2018-07-06 20:51:54

by Alexey Khoroshilov

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

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 <[email protected]>
---
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);
--
2.7.4

2018-07-12 01:26:51

by Alex Williamson

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

On Fri, 6 Jul 2018 23:50:06 +0300
Alexey Khoroshilov <[email protected]> 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 <[email protected]>
> ---
> 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