Re: [RESEND PATCH] hw/dma: fix crash caused by race condition

[Stefan Hajnoczi]

On Wed, Jun 01, 2022 at 10:00:50AM +0200, David Hildenbrand wrote:
> On 01.06.22 02:20, Tong Zhang wrote:
> > Hi David,
> > 
> > On Mon, May 30, 2022 at 9:19 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> On 27.04.22 22:51, Tong Zhang wrote:
> >>> assert(dbs->acb) is meant to check the return value of io_func per
> >>> documented in commit 6bee44ea34 ("dma: the passed io_func does not
> >>> return NULL"). However, there is a chance that after calling
> >>> aio_context_release(dbs->ctx); the dma_blk_cb function is called before
> >>> the assertion and dbs->acb is set to NULL again at line 121. Thus when
> >>> we run assert at line 181 it will fail.
> >>>
> >>>   softmmu/dma-helpers.c:181: dma_blk_cb: Assertion `dbs->acb' failed.
> >>>
> >>> Reported-by: Francisco Londono <f.londono@samsung.com>
> >>> Signed-off-by: Tong Zhang <t.zhang2@samsung.com>
> >>> ---
> >>>  softmmu/dma-helpers.c | 2 +-
> >>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
> >>> index 7820fec54c..cb81017928 100644
> >>> --- a/softmmu/dma-helpers.c
> >>> +++ b/softmmu/dma-helpers.c
> >>> @@ -177,8 +177,8 @@ static void dma_blk_cb(void *opaque, int ret)
> >>>      aio_context_acquire(dbs->ctx);
> >>>      dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,
> >>>                              dma_blk_cb, dbs, dbs->io_func_opaque);
> >>> -    aio_context_release(dbs->ctx);
> >>>      assert(dbs->acb);
> >>> +    aio_context_release(dbs->ctx);
> >>>  }
> >>>
> >>>  static void dma_aio_cancel(BlockAIOCB *acb)
> >>
> >> I'm fairly new to that code, but I wonder what prevents dma_blk_cb() to
> >> run after you reshuffled the code?
> >>
> > 
> > IMO if the assert is to test whether io_func returns a non-NULL value
> > shouldn't it be immediately after calling io_func.
> > Also... as suggested by commit 6bee44ea346aed24e12d525daf10542d695508db
> >   >     dma: the passed io_func does not return NULL
> 
> Yes, but I just don't see how it would fix the assertion you document in
> the patch description. The locking change to fix the assertion doesn't
> make any sense to me, and most probably I am missing something important :)

The other thread will invoke dma_blk_cb(), which modifies dbs->acb, when
it can take the lock. Therefore dbs->acb may contain a value different
from our io_func()'s return value by the time we perform the assertion
check (that's the race).

This patch makes sense to me. Can you rephrase your concern?

Stefan

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