From f5d22e372f4ac3eb287037a488c29691d52a6330 Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Tue, 26 May 2026 12:24:48 -0400 Subject: [PATCH] nfsd: don't free session slots that are still in use 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. Additionally, a concurrent thread processing SEQUENCE on a different high-numbered slot can have its slot freed out from under it. NFSD4_SLOT_INUSE is set under nn->client_lock before the lock is released, so any concurrent thread past SEQUENCE will have its slot marked. However, free_session_slots() does not check NFSD4_SLOT_INUSE before freeing. Fix both problems by: 1. Checking that the current request's slotid is below the shrink boundary. 2. Scanning slots in the to-be-freed range for NFSD4_SLOT_INUSE and deferring the shrink if any are active. Fixes: fc8738c68d0b ("nfsd: add support for freeing unused session-DRC slots") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-6 Signed-off-by: Jeff Layton Link: https://patch.msgid.link/20260526-nfsd4_sequence_shrink_uaf_on_loaded_slot-v2-1-74a89db0639e@kernel.org Signed-off-by: Chuck Lever --- fs/nfsd/nfs4state.c | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c index 8f90fba53357..0d6a7440a4e2 100644 --- a/fs/nfsd/nfs4state.c +++ b/fs/nfsd/nfs4state.c @@ -4496,6 +4496,19 @@ static void nfsd4_construct_sequence_response(struct nfsd4_session *session, seq->status_flags |= SEQ4_STATUS_ADMIN_STATE_REVOKED; } +static bool nfsd4_slots_inuse(struct nfsd4_session *ses, int from) +{ + int i; + + for (i = from; i < ses->se_fchannel.maxreqs; i++) { + struct nfsd4_slot *slot = xa_load(&ses->se_slots, i); + + if (slot->sl_flags & NFSD4_SLOT_INUSE) + return true; + } + return false; +} + __be32 nfsd4_sequence(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate, union nfsd4_op_u *u) @@ -4575,7 +4588,9 @@ 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 && + !nfsd4_slots_inuse(session, session->se_target_maxslots)) /* Client acknowledged our reduce maxreqs */ free_session_slots(session, session->se_target_maxslots);