security: make inode_follow_link RCU-walk aware

inode_follow_link now takes an inode and rcu flag as well as the
dentry.

inode is used in preference to d_backing_inode(dentry), particularly
in RCU-walk mode.

selinux_inode_follow_link() gets dentry_has_perm() and
inode_has_perm() open-coded into it so that it can call
avc_has_perm_flags() in way that is safe if LOOKUP_RCU is set.

Calling avc_has_perm_flags() with rcu_read_lock() held means
that when avc_has_perm_noaudit calls avc_compute_av(), the attempt
to rcu_read_unlock() before calling security_compute_av() will not
actually drop the RCU read-lock.

However as security_compute_av() is completely in a read_lock()ed
region, it should be safe with the RCU read-lock held.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 3 insertions, 2 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 33b655d..0fa7af2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -881,8 +881,9 @@ const char *get_link(struct nameidata *nd)
 
 	touch_atime(&last->link);
 
-	error = security_inode_follow_link(dentry);
-	if (error)
+	error = security_inode_follow_link(dentry, inode,
+					   nd->flags & LOOKUP_RCU);
+	if (unlikely(error))
 		return ERR_PTR(error);
 
 	nd->last_type = LAST_BIND;