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

[Berkant Koc <me@berkoc.com>]

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