.

The preparser may also propose a description for the key by attaching it as a
string to the description field.  This can be used by passing a NULL or ""
description to the add_key() system call or the key_create_or_update()
function.  This cannot work with request_key() as that required the description
to tell the upcall about the key to be created.

This, for example permits keys that store PGP public keys to generate their own
name from the user ID and public key fingerprint in the key.

The instantiate() and update() operations are then modified to look like this:

	int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);
	int (*update)(struct key *key, struct key_preparsed_payload *prep);

and the new payload data is passed in *prep, whether or not it was preparsed.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>

Use keyring_alloc() to create special keyrings now that it has a permissions
parameter rather than using key_alloc() + key_instantiate_and_link().

Also document and export keyring_alloc() so that modules can use it too.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

In net/dns_resolver/dns_key.c and net/rxrpc/ar-key.c make them
work with user namespaces enabled where key_alloc takes kuids and kgids.
Pass GLOBAL_ROOT_UID and GLOBAL_ROOT_GID instead of bare 0's.

Cc: Sage Weil <sage@inktank.com>
Cc: ceph-devel@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Cc: David Miller <davem@davemloft.net>
Cc: linux-afs@lists.infradead.org
Acked-by: NSerge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>

Give the key type the opportunity to preparse the payload prior to the
instantiation and update routines being called.  This is done with the
provision of two new key type operations:

	int (*preparse)(struct key_preparsed_payload *prep);
	void (*free_preparse)(struct key_preparsed_payload *prep);

If the first operation is present, then it is called before key creation (in
the add/update case) or before the key semaphore is taken (in the update and
instantiate cases).  The second operation is called to clean up if the first
was called.

preparse() is given the opportunity to fill in the following structure:

	struct key_preparsed_payload {
		char		*description;
		void		*type_data[2];
		void		*payload;
		const void	*data;
		size_t		datalen;
		size_t		quotalen;
	};

Before the preparser is called, the first three fields will have been cleared,
the payload pointer and size will be stored in data and datalen and the default
quota size from the key_type struct will be stored into quotalen.

The preparser may parse the payload in any way it likes and may store data in
the type_data[] and payload fields for use by the instantiate() and update()
ops.

The preparser may also propose a description for the key by attaching it as a
string to the description field.  This can be used by passing a NULL or ""
description to the add_key() system call or the key_create_or_update()
function.  This cannot work with request_key() as that required the description
to tell the upcall about the key to be created.

This, for example permits keys that store PGP public keys to generate their own
name from the user ID and public key fingerprint in the key.

The instantiate() and update() operations are then modified to look like this:

	int (*instantiate)(struct key *key, struct key_preparsed_payload *prep);
	int (*update)(struct key *key, struct key_preparsed_payload *prep);

and the new payload data is passed in *prep, whether or not it was preparsed.
Signed-off-by: NDavid Howells <dhowells@redhat.com>

Announce the (un)registration of a key type in the core key code rather than
in the callers.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NMimi Zohar <zohar@us.ibm.com>

Use of "unsigned int" is preferred to bare "unsigned" in net tree.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The kernel contains some special internal keyrings, for instance the DNS
resolver keyring :

2a93faf1 I-----     1 perm 1f030000     0     0 keyring   .dns_resolver: empty

It would occasionally be useful to allow the contents of such keyrings to be
flushed by root (cache invalidation).

Allow a flag to be set on a keyring to mark that someone possessing the
sysadmin capability can clear the keyring, even without normal write access to
the keyring.

Set this flag on the special keyrings created by the DNS resolver, the NFS
identity mapper and the CIFS identity mapper.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NJeff Layton <jlayton@redhat.com>
Acked-by: NSteve Dickson <steved@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

Improve /proc/keys by:

 (1) Don't attempt to summarise the payload of a negated key.  It won't have
     one.  To this end, a helper function - key_is_instantiated() has been
     added that allows the caller to find out whether the key is positively
     instantiated (as opposed to being uninstantiated or negatively
     instantiated).

 (2) Do show keys that are negative, expired or revoked rather than hiding
     them.  This requires an override flag (no_state_check) to be passed to
     search_my_process_keyrings() and keyring_search_aux() to suppress this
     check.

     Without this, keys that are possessed by the caller, but only grant
     permissions to the caller if possessed are skipped as the possession check
     fails.

     Keys that are visible due to user, group or other checks are visible with
     or without this patch.
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

When a DNS resolver key is instantiated with an error indication, attempts to
read that key will result in an oops because user_read() is expecting there to
be a payload - and there isn't one [CVE-2011-1076].

Give the DNS resolver key its own read handler that returns the error cached in
key->type_data.x[0] as an error rather than crashing.

Also make the kenter() at the beginning of dns_resolver_instantiate() limit the
amount of data it prints, since the data is not necessarily NUL-terminated.

The buggy code was added in:

	commit 4a2d7892
	Author: Wang Lei <wang840925@gmail.com>
	Date:   Wed Aug 11 09:37:58 2010 +0100
	Subject: DNS: If the DNS server returns an error, allow that to be cached [ver #2]

This can trivially be reproduced by any user with the following program
compiled with -lkeyutils:

	#include <stdlib.h>
	#include <keyutils.h>
	#include <err.h>
	static char payload[] = "#dnserror=6";
	int main()
	{
		key_serial_t key;
		key = add_key("dns_resolver", "a", payload, sizeof(payload),
			      KEY_SPEC_SESSION_KEYRING);
		if (key == -1)
			err(1, "add_key");
		if (keyctl_read(key, NULL, 0) == -1)
			err(1, "read_key");
		return 0;
	}

What should happen is that keyctl_read() reports error 6 (ENXIO) to the user:

	dns-break: read_key: No such device or address

but instead the kernel oopses.

This cannot be reproduced with the 'keyutils add' or 'keyutils padd' commands
as both of those cut the data down below the NUL termination that must be
included in the data.  Without this dns_resolver_instantiate() will return
-EINVAL and the key will not be instantiated such that it can be read.

The oops looks like:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffff811b99f7>] user_read+0x4f/0x8f
PGD 3bdf8067 PUD 385b9067 PMD 0
Oops: 0000 [#1] SMP
last sysfs file: /sys/devices/pci0000:00/0000:00:19.0/irq
CPU 0
Modules linked in:

Pid: 2150, comm: dns-break Not tainted 2.6.38-rc7-cachefs+ #468                  /DG965RY
RIP: 0010:[<ffffffff811b99f7>]  [<ffffffff811b99f7>] user_read+0x4f/0x8f
RSP: 0018:ffff88003bf47f08  EFLAGS: 00010246
RAX: 0000000000000001 RBX: ffff88003b5ea378 RCX: ffffffff81972368
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff88003b5ea378
RBP: ffff88003bf47f28 R08: ffff88003be56620 R09: 0000000000000000
R10: 0000000000000395 R11: 0000000000000002 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: ffffffffffffffa1
FS:  00007feab5751700(0000) GS:ffff88003e000000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000010 CR3: 000000003de40000 CR4: 00000000000006f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Process dns-break (pid: 2150, threadinfo ffff88003bf46000, task ffff88003be56090)
Stack:
 ffff88003b5ea378 ffff88003b5ea3a0 0000000000000000 0000000000000000
 ffff88003bf47f68 ffffffff811b708e ffff88003c442bc8 0000000000000000
 00000000004005a0 00007fffba368060 0000000000000000 0000000000000000
Call Trace:
 [<ffffffff811b708e>] keyctl_read_key+0xac/0xcf
 [<ffffffff811b7c07>] sys_keyctl+0x75/0xb6
 [<ffffffff81001f7b>] system_call_fastpath+0x16/0x1b
Code: 75 1f 48 83 7b 28 00 75 18 c6 05 58 2b fb 00 01 be bb 00 00 00 48 c7 c7 76 1c 75 81 e8 13 c2 e9 ff 4c 8b b3 e0 00 00 00 4d 85 ed <41> 0f b7 5e 10 74 2d 4d 85 e4 74 28 e8 98 79 ee ff 49 39 dd 48
RIP  [<ffffffff811b99f7>] user_read+0x4f/0x8f
 RSP <ffff88003bf47f08>
CR2: 0000000000000010
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NJeff Layton <jlayton@redhat.com>
cc: Wang Lei <wang840925@gmail.com>
Signed-off-by: NJames Morris <jmorris@namei.org>

Changed Makefile to use <modules>-y instead of <modules>-objs
because -objs is deprecated and not mentioned in
Documentation/kbuild/makefiles.txt.
Signed-off-by: NTracey Dent <tdent48227@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

If the DNS server returns an error, allow that to be cached in the DNS resolver
key in lieu of a value.  Userspace passes the desired error number as an option
in the payload:

	"#dnserror=<number>"

Userspace must map h_errno from the name resolution routines to an appropriate
Linux error before passing it up.  Something like the following mapping is
recommended:

	[HOST_NOT_FOUND]	= ENODATA,
	[TRY_AGAIN]		= EAGAIN,
	[NO_RECOVERY]		= ECONNREFUSED,
	[NO_DATA]		= ENODATA,

in lieu of Linux errors specifically for representing name service errors.  The
filesystem must map these errors appropropriately before passing them to
userspace.  AFS is made to map ENODATA and EAGAIN to EDESTADDRREQ for the
return to userspace; ECONNREFUSED is allowed to stand as is.

The error can be seen in /proc/keys as a negative number after the description
of the key.  Compare, for example, the following key entries:

2f97238c I--Q--     1  53s 3f010000     0     0 dns_resol afsdb:grand.centrall.org: -61
338bfbbe I--Q--     1  59m 3f010000     0     0 dns_resol afsdb:grand.central.org: 37

If the error option is supplied in the payload, the main part of the payload is
discarded.  The key should have an expiry time set by userspace.
Signed-off-by: NWang Lei <wang840925@gmail.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

CC: Dave Howells <dhowells@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

Fixes for the DNS query module, including:

 (1) Use 'negative' instead of '-ve' in the documentation.

 (2) Mark the kdoc comment with '/**' on dns_query().
Reported-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

Fixes build errors:

net/dns_resolver/dns_key.c: In function 'init_dns_resolver':
net/dns_resolver/dns_key.c:170: error: implicit declaration of function 'IS_ERR'
net/dns_resolver/dns_key.c:171: error: implicit declaration of function 'PTR_ERR'
net/dns_resolver/dns_query.c: In function 'dns_query':
net/dns_resolver/dns_query.c:126: error: implicit declaration of function 'IS_ERR'
net/dns_resolver/dns_query.c:127: error: implicit declaration of function 'PTR_ERR'
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

Separate out the DNS resolver key type from the CIFS filesystem into its own
module so that it can be made available for general use, including the AFS
filesystem module.

This facility makes it possible for the kernel to upcall to userspace to have
it issue DNS requests, package up the replies and present them to the kernel
in a useful form.  The kernel is then able to cache the DNS replies as keys
can be retained in keyrings.

Resolver keys are of type "dns_resolver" and have a case-insensitive
description that is of the form "[<type>:]<domain_name>".  The optional <type>
indicates the particular DNS lookup and packaging that's required.  The
<domain_name> is the query to be made.

If <type> isn't given, a basic hostname to IP address lookup is made, and the
result is stored in the key in the form of a printable string consisting of a
comma-separated list of IPv4 and IPv6 addresses.

This key type is supported by userspace helpers driven from /sbin/request-key
and configured through /etc/request-key.conf.  The cifs.upcall utility is
invoked for UNC path server name to IP address resolution.

The CIFS functionality is encapsulated by the dns_resolve_unc_to_ip() function,
which is used to resolve a UNC path to an IP address for CIFS filesystem.  This
part remains in the CIFS module for now.

See the added Documentation/networking/dns_resolver.txt for more information.
Signed-off-by: NWang Lei <wang840925@gmail.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Acked-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>