blk_mq_realloc_hw_ctxs() will free the 'queue_hw_ctx'(e.g. undate
submit_queues through configfs for null_blk), while it might still be
used from other context(e.g. switch elevator to none):
t1 t2
elevator_switch
blk_mq_unquiesce_queue
blk_mq_run_hw_queues
queue_for_each_hw_ctx
// assembly code for hctx = (q)->queue_hw_ctx[i]
mov 0x48(%rbp),%rdx -> read old queue_hw_ctx
__blk_mq_update_nr_hw_queues
blk_mq_realloc_hw_ctxs
hctxs = q->queue_hw_ctx
q->queue_hw_ctx = new_hctxs
kfree(hctxs)
movslq %ebx,%rax
mov (%rdx,%rax,8),%rdi ->uaf
This problem was found by code review, and I comfirmed that the concurrent
scenarios do exist(specifically 'q->queue_hw_ctx' can be changed during
blk_mq_run_hw_queues), however, the uaf problem hasn't been repoduced yet
without hacking the kernel.
Sicne the queue is freezed in __blk_mq_update_nr_hw_queues, fix the
problem by protecting 'queue_hw_ctx' through rcu where it can be accessed
without grabbing 'q_usage_counter'.
Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
block/blk-mq.c | 8 +++++++-
include/linux/blk-mq.h | 2 +-
include/linux/blkdev.h | 13 ++++++++++++-
3 files changed, 20 insertions(+), 3 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6c59ffe765fd..79367457d555 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3955,7 +3955,13 @@ static void blk_mq_realloc_hw_ctxs(struct blk_mq_tag_set *set,
if (hctxs)
memcpy(new_hctxs, hctxs, q->nr_hw_queues *
sizeof(*hctxs));
- q->queue_hw_ctx = new_hctxs;
+
+ rcu_assign_pointer(q->queue_hw_ctx, new_hctxs);
+ /*
+ * Make sure reading the old queue_hw_ctx from other
+ * context concurrently won't trigger uaf.
+ */
+ synchronize_rcu();
kfree(hctxs);
hctxs = new_hctxs;
}
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index d319ffa59354..edcf8ead76c6 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -918,7 +918,7 @@ static inline void *blk_mq_rq_to_pdu(struct request *rq)
#define queue_for_each_hw_ctx(q, hctx, i) \
for ((i) = 0; (i) < (q)->nr_hw_queues && \
- ({ hctx = (q)->queue_hw_ctx[i]; 1; }); (i)++)
+ ({ hctx = queue_hctx((q), i); 1; }); (i)++)
#define hctx_for_each_ctx(hctx, ctx, i) \
for ((i) = 0; (i) < (hctx)->nr_ctx && \
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3bfc75a2a450..2018a4dd2028 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -354,7 +354,7 @@ struct request_queue {
unsigned int queue_depth;
/* hw dispatch queues */
- struct blk_mq_hw_ctx **queue_hw_ctx;
+ struct blk_mq_hw_ctx __rcu **queue_hw_ctx;
unsigned int nr_hw_queues;
/*
@@ -622,6 +622,17 @@ static inline bool queue_is_mq(struct request_queue *q)
return q->mq_ops;
}
+static inline struct blk_mq_hw_ctx *queue_hctx(struct request_queue *q, int id)
+{
+ struct blk_mq_hw_ctx *hctx;
+
+ rcu_read_lock();
+ hctx = *(rcu_dereference(q->queue_hw_ctx) + id);
+ rcu_read_unlock();
+
+ return hctx;
+}
+
#ifdef CONFIG_PM
static inline enum rpm_status queue_rpm_status(struct request_queue *q)
{
--
2.31.1
