[PATCH v4] staging: vme_user: validate slave window size against buffer size

[Rion Kiguchi <kiguchi.r.sec@gmail.com>]

The VME_SET_SLAVE ioctl in drivers/staging/vme_user/vme_user.c accepts
a user-controlled slave.size and forwards it to vme_slave_set() without
comparing it against image[minor].size_buf. The slave-image kernel
buffer is allocated at probe time with a fixed size of PCI_BUF_SIZE
(0x20000 / 128 KiB), but the configured VME window size can be made
much larger via the ioctl.

Additionally, a slave.size of 0 is permitted, which causes vme_get_size()
to return 0. In vme_user_read() and vme_user_write(), the boundary check
(*ppos > image_size - 1) suffers from an integer underflow because
image_size is size_t. This bypasses the bounds check entirely, allowing
offsets beyond the actual allocation.

Result: a local user with read/write access to /dev/bus/vme/s* can
trigger out-of-bounds read and write of the kernel slab adjacent to
the slave-image buffer.

Fix: reject slave.size == 0 and slave.size > size_buf in the VME_SET_SLAVE
handler. With this check in place, the existing bounds checks in
vme_user_read() / vme_user_write() against vme_get_size() are
sufficient to prevent OOB access.

Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Rion Kiguchi <kiguchi.r.sec@gmail.com>
---
Changes in v4:
 - Add check for slave.size == 0 to prevent integer underflow in bounds check.
 - Remove trailing whitespaces inadvertently added in previous versions.
 - Remove Fixes and Cc: stable tags to resolve checkpatch warnings based on maintainer feedback.

Changes in v3:
 - Drop redundant checks in buffer_to_user() / buffer_from_user();
   the existing vme_get_size()-based bounds checks in vme_user_read()
   / vme_user_write() are sufficient once VME_SET_SLAVE rejects
   oversized windows.

Changes in v2:
 - Use git send-email instead of Gmail web compose.
 - Drop redundant Reported-by tag.
 - Add Assisted-by tag.

 drivers/staging/vme_user/vme_user.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/staging/vme_user/vme_user.c b/drivers/staging/vme_user/vme_user.c
index 11e25c2f6..ba4c9a6d0 100644
--- a/drivers/staging/vme_user/vme_user.c
+++ b/drivers/staging/vme_user/vme_user.c
@@ -394,6 +394,16 @@ static int vme_user_ioctl(struct inode *inode, struct file *file,
 				return -EFAULT;
 			}
 
+			/*
+			 * Reject window sizes larger than the kernel buffer
+			 * allocated at probe time, otherwise subsequent
+			 * read/write would access memory beyond kern_buf.
+			 * Also reject size == 0 to prevent integer underflow
+			 * in the boundary check (*ppos > image_size - 1).
+			 */
+			if (slave.size == 0 || slave.size > image[minor].size_buf)
+				return -EINVAL;
+
 			/* XXX	We do not want to push aspace, cycle and width
 			 *	to userspace as they are
 			 */
-- 
2.43.0