[PATCH] 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.

The subsequent read() / write() handlers (vme_user_read /
vme_user_write) clamp the I/O range against vme_get_size() (the
configured window size, attacker-controlled) but never consult
size_buf. The slave I/O paths buffer_to_user() and buffer_from_user()
then index image[minor].kern_buf with *ppos values up to
image_size - 1, well 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 > size_buf in the VME_SET_SLAVE handler. Also
add defensive bounds checks against size_buf in buffer_to_user() and
buffer_from_user() so that the I/O paths cannot exceed the
allocation even if a future ioctl path forgets to validate.

Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Rion Kiguchi <kiguchi.r.sec@gmail.com>
---
 drivers/staging/vme_user/vme_user.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/vme_user/vme_user.c b/drivers/staging/vme_user/vme_user.c
index 11e25c2f6..41b8d5b51 100644
--- a/drivers/staging/vme_user/vme_user.c
+++ b/drivers/staging/vme_user/vme_user.c
@@ -156,6 +156,11 @@ static ssize_t buffer_to_user(unsigned int minor, char __user *buf,
 {
 	void *image_ptr;
 
+	if (*ppos < 0 || (u64)*ppos >= image[minor].size_buf ||
+	    count > image[minor].size_buf - (u64)*ppos) {
+		pr_warn_ratelimited("%s: out-of-bounds access\n", __func__);
+		return -EINVAL;
+	}
 	image_ptr = image[minor].kern_buf + *ppos;
 	if (copy_to_user(buf, image_ptr, (unsigned long)count))
 		return -EFAULT;
@@ -168,6 +173,11 @@ static ssize_t buffer_from_user(unsigned int minor, const char __user *buf,
 {
 	void *image_ptr;
 
+	if (*ppos < 0 || (u64)*ppos >= image[minor].size_buf ||
+	    count > image[minor].size_buf - (u64)*ppos) {
+		pr_warn_ratelimited("%s: out-of-bounds access\n", __func__);
+		return -EINVAL;
+	}
 	image_ptr = image[minor].kern_buf + *ppos;
 	if (copy_from_user(image_ptr, buf, (unsigned long)count))
 		return -EFAULT;
@@ -394,6 +404,14 @@ 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.
+			 */
+			if (slave.size > image[minor].size_buf)
+				return -EINVAL;
+
 			/* XXX	We do not want to push aspace, cycle and width
 			 *	to userspace as they are
 			 */
@@ -401,7 +419,6 @@ static int vme_user_ioctl(struct inode *inode, struct file *file,
 				slave.enable, slave.vme_addr, slave.size,
 				image[minor].pci_buf, slave.aspace,
 				slave.cycle);
-
 			break;
 		}
 		break;
-- 
2.43.0