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Re: [Qemu-block] [PATCH v2] block: Fix qemu crash when using scsi-block

From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [PATCH v2] block: Fix qemu crash when using scsi-block
Date: Mon, 18 Dec 2017 13:22:08 +0000
User-agent: Mutt/1.9.1 (2017-09-22)

On Fri, Dec 15, 2017 at 04:59:13PM -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 change blk_aio_prwv() to accept a void pointer for the IO buffer
> rather than a QEMUIOVector. blk_aio_prwv() passes this through in BlkRwCo and 
> the
> coroutine function casts it to QEMUIOVector or uses the void pointer directly.
> Signed-off-by: Deepa Srinivasan <address@hidden>
> Signed-off-by: Konrad Rzeszutek Wilk <address@hidden>
> Reviewed-by: Mark Kanda <address@hidden>
> ---
>  block/block-backend.c | 51 
> +++++++++++++++++++++++++--------------------------
>  1 file changed, 25 insertions(+), 26 deletions(-)

Reviewed-by: Stefan Hajnoczi <address@hidden>

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