nfsd4_sequence() can free the very slot it is currently processing.
When the session shrinker has reduced se_target_maxslots below
se_fchannel.maxreqs, the shrink path checks three conditions before
calling free_session_slots():

  1. se_target_maxslots < maxreqs  (shrink was advertised)
  2. slot->sl_generation == se_slot_gen  (slot is up-to-date)
  3. seq->maxslots <= se_target_maxslots  (client acknowledges)

However, seq->slotid is never checked against se_target_maxslots.
A client using a slot in the range [se_target_maxslots, maxreqs) can
satisfy all three conditions: its slot has the current generation
(set by a prior SEQUENCE), and it sends sa_highest_slotid <=
se_target_maxslots to acknowledge the reduction.

free_session_slots() then kfrees every slot at index >=
se_target_maxslots, including the caller's own slot. The function
continues to write sl_seqid, sl_flags, sl_generation, and stores the
dangling pointer in cstate->slot. Later, nfsd4_store_cache_entry()
copies up to maxresp_cached bytes of the compound reply into the freed
sl_data[] array, corrupting whatever slab object now occupies that
address.

This is a remotely triggerable heap use-after-free from any
authenticated NFSv4.1+ client with an established session, requiring
only that the kernel memory shrinker has run against nfsd session slots
(a normal occurrence under memory pressure).

Fix this by adding a check that the current request's slotid is below
the shrink boundary before allowing the slot reduction to proceed.

Fixes: 8fb77d12c76e ("nfsd: add support for freeing unused session-DRC slots")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/nfsd/nfs4state.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index d5cbf626ab9b..f90ad8281a95 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -4848,7 +4848,8 @@ nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 
 	if (session->se_target_maxslots < session->se_fchannel.maxreqs &&
 	    slot->sl_generation == session->se_slot_gen &&
-	    seq->maxslots <= session->se_target_maxslots)
+	    seq->maxslots <= session->se_target_maxslots &&
+	    seq->slotid < session->se_target_maxslots)
 		/* Client acknowledged our reduce maxreqs */
 		free_session_slots(session, session->se_target_maxslots);
 

---
base-commit: 97bac3c7a039675d7ae71fbdf3a7c39e840339b6
change-id: 20260526-nfsd4_sequence_shrink_uaf_on_loaded_slot-19843be018ac

Best regards,
-- 
Jeff Layton <jlayton@kernel.org>