From: Dylan Yudaken dylany@meta.com
[ Upstream commit 0fc8c2acbfc789a977a50a4a9812a8e4b37958ce ]
io_cqring_wait (and it's wake function io_has_work) used cached_cq_tail in order to calculate the number of CQEs. cached_cq_tail is set strictly before the user visible rings->cq.tail
However as far as userspace is concerned, if io_uring_enter(2) is called with a minimum number of events, they will verify by checking rings->cq.tail.
It is therefore possible for io_uring_enter(2) to return early with fewer events visible to the user.
Instead make the wait functions read from the user visible value, so there will be no discrepency.
This is triggered eventually by the following reproducer:
struct io_uring_sqe *sqe; struct io_uring_cqe *cqe; unsigned int cqe_ready; struct io_uring ring; int ret, i;
ret = io_uring_queue_init(N, &ring, 0); assert(!ret); while(true) { for (i = 0; i < N; i++) { sqe = io_uring_get_sqe(&ring); io_uring_prep_nop(sqe); sqe->flags |= IOSQE_ASYNC; } ret = io_uring_submit(&ring); assert(ret == N);
do { ret = io_uring_wait_cqes(&ring, &cqe, N, NULL, NULL); } while(ret == -EINTR); cqe_ready = io_uring_cq_ready(&ring); assert(!ret); assert(cqe_ready == N); io_uring_cq_advance(&ring, N); }
Fixes: ad3eb2c89fb2 ("io_uring: split overflow state into SQ and CQ side") Signed-off-by: Dylan Yudaken dylany@meta.com Link: https://lore.kernel.org/r/20221108153016.1854297-1-dylany@meta.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Sasha Levin sashal@kernel.org --- io_uring/io_uring.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-)
diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index d29f397f095e..f347e81e2d98 100644 --- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -171,6 +171,11 @@ static inline unsigned int __io_cqring_events(struct io_ring_ctx *ctx) return ctx->cached_cq_tail - READ_ONCE(ctx->rings->cq.head); }
+static inline unsigned int __io_cqring_events_user(struct io_ring_ctx *ctx) +{ + return READ_ONCE(ctx->rings->cq.tail) - READ_ONCE(ctx->rings->cq.head); +} + static bool io_match_linked(struct io_kiocb *head) { struct io_kiocb *req; @@ -2163,7 +2168,7 @@ struct io_wait_queue { static inline bool io_should_wake(struct io_wait_queue *iowq) { struct io_ring_ctx *ctx = iowq->ctx; - int dist = ctx->cached_cq_tail - (int) iowq->cq_tail; + int dist = READ_ONCE(ctx->rings->cq.tail) - (int) iowq->cq_tail;
/* * Wake up if we have enough events, or if a timeout occurred since we @@ -2240,7 +2245,8 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events, do { io_cqring_overflow_flush(ctx);
- if (io_cqring_events(ctx) >= min_events) + /* if user messes with these they will just get an early return */ + if (__io_cqring_events_user(ctx) >= min_events) return 0; if (!io_run_task_work()) break;