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Re: [Qemu-devel] [PATCH] block: Fix qemu crash when using scsi-block

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH] block: Fix qemu crash when using scsi-block
Date: Wed, 22 Nov 2017 17:06:07 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

On Wed, Nov 22, 2017 at 07:33:28AM -0800, Deepa Srinivasan wrote:
> Starting qemu with the following arguments causes qemu to segfault:
> ... -device lsi,id=lsi0 -drive file=iscsi:<...>,format=raw,if=none,node-name=
> iscsi1 -device scsi-block,bus=lsi0.0,id=<...>,drive=iscsi1
> This patch fixes blk_aio_ioctl() so it does not pass stack addresses to
> blk_aio_ioctl_entry() which may be invoked after blk_aio_ioctl() returns. More
> details about the bug follow.
> blk_aio_ioctl() invokes blk_aio_prwv() with blk_aio_ioctl_entry as the
> coroutine parameter. blk_aio_prwv() ultimately calls aio_co_enter().
> When blk_aio_ioctl() is executed from within a coroutine context (e.g.
> iscsi_bh_cb()), aio_co_enter() adds the coroutine (blk_aio_ioctl_entry) to
> the current coroutine's wakeup queue. blk_aio_ioctl() then returns.
> When blk_aio_ioctl_entry() executes later, it accesses an invalid pointer:
> ....
>     BlkRwCo *rwco = &acb->rwco;
>     rwco->ret = blk_co_ioctl(rwco->blk, rwco->offset,
>                              rwco->qiov->iov[0].iov_base);  <--- qiov is
>                                                                  invalid here
> ...
> In the case when blk_aio_ioctl() is called from a non-coroutine context,
> blk_aio_ioctl_entry() executes immediately. But if bdrv_co_ioctl() calls
> qemu_coroutine_yield(), blk_aio_ioctl() will return. When the coroutine
> execution is complete, control returns to blk_aio_ioctl_entry() after the call
> to blk_co_ioctl(). There is no invalid reference after this point, but the
> function is still holding on to invalid pointers.
> The fix is to allocate memory for the QEMUIOVector and struct iovec as part of
> the request struct which the IO buffer is part of. The memory for this struct 
> is
> guaranteed to be valid till the AIO is completed.

Thanks for the patch!

AIO APIs currently don't require the caller to match qiov's lifetime to
the I/O request lifetime.  This patch changes that for blk_aio_ioctl()
only.  If we want to do this consistently then all aio callers need to
be audited and fixed.

The alternative is to make the API copy qiov when necessary.  That is
less efficient but avoids modifying all callers.

Either way, the lifetime of qiov must be consistent across all aio APIs,
not just blk_aio_ioctl().

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