Re: [PATCH] sample: vfio-mdev: avoid deadlock in mdev_access()

From: Alex Williamson <alex.williamson@redhat.com>
To: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Gerd Hoffmann <kraxel@redhat.com>,
	Kirti Wankhede <kwankhede@nvidia.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	ldv-project@linuxtesting.org
Subject: Re: [PATCH] sample: vfio-mdev: avoid deadlock in mdev_access()
Date: Wed, 11 Jul 2018 13:30:26 -0600	[thread overview]
Message-ID: <20180711133026.5339589d@t450s.home> (raw)
In-Reply-To: <1530910206-24948-1-git-send-email-khoroshilov@ispras.ru>

On Fri,  6 Jul 2018 23:50:06 +0300
Alexey Khoroshilov <khoroshilov@ispras.ru> wrote:

> mdev_access() calls mbochs_get_page() with mdev_state->ops_lock held,
> while mbochs_get_page() locks the mutex by itself.
> It leads to unavoidable deadlock.
> 
> Found by Linux Driver Verification project (linuxtesting.org).
> 
> Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
> ---
>  samples/vfio-mdev/mbochs.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/samples/vfio-mdev/mbochs.c b/samples/vfio-mdev/mbochs.c
> index 2960e26c6ea4..6295a21381eb 100644
> --- a/samples/vfio-mdev/mbochs.c
> +++ b/samples/vfio-mdev/mbochs.c
> @@ -178,6 +178,8 @@ static const char *vbe_name(u32 index)
>  	return "(invalid)";
>  }
>  
> +static struct page *__mbochs_get_page(struct mdev_state *mdev_state,
> +				      pgoff_t pgoff);
>  static struct page *mbochs_get_page(struct mdev_state *mdev_state,
>  				    pgoff_t pgoff);
>  
> @@ -394,7 +396,7 @@ static ssize_t mdev_access(struct mdev_device *mdev, char *buf, size_t count,
>  		   MBOCHS_MEMORY_BAR_OFFSET + mdev_state->memsize) {
>  		pos -= MBOCHS_MMIO_BAR_OFFSET;
>  		poff = pos & ~PAGE_MASK;
> -		pg = mbochs_get_page(mdev_state, pos >> PAGE_SHIFT);
> +		pg = __mbochs_get_page(mdev_state, pos >> PAGE_SHIFT);
>  		map = kmap(pg);
>  		if (is_write)
>  			memcpy(map + poff, buf, count);

Looks good, applied to vfio for-linus branch for v4.18.  Thanks,

Alex