Re: [PATCH 2/2] xfs: make sure link path does not go away at access

On Thu, 2021-11-11 at 11:08 -0500, Brian Foster wrote:

Hi Brian,

On Thu, Nov 11, 2021 at 11:39:30AM +0800, Ian Kent wrote:

quoted

When following a trailing symlink in rcu-walk mode it's possible to
succeed in getting the ->get_link() method pointer but the link
path
string be deallocated while it's being used.

Utilize the rcu mechanism to mitigate this risk.

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Ian Kent <raven@themaw.net>
---
 fs/xfs/kmem.h      |    4 ++++
 fs/xfs/xfs_inode.c |    4 ++--
 fs/xfs/xfs_iops.c  |   10 ++++++++--
 3 files changed, 14 insertions(+), 4 deletions(-)

...

quoted

diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
index a607d6aca5c4..2977e19da7b7 100644
--- a/fs/xfs/xfs_iops.c
+++ b/fs/xfs/xfs_iops.c

@@ -524,11 +524,17 @@ xfs_vn_get_link_inline(

 
        /*
         * The VFS crashes on a NULL pointer, so return -
EFSCORRUPTED if
-        * if_data is junk.
+        * if_data is junk. Also, if the path walk is in rcu-walk
mode
+        * and the inode link path has gone away due inode re-use
we have
+        * no choice but to tell the VFS to redo the lookup.
         */
-       link = ip->i_df.if_u1.if_data;
+       link = rcu_dereference(ip->i_df.if_u1.if_data);
+       if (!dentry && !link)
+               return ERR_PTR(-ECHILD);
+

One thing that concerns me slightly about this approach is that inode
reuse does not necessarily guarantee that if_data is NULL. It seems
technically just as possible (even if exceedingly unlikely) for link
to
point at newly allocated memory since the previous sequence count
validation check. The inode could be reused as another inline symlink
for example, though it's not clear to me if that is really a problem
for
the vfs (assuming a restart would just land on the new link
anyways?).
But the inode could also be reallocated as something like a shortform
directory, which means passing directory header data or whatever that
it
stores in if_data back to pick_link(), which is then further
processed
as a string.

This is the sort of feedback I was hoping for.

This sounds related to the life-cycle of xfs inodes and re-use.
Hopefully someone here on the list can enlighten me on this.

The thing that comes to mind is that the inode re-use would
need to occur between the VFS check that validates the inode
is still ok and the use of link string. I think that can still
go away even with the above check.

Hopefully someone can clarify what happens here.

With that, I wonder why we wouldn't just return -ECHILD here like we
do
for the non-inline case to address the immediate problem, and then
perhaps separately consider if we can rework bits of the
reuse/reclaim
code to allow rcu lookup of inline symlinks under certain conditions.

Always switching to ref-walk mode would certainly resolve the
problem too, yes, perhaps we have no choice ...

Ian

FWIW, I'm also a little curious why we don't set i_link for inline
symlinks. I don't think that addresses this validation problem, but
perhaps might allow rcu lookups in the inline symlink common case
where
things don't change during the lookup (and maybe even eliminate the
need
for this custom inline callback)..?

Brian

quoted

        if (XFS_IS_CORRUPT(ip->i_mount, !link))
                return ERR_PTR(-EFSCORRUPTED);
+
        return link;
 }