bpf: relax inode permission check for retrieving bpf program

commit e547ff3f803e779a3898f1f48447b29f43c54085 upstream.

For iptable module to load a bpf program from a pinned location, it
only retrieve a loaded program and cannot change the program content so
requiring a write permission for it might not be necessary.
Also when adding or removing an unrelated iptable rule, it might need to
flush and reload the xt_bpf related rules as well and triggers the inode
permission check. It might be better to remove the write premission
check for the inode so we won't need to grant write access to all the
processes that flush and restore iptables rules.
Signed-off-by: NChenbo Feng <fengc@google.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/bpf/inode.c

@@ -518,7 +518,7 @@ int bpf_obj_get_user(const char __user *pathname, int flags)
 static struct bpf_prog *__get_prog_inode(struct inode *inode, enum bpf_prog_type type)
 {
 	struct bpf_prog *prog;
-	int ret = inode_permission(inode, MAY_READ | MAY_WRITE);
+	int ret = inode_permission(inode, MAY_READ);
 	if (ret)
 		return ERR_PTR(ret);