linux-kernel - Re: [REGRESSION] virtio: reject shm region if length is zero

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Message-ID: <87y0rkie9l.fsf@alyssa.is>
Date: Fri, 15 Aug 2025 21:19:34 +0200
From: Alyssa Ross <hi@...ssa.is>
To: SamiUddinsami.md.ko@...il.com
Cc: mst@...hat.com, jasowang@...hat.com, xuanzhuo@...ux.alibaba.com,
 eperezma@...hat.com, virtualization@...ts.linux.dev,
 linux-kernel@...r.kernel.org, Sami Uddin <sami.md.ko@...il.com>,
 regressions@...ts.linux.dev
Subject: Re: [REGRESSION] virtio: reject shm region if length is zero

Alyssa Ross <hi@...ssa.is> writes:

> On Mon, May 12, 2025 at 07:51:53AM +0930, SamiUddinsami.md.ko@...il.com wrote:
>> From: Sami Uddin <sami.md.ko@...il.com>
>>
>> Prevent usage of shared memory regions where the length is zero,
>> as such configurations are not valid and may lead to unexpected behavior.
>>
>> Signed-off-by: Sami Uddin <sami.md.ko@...il.com>
>> ---
>> v3:
>> - Use idiomatic 'if (!region->len)' as suggested by reviewer
>> v2:
>> - Fixed coding style issue: added space after 'if' statement
>>
>>  include/linux/virtio_config.h | 2 ++
>>  1 file changed, 2 insertions(+)
>
> Hi, I'm sorry to be the bearer of bad news, but since this patch my VM
> no longer works.  The system is running wayland-proxy-virtwl[1] inside
> a crosvm[2] VM, using crosvm's virtio-gpu device to do cross-domain
> Wayland forwarding.
>
> Since this change, wayland-proxy-virtwl crashes with the following log
> message:
>
> 	wl-proxy [WARNING]: Error handling client: Unix.Unix_error(Unix.EINVAL, "DRM_IOCTL_VIRTGPU_RESOURCE_CREATE_BLOB", "")
>
> I'm pretty confused by what this change was supposed to do in the first
> place…  Looking at how virtio_get_shm_region() is used in
> virtio_gpu_init(), it's called with a pointer to zeroed memory, and then
> the get_shm_region() implementation is supposed to write to the region,
> without ever reading from it as far as I can tell.  Why is the initial
> value of an out parameter being checked at all?  How does this prevent
> using zero-length shared memory regions?
>
> [1]: https://crosvm.dev/
> [2]: https://github.com/talex5/wayland-proxy-virtwl
>
> #regzbot introduced: 206cc44588f72b49ad4d7e21a7472ab2a72a83df

Okay, just found that it's already been reverted:
https://lore.kernel.org/all/20250808072533-mutt-send-email-mst@kernel.org/

Still, I'm confused how this was supposed to fix anything…

#regzbot fix: Revert "virtio: reject shm region if length is zero"

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