Re: [PATCH] drm/vmwgfx: validate execbuf header.size lower bound

[Zack Rusin <zack.rusin@broadcom.com>]

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On Sun, May 17, 2026 at 10:07 AM Berkant Koc <me@berkoc.com> wrote:
>
> Commit 32b415a9dc2c ("drm/vmwgfx: Validate command header size against
> SVGA_CMD_MAX_DATASIZE") added an upper bound on the user-supplied
> SVGA3dCmdHeader.size field but no matching lower bound. When
> header->size is smaller than sizeof(cmd->body), the size_t subtraction
> in expressions like
>
>   maxnum = (header->size - sizeof(cmd->body)) / sizeof(*decl);
>
> underflows. The subsequent bound check
>
>   if (cmd->body.numVertexDecls > maxnum) return -EINVAL;
>
> is bypassed because maxnum is ~SIZE_MAX, and the loop walks
> attacker-chosen entries past the command buffer.
>
> In vmw_cmd_draw this leads to a 4-byte OOB-read per iteration via
> vmw_cmd_res_check(&decl[i].array.surfaceId, ...); on a surface-handle
> collision, vmw_resource_relocation_add records the OOB address as
> rel->offset (29-bit bitfield), and vmw_resource_relocations_apply
> later performs a 32-bit kernel write at cb + rel->offset.
>
> The same root cause is present in vmw_cmd_dma (suffix pointer-arith
> underflow leading to OOB-read of suffix->suffixSize) and
> vmw_cmd_shader_define (size_t wraparound passed to
> vmw_compat_shader_add).
>
> Reachable via DRM_VMW_EXECBUF (DRM_RENDER_ALLOW). Reject undersized
> headers at all three sites before the subtraction.
>
> Cc: stable@vger.kernel.org # v6.18+
> Fixes: 32b415a9dc2c ("drm/vmwgfx: Validate command header size against SVGA_CMD_MAX_DATASIZE")
> Signed-off-by: Berkant Koc <me@berkoc.com>
> ---
>  drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> index e1f18020170a..6f9c7d61cc66 100644
> --- a/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> +++ b/drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c
> @@ -1506,6 +1506,12 @@ static int vmw_cmd_dma(struct vmw_private *dev_priv,
>         bool dirty;
>
>         cmd = container_of(header, typeof(*cmd), header);
> +
> +       if (unlikely(header->size < sizeof(cmd->body) + sizeof(*suffix))) {
> +               VMW_DEBUG_USER("DMA cmd header.size too small.\n");
> +               return -EINVAL;
> +       }
> +
>         suffix = (SVGA3dCmdSurfaceDMASuffix *)((unsigned long) &cmd->body +
>                                                header->size - sizeof(*suffix));
>
> @@ -1572,6 +1578,12 @@ static int vmw_cmd_draw(struct vmw_private *dev_priv,
>                 return ret;
>
>         cmd = container_of(header, typeof(*cmd), header);
> +
> +       if (unlikely(header->size < sizeof(cmd->body))) {
> +               VMW_DEBUG_USER("Draw cmd header.size smaller than body.\n");
> +               return -EINVAL;
> +       }
> +
>         maxnum = (header->size - sizeof(cmd->body)) / sizeof(*decl);
>
>         if (unlikely(cmd->body.numVertexDecls > maxnum)) {
> @@ -1915,6 +1927,11 @@ static int vmw_cmd_shader_define(struct vmw_private *dev_priv,
>         if (unlikely(!dev_priv->has_mob))
>                 return 0;
>
> +       if (unlikely(cmd->header.size < sizeof(cmd->body))) {
> +               VMW_DEBUG_USER("Shader define cmd header.size smaller than body.\n");
> +               return -EINVAL;
> +       }
> +
>         size = cmd->header.size - sizeof(cmd->body);
>         ret = vmw_compat_shader_add(dev_priv, vmw_context_res_man(ctx),
>                                     cmd->body.shid, cmd + 1, cmd->body.type,
> --
> 2.47.3
>

I think you might have forgotten to disclose the tool/llm you've used
for this. Using Claude Opus 4.7 I've found the same issues and the
series fixing those bugs is available on dri-devel at
https://patchwork.freedesktop.org/series/166024/ . If you've used a
different llm I'd love to know which one. And if you have a completely
different tool for detecting those issues that'd be great to know as
well.

z

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