io_uring: Use WRITE_ONCE() when writing to sq_flags

The compiler should be forbidden from any strange optimization for async
writes to user visible data-structures. Without proper protection, the
compiler can cause write-tearing or invent writes that would confuse the
userspace.

However, there are writes to sq_flags which are not protected by
WRITE_ONCE(). Use WRITE_ONCE() for these writes.

This is purely a theoretical issue. Presumably, any compiler is very
unlikely to do such optimizations.

Fixes: 75b28affdd6a ("io_uring: allocate the two rings together")
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Nadav Amit <namit@vmware.com>
Link: https://lore.kernel.org/r/20210808001342.964634-3-namit@vmware.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 1093df3..ca06448 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1500,7 +1500,8 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
        all_flushed = list_empty(&ctx->cq_overflow_list);
        if (all_flushed) {
                clear_bit(0, &ctx->check_cq_overflow);
-               ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
+               WRITE_ONCE(ctx->rings->sq_flags,
+                          ctx->rings->sq_flags & ~IORING_SQ_CQ_OVERFLOW);
        }
 
        if (posted)
@@ -1579,7 +1580,9 @@ static bool io_cqring_event_overflow(struct io_ring_ctx *ctx, u64 user_data,
        }
        if (list_empty(&ctx->cq_overflow_list)) {
                set_bit(0, &ctx->check_cq_overflow);
-               ctx->rings->sq_flags |= IORING_SQ_CQ_OVERFLOW;
+               WRITE_ONCE(ctx->rings->sq_flags,
+                          ctx->rings->sq_flags | IORING_SQ_CQ_OVERFLOW);
+
        }
        ocqe->cqe.user_data = user_data;
        ocqe->cqe.res = res;
@@ -6804,14 +6807,16 @@ static inline void io_ring_set_wakeup_flag(struct io_ring_ctx *ctx)
 {
        /* Tell userspace we may need a wakeup call */
        spin_lock_irq(&ctx->completion_lock);
-       ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
+       WRITE_ONCE(ctx->rings->sq_flags,
+                  ctx->rings->sq_flags | IORING_SQ_NEED_WAKEUP);
        spin_unlock_irq(&ctx->completion_lock);
 }
 
 static inline void io_ring_clear_wakeup_flag(struct io_ring_ctx *ctx)
 {
        spin_lock_irq(&ctx->completion_lock);
-       ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+       WRITE_ONCE(ctx->rings->sq_flags,
+                  ctx->rings->sq_flags & ~IORING_SQ_NEED_WAKEUP);
        spin_unlock_irq(&ctx->completion_lock);
 }