Re: [PATCH v3] lib/rhashtable: clear stale iter->p on table restart

[Herbert Xu]

On Tue, Jul 07, 2026 at 12:41:15PM -0400, Cen Zhang (Microsoft) wrote:
> rhashtable_walk_start_check() has two restart paths when resuming a walk.
> When iter->walker.tbl is valid, it re-validates iter->p against the table
> and sets iter->p = NULL if the object is gone.  When iter->walker.tbl is
> NULL (table was freed during resize), it resets slot and skip but forgets
> to clear iter->p.
> 
> rhashtable_walk_next() then dereferences the stale iter->p, reading
> freed memory.  This is a use-after-free.

Maybe I'm misreading the original patch (in the Fixes header).  But
it seems the whole point of having it is to look for iter->p in the
new table.  Even if the hash table remains the same iter->p could have
been freed since we hold no reference to that object.

If that is the case, then resetting iter->p on a resize doesn't
fix this at all since the root cause is that iter->p is being
held with no reference.

I think we should just revert the original patch since the whole
concept doesn't seem to work (although it's salvageable for the
non-rhlist case).

Thanks,
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt