[PATCH v3] media: v4l2-isp: reject zero-sized parameter blocks

[David Carlier <devnexen@gmail.com>]

v4l2_isp_params_validate_buffer() walks the blocks of a parameters
buffer by adding block->size to the current offset, but never bounds
that size from below. A block with size 0 is not caught by the
block->size > buffer_size test, and the comparison against info->size
passes as well when the driver's type_info[] entry is an uninitialised
hole, both sizes being 0. The walk then makes no forward progress and
loops forever.

Drivers build their type_info[] arrays with designated initialisers
indexed by their block type enumeration, so an enumerator left without
an entry leaves a zeroed hole rather than failing the build. Drivers
call the validator from vb2 .buf_prepare, so such a hole turns a
VIDIOC_QBUF on the parameters video device into an unkillable task
spinning with the queue mutex held.

Reject a block smaller than its own header. A block's size includes its
header, so anything below that is malformed whatever the driver table
contains, and rejecting it is what keeps the walk moving. Blocks
carrying only a header to disable a block are exactly that size and
still pass.

An empty type info entry can then no longer stall the walk, but a block
matched against it is only constrained by that header size check. Reject
such a block explicitly: the driver does not implement the type and
cannot tell whether the block content is meaningful, and accepting it
silently would leave that content unconstrained until a later kernel
implements the type and starts validating it.

Fixes: 3cb6de6fafb8 ("media: v4l2-core: Introduce v4l2-isp.c")
Cc: stable@vger.kernel.org
Suggested-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
---
v3:
- reject a block whose type info entry is empty instead of skipping
  it, so a type the driver does not implement cannot become
  unconstrained uAPI (Jacopo)

v2:
- skip an empty type info entry instead of matching the block against
  a zeroed one
- reworded the commit message, which no longer leans on rppx1

 drivers/media/v4l2-core/v4l2-isp.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/media/v4l2-core/v4l2-isp.c b/drivers/media/v4l2-core/v4l2-isp.c
index 1eb46e080afa..efe994b4c4d7 100644
--- a/drivers/media/v4l2-core/v4l2-isp.c
+++ b/drivers/media/v4l2-core/v4l2-isp.c
@@ -84,6 +84,13 @@ int v4l2_isp_params_validate_buffer(struct device *dev, struct vb2_buffer *vb,
 			return -EINVAL;
 		}
 
+		if (block->size < sizeof(*block)) {
+			dev_dbg(dev,
+				"Invalid block size %u at offset %zu\n",
+				block->size, block_offset);
+			return -EINVAL;
+		}
+
 		if (block->size > buffer_size) {
 			dev_dbg(dev, "Premature end of parameters data\n");
 			return -EINVAL;
@@ -99,12 +106,25 @@ int v4l2_isp_params_validate_buffer(struct device *dev, struct vb2_buffer *vb,
 			return -EINVAL;
 		}
 
+		/*
+		 * An empty type info entry denotes a block type the driver
+		 * does not support. Reject the buffer instead of ignoring the
+		 * block: accepting it silently would let userspace fill it
+		 * with data that a later kernel, once it implements the type,
+		 * would validate and possibly reject.
+		 */
+		info = &type_info[block->type];
+		if (!info->size) {
+			dev_dbg(dev, "Unsupported block type %u at offset %zu\n",
+				block->type, block_offset);
+			return -EINVAL;
+		}
+
 		/*
 		 * Match the block reported size against the type info provided
 		 * one, but allow the block to only contain the header in
 		 * case it is going to be disabled.
 		 */
-		info = &type_info[block->type];
 		if (block->size != info->size &&
 		    (!(block->flags & V4L2_ISP_PARAMS_FL_BLOCK_DISABLE) ||
 		    block->size != sizeof(*block))) {
-- 
2.55.0