🛡️ CVE-2024-57924 — kernel

fs: relax assertions on failure to encode file handles

In the Linux kernel, the following vulnerability has been resolved:

fs: relax assertions on failure to encode file handles

Encoding file handles is usually performed by a filesystem >encode_fh()

method that may fail for various reasons.

The legacy users of exportfs_encode_fh(), namely, nfsd and

name_to_handle_at(2) syscall are ready to cope with the possibility

of failure to encode a file handle.

There are a few other users of exportfs_encode_{fh,fid}() that

currently have a WARN_ON() assertion when ->encode_fh() fails.

Relax those assertions because they are wrong.

The second linked bug report states commit 16aac5ad1fa9 ("ovl: support

encoding non-decodable file handles") in v6.6 as the regressing commit,

but this is not accurate.

The aforementioned commit only increases the chances of the assertion

and allows triggering the assertion with the reproducer using overlayfs,

inotify and drop_caches.

Triggering this assertion was always possible with other filesystems and

other reasons of ->encode_fh() failures and more particularly, it was

also possible with the exact same reproducer using overlayfs that is

mounted with options index=on,nfs_export=on also on kernels < v6.6.

Therefore, I am not listing the aforementioned commit as a Fixes commit.

Backport hint: this patch will have a trivial conflict applying to

v6.6.y, and other trivial conflicts applying to stable kernels < v6.6.