[PATCH] sched/mmcid: fix OOB clear_bit when CID is MM_CID_UNSET in fixup path

[Rik van Riel <riel@surriel.com>]

In mm_cid_fixup_cpus_to_tasks(), when rq->curr has the target mm and
mm_cid.active is set, the CID is checked with cid_in_transit() before
setting the transition bit.  In per-CPU mode a newly forked or exec'd
task can be running with mm_cid.cid == MM_CID_UNSET because CIDs are
assigned lazily on schedule-in.  With cid_in_transit() the guard passes
for MM_CID_UNSET (no transit bit), converts it to MM_CID_UNSET |
MM_CID_TRANSIT and stores it back; later mm_cid_schedout() feeds this
to clear_bit() with MM_CID_UNSET as the bit number, triggering an
out-of-bounds write.

Symptoms: this is genuine memory corruption, but a bounded out-of-bounds
write, not an arbitrary one.  MM_CID_UNSET is the fixed sentinel BIT(31),
so once the bad value reaches mm_cid_schedout() the cid_from_transit_cid()
strip leaves MM_CID_UNSET, which fails the "cid < max_cids" convergence
test and falls into mm_drop_cid() -> clear_bit(MM_CID_UNSET,
mm_cidmask(mm)).  The cid bitmap is embedded in the mm_struct slab object
(after cpu_bitmap and mm_cpus_allowed) and is only num_possible_cpus()
bits wide, so clearing bit 31 is a deterministic OOB bit-clear at a
fixed offset of 2^31 / 8 == 256 MiB past the bitmap base.  The address is
not attacker-influenced (fixed sentinel -> fixed offset) and the op only
clears a single bit; what sits 256 MiB further along the direct map is
whatever kernel object happens to live there, so this corrupts one bit of
unpredictable kernel memory -- it is not an arbitrary-address or
arbitrary-value write.

It triggers only in per-CPU CID mode, when a CPU is running an active
task of the target mm whose cid is still MM_CID_UNSET -- the
fork()/execve() window before that task's next schedule-in assigns it a
real CID -- and a per-CPU -> per-task fixup walks over it (the mode
fallback driven by a thread exit, sched_mm_cid_exit(), or by the deferred
max_cids recompute in mm_cid_work_fn()).

In practice syzkaller surfaced it as a KASAN use-after-free reported in
__schedule -> mm_cid_switch_to, where the offending clear_bit() is inlined
via mm_cid_schedout() -> mm_drop_cid().

Switch to cid_on_task() which excludes MM_CID_UNSET, MM_CID_ONCPU, and
MM_CID_TRANSIT, so we only set the transition bit on a genuine
task-owned CID.

Fixes: fbd0e71dc370 ("sched/mmcid: Provide CID ownership mode fixup functions")
Assisted-by: Claude:claude-opus-4-8 syzkaller
Signed-off-by: Rik van Riel <riel@surriel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: K Prateek Nayak <kprateek.nayak@amd.com>
Cc: linux-kernel@vger.kernel.org
---
 kernel/sched/core.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 8b791e9e9f67..4c8b6ca254ce 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -10909,8 +10909,19 @@ static void mm_cid_fixup_cpus_to_tasks(struct mm_struct *mm)
 		} else if (rq->curr->mm == mm && rq->curr->mm_cid.active) {
 			unsigned int cid = rq->curr->mm_cid.cid;
 
-			/* Ensure it has the transition bit set */
-			if (!cid_in_transit(cid)) {
+			/*
+			 * Only a genuine task-owned CID needs the transition
+			 * bit. A running active task can legitimately have
+			 * MM_CID_UNSET here: in per-CPU mode CIDs are assigned
+			 * lazily on schedule-in, so fork()/execve() leave the
+			 * task active with no owned CID until its next
+			 * schedule-in. cid_on_task() excludes the
+			 * MM_CID_UNSET/ONCPU/TRANSIT bits, so we never turn
+			 * e.g. MM_CID_UNSET into MM_CID_UNSET|MM_CID_TRANSIT,
+			 * which mm_cid_schedout() would later feed to
+			 * clear_bit() as an out-of-bounds bit number.
+			 */
+			if (cid_on_task(cid)) {
 				cid = cid_to_transit_cid(cid);
 				rq->curr->mm_cid.cid = cid;
 				pcp->cid = cid;
-- 
2.53.0-Meta