From: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
This patch does NOT apply to the 3.12 stable tree. If you still want
it applied, please provide a backport.
===============
commit b37199e626b31e1175fb06764c5d1d687723aac2 upstream.
We can get false negative from __lookup_mnt() if an unrelated vfsmount
gets moved. In that case legitimize_mnt() is guaranteed to fail,
and we will fall back to non-RCU walk... unless we end up running
into a hard error on a filesystem object we wouldn't have reached
if not for that false negative. IOW, delaying that check until
the end of pathname resolution is wrong - we should recheck right
after we attempt to cross the mountpoint. We don't need to recheck
unless we see d_mountpoint() being true - in that case even if
we have just raced with mount/umount, we can simply go on as if
we'd come at the moment when the sucker wasn't a mountpoint; if we
run into a hard error as the result, it was a legitimate outcome.
__lookup_mnt() returning NULL is different in that respect, since
it might've happened due to operation on completely unrelated
mountpoint.
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
fs/namei.c | 29 +++++++++++++----------------
1 file changed, 13 insertions(+), 16 deletions(-)
diff --git a/fs/namei.c b/fs/namei.c
index 2f730ef9b4b3..4b491b431990 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1109,7 +1109,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
return false;
if (!d_mountpoint(path->dentry))
- break;
+ return true;
mounted = __lookup_mnt(path->mnt, path->dentry);
if (!mounted)
@@ -1125,20 +1125,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
*/
*inode = path->dentry->d_inode;
}
- return true;
-}
-
-static void follow_mount_rcu(struct nameidata *nd)
-{
- while (d_mountpoint(nd->path.dentry)) {
- struct mount *mounted;
- mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
- if (!mounted)
- break;
- nd->path.mnt = &mounted->mnt;
- nd->path.dentry = mounted->mnt.mnt_root;
- nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
- }
+ return read_seqretry(&mount_lock, nd->m_seq);
}
static int follow_dotdot_rcu(struct nameidata *nd)
@@ -1166,7 +1153,17 @@ static int follow_dotdot_rcu(struct nameidata *nd)
break;
nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
}
- follow_mount_rcu(nd);
+ while (d_mountpoint(nd->path.dentry)) {
+ struct mount *mounted;
+ mounted = __lookup_mnt(nd->path.mnt, nd->path.dentry);
+ if (!mounted)
+ break;
+ nd->path.mnt = &mounted->mnt;
+ nd->path.dentry = mounted->mnt.mnt_root;
+ nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
+ if (!read_seqretry(&mount_lock, nd->m_seq))
+ goto failed;
+ }
nd->inode = nd->path.dentry->d_inode;
return 0;
--
1.9.1
--
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