fget: check that the fd still exists after getting a ref to it

stable inclusion
from stable-5.10.84
commit 4baba6ba56eb91a735a027f783cc4b9276b48d5b
category: bugfix
bugzilla: 185907 https://gitee.com/openeuler/kernel/issues/I4DDEL
CVE: CVE-2021-4083

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.10.y&id=4baba6ba56eb91a735a027f783cc4b9276b48d5b

-------------------------------------------------

commit 054aa8d4 upstream.

Jann Horn points out that there is another possible race wrt Unix domain
socket garbage collection, somewhat reminiscent of the one fixed in
commit cbcf0112 ("af_unix: fix garbage collect vs MSG_PEEK").

See the extended comment about the garbage collection requirements added
to unix_peek_fds() by that commit for details.

The race comes from how we can locklessly look up a file descriptor just
as it is in the process of being closed, and with the right artificial
timing (Jann added a few strategic 'mdelay(500)' calls to do that), the
Unix domain socket garbage collector could see the reference count
decrement of the close() happen before fget() took its reference to the
file and the file was attached onto a new file descriptor.

This is all (intentionally) correct on the 'struct file *' side, with
RCU lookups and lockless reference counting very much part of the
design.  Getting that reference count out of order isn't a problem per
se.

But the garbage collector can get confused by seeing this situation of
having seen a file not having any remaining external references and then
seeing it being attached to an fd.

In commit cbcf0112 ("af_unix: fix garbage collect vs MSG_PEEK") the
fix was to serialize the file descriptor install with the garbage
collector by taking and releasing the unix_gc_lock.

That's not really an option here, but since this all happens when we are
in the process of looking up a file descriptor, we can instead simply
just re-check that the file hasn't been closed in the meantime, and just
re-do the lookup if we raced with a concurrent close() of the same file
descriptor.
Reported-and-tested-by: NJann Horn <jannh@google.com>
Acked-by: NMiklos Szeredi <mszeredi@redhat.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NBaokun Li <libaokun1@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

fs/file.c

@@ -883,6 +883,10 @@ static struct file *__fget_files(struct files_struct *files, unsigned int fd,
 			file = NULL;
 		else if (!get_file_rcu_many(file, refs))
 			goto loop;
+		else if (__fcheck_files(files, fd) != file) {
+			fput_many(file, refs);
+			goto loop;
+		}
 	}
 	rcu_read_unlock();