Re: [PATCH 0/2] uring_cmd SQE corruptions

Caleb Sander
2025-02-12T21:58:52Z

On Wed, Feb 12, 2025 at 1:02 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> 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)

I believe this still misses the last request in a linked chain, which
won't have REQ_F_LINK/REQ_F_HARDLINK set?
IOSQE_IO_DRAIN also causes subsequent operations to be issued async;
is REQ_F_IO_DRAIN set on those operations too?

Thanks,
Caleb

> +
> 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