syzbot is reporting that use of SOCKET_I()->sk from open() can result in
use after free problem [1], for socket's inode is still reachable via
/proc/pid/fd/n despite destruction of SOCKET_I()->sk already completed.

But there is no point with calling security_file_open() on sockets
because open("/proc/pid/fd/n", !O_PATH) on sockets fails with -ENXIO.

There is some point with calling security_inode_getattr() on sockets
because stat("/proc/pid/fd/n") and fstat(open("/proc/pid/fd/n", O_PATH))
are valid. If we want to access "struct sock"->sk_{family,type,protocol}
fields, we will need to use security_socket_post_create() hook and
security_inode_free() hook in order to remember these fields because
security_sk_free() hook is called before the inode is destructed. But
since information which can be protected by checking
security_inode_getattr() on sockets is trivial, let's not be bothered by
"struct inode"->i_security management.

There is point with calling security_file_ioctl() on sockets. Since
ioctl(open("/proc/pid/fd/n", O_PATH)) is invalid, security_file_ioctl()
on sockets should remain safe.

[1] https://syzkaller.appspot.com/bug?id=73d590010454403d55164cca23bd0565b1eb3b74

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <redacted>
---
 security/tomoyo/tomoyo.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/security/tomoyo/tomoyo.c b/security/tomoyo/tomoyo.c
index 716c92e..8ea3f5d 100644
--- a/security/tomoyo/tomoyo.c
+++ b/security/tomoyo/tomoyo.c

@@ -126,6 +126,9 @@ static int tomoyo_bprm_check_security(struct linux_binprm *bprm)
  */
 static int tomoyo_inode_getattr(const struct path *path)
 {
+	/* It is not safe to call tomoyo_get_socket_name(). */
+	if (S_ISSOCK(d_inode(path->dentry)->i_mode))
+		return 0;
 	return tomoyo_path_perm(TOMOYO_TYPE_GETATTR, path, NULL);
 }
 

@@ -316,6 +319,9 @@ static int tomoyo_file_open(struct file *f)
 	/* Don't check read permission here if called from do_execve(). */
 	if (current->in_execve)
 		return 0;
+	/* Sockets can't be opened by open(). */
+	if (S_ISSOCK(file_inode(f)->i_mode))
+		return 0;
 	return tomoyo_check_open_permission(tomoyo_domain(), &f->f_path,
 					    f->f_flags);
 }

-- 
1.8.3.1