From f35e3b5784221654f9cdbd6222275a7fa203f6c3 Mon Sep 17 00:00:00 2001 From: Chris Mason Date: Thu, 1 Oct 2026 13:50:22 +0000 Subject: futex: Fix private hash use-after-free on resize poll_state_synchronize_rcu(mm->futex.phash.batches) is used by futex_ref_drop() to check that a grace period has passed since the current hash was published. This relies on batches referencing a grace period which started after the hash pointer was assigned. __futex_pivot_hash() sets mmph->batches before it replaces mmph->hash: scoped_guard(rcu) { mmph->batches = get_state_synchronize_rcu(); rcu_assign_pointer(mmph->hash, new); } The scoped_guard(rcu) doesn't stop new grace periods from starting, and if one starts between those two assignments, futex_ref_drop() can move forward while a reader still holds a pointer to the old hash. Fix things by setting mmph->batches after assigning mmph->hash. The scoped_guard(rcu) isn't needed, so let's drop that as well. Fixes: 56180dd20c19 ("futex: Use RCU-based per-CPU reference counting instead of rcuref_t") Assisted-by: kres Signed-off-by: Chris Mason Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Paul E. McKenney Link: https://patch.msgid.link/20261001135022.2220288-1-mason@kernel.org --- kernel/futex/core.c | 10 ++++++---- 1 file changed, 6 insertions(+), 4 deletions(-) diff --git a/kernel/futex/core.c b/kernel/futex/core.c index a061f54b6..095f9fe44 100644 --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -213,10 +213,12 @@ static bool __futex_pivot_hash(struct mm_struct *mm, struct futex_private_hash * futex_rehash_private(fph, new); } new->state = FR_PERCPU; - scoped_guard(rcu) { - mmph->batches = get_state_synchronize_rcu(); - rcu_assign_pointer(mmph->hash, new); - } + rcu_assign_pointer(mmph->hash, new); + /* + * mmph->batches must reference a grace period which started after + * mmph->hash was assigned. See futex_ref_drop(). + */ + mmph->batches = get_state_synchronize_rcu(); kvfree_rcu(fph, rcu); return true; } -- cgit v1.3.1