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To:     Jens Axboe <axboe@kernel.dk>, Hao Xu <haoxu@linux.alibaba.com>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Eric Dumazet <edumazet@google.com>
From:   Eric Dumazet <eric.dumazet@gmail.com>
Subject: [BUG] About "io_uring: add more uring info to fdinfo for debug"
Message-ID: <9ac22d4d-c841-651a-fdd5-9fb3a65c57ab@gmail.com>
Date:   Thu, 28 Oct 2021 14:24:38 -0700
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Hi

I was looking at commit 83f84356bc8f2d
("io_uring: add more uring info to fdinfo for debug") after receiving
syzbot reports.

I suspect that the following :

+       for (i = cached_sq_head; i < sq_tail; i++) {
+               unsigned int sq_idx = READ_ONCE(ctx->sq_array[i & sq_mask]);
+
+               if (likely(sq_idx <= sq_mask)) {
+                       struct io_uring_sqe *sqe = &ctx->sq_sqes[sq_idx];
+
+                       seq_printf(m, "%5u: opcode:%d, fd:%d, flags:%x, user_data:%llu\n",
+                                  sq_idx, sqe->opcode, sqe->fd, sqe->flags, sqe->user_data);
+               }
+       }


Can loop around ~2^32 times if sq_tail is close to ~0U

I see various READ_ONCE(), which are probably not good enough.

At very minimum I would handling wrapping...

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f37de99254bf8e0b8364b267388d1056fce436a4..50493f5c004b70cff103e20bf4b1ba92304eddb9 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -10117,7 +10117,7 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
        struct io_overflow_cqe *ocqe;
        struct io_rings *r = ctx->rings;
        unsigned int sq_mask = ctx->sq_entries - 1, cq_mask = ctx->cq_entries - 1;
-       unsigned int cached_sq_head = ctx->cached_sq_head;
+       unsigned int cached_sq_head = READ_ONCE(ctx->cached_sq_head);
        unsigned int cached_cq_tail = ctx->cached_cq_tail;
        unsigned int sq_head = READ_ONCE(r->sq.head);
        unsigned int sq_tail = READ_ONCE(r->sq.tail);
@@ -10139,7 +10139,7 @@ static __cold void __io_uring_show_fdinfo(struct io_ring_ctx *ctx,
        seq_printf(m, "CqTail:\t%u\n", cq_tail & cq_mask);
        seq_printf(m, "CachedCqTail:\t%u\n", cached_cq_tail & cq_mask);
        seq_printf(m, "SQEs:\t%u\n", sq_tail - cached_sq_head);
-       for (i = cached_sq_head; i < sq_tail; i++) {
+       for (i = cached_sq_head; (int)(i - sq_tail) < 0; i++) {
                unsigned int sq_idx = READ_ONCE(ctx->sq_array[i & sq_mask]);
 
                if (likely(sq_idx <= sq_mask)) {