Re: [PATCH 0/2] uring_cmd SQE corruptions

Jens Axboe
2025-02-12T21:02:02Z

On 2/12/25 1:55 PM, Jens Axboe wrote:
> On 2/12/25 1:45 PM, Caleb Sander Mateos wrote:
>> In our application issuing NVMe passthru commands, we have observed
>> nvme_uring_cmd fields being corrupted between when userspace initializes
>> the io_uring SQE and when nvme_uring_cmd_io() processes it.
>>
>> We hypothesized that the uring_cmd's were executing asynchronously after
>> the io_uring_enter() syscall returned, yet were still reading the SQE in
>> the userspace-mapped SQ. Since io_uring_enter() had already incremented
>> the SQ head index, userspace reused the SQ slot for a new SQE once the
>> SQ wrapped around to it.
>>
>> We confirmed this hypothesis by "poisoning" all SQEs up to the SQ head
>> index in userspace upon return from io_uring_enter(). By overwriting the
>> nvme_uring_cmd nsid field with a known garbage value, we were able to
>> trigger the err message in nvme_validate_passthru_nsid(), which logged
>> the garbage nsid value.
>>
>> The issue is caused by commit 5eff57fa9f3a ("io_uring/uring_cmd: defer
>> SQE copying until it's needed"). With this commit reverted, the poisoned
>> values in the SQEs are no longer seen by nvme_uring_cmd_io().
>>
>> Prior to the commit, each uring_cmd SQE was unconditionally memcpy()ed
>> to async_data at prep time. The commit moved this memcpy() to 2 cases
>> when the request goes async:
>> - If REQ_F_FORCE_ASYNC is set to force the initial issue to go async
>> - If ->uring_cmd() returns -EAGAIN in the initial non-blocking issue
>>
>> This patch set fixes a bug in the EAGAIN case where the uring_cmd's sqe
>> pointer is not updated to point to async_data after the memcpy(),
>> as it correctly is in the REQ_F_FORCE_ASYNC case.
>>
>> However, uring_cmd's can be issued async in other cases not enumerated
>> by 5eff57fa9f3a, also leading to SQE corruption. These include requests
>> besides the first in a linked chain, which are only issued once prior
>> requests complete. Requests waiting for a drain to complete would also
>> be initially issued async.
>>
>> While it's probably possible for io_uring_cmd_prep_setup() to check for
>> each of these cases and avoid deferring the SQE memcpy(), we feel it
>> might be safer to revert 5eff57fa9f3a to avoid the corruption risk.
>> As discussed recently in regard to the ublk zero-copy patches[1], new
>> async paths added in the future could break these delicate assumptions.
>
> I don't think it's particularly delicate - did you manage to catch the
> case queueing a request for async execution where the sqe wasn't already
> copied? I did take a quick look after our out-of-band conversation, and
> the only missing bit I immediately spotted is using SQPOLL. But I don't
> think you're using that, right? And in any case, lifetime of SQEs with
> SQPOLL is the duration of the request anyway, so should not pose any
> risk of overwriting SQEs. But I do think the code should copy for that
> case too, just to avoid it being a harder-to-use thing than it should
> be.
>
> The two patches here look good, I'll go ahead with those. That'll give
> us a bit of time to figure out where this missing copy is.

Can you try this on top of your 2 and see if you still hit anything odd?

diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
index bcfca18395c4..15a8a67f556e 100644
--- a/io_uring/uring_cmd.c
+++ b/io_uring/uring_cmd.c
@@ -177,10 +177,13 @@ static void io_uring_cmd_cache_sqes(struct io_kiocb *req)
ioucmd->sqe = cache->sqes;
}

+#define SQE_COPY_FLAGS (REQ_F_FORCE_ASYNC|REQ_F_LINK|REQ_F_HARDLINK|REQ_F_IO_DRAIN)
+
static int io_uring_cmd_prep_setup(struct io_kiocb *req,
const struct io_uring_sqe *sqe)
{
struct io_uring_cmd *ioucmd = io_kiocb_to_cmd(req, struct io_uring_cmd);
+ struct io_ring_ctx *ctx = req->ctx;
struct io_uring_cmd_data *cache;

cache = io_uring_alloc_async_data(&req->ctx->uring_cache, req);
@@ -190,7 +193,7 @@ static int io_uring_cmd_prep_setup(struct io_kiocb *req,

ioucmd->sqe = sqe;
/* defer memcpy until we need it */
- if (unlikely(req->flags & REQ_F_FORCE_ASYNC))
+ if (unlikely(ctx->flags & IORING_SETUP_SQPOLL || req->flags & SQE_COPY_FLAGS))
io_uring_cmd_cache_sqes(req);
return 0;
}

--
Jens Axboe