staging: vme_user: bound slave read/write to the kern_buf size

The SLAVE-path helpers buffer_to_user() and buffer_from_user() copy
'count' bytes into/out of the fixed-size kern_buf (size_buf ==
PCI_BUF_SIZE == 0x20000, 128 KiB) using *ppos as the offset, without
bounding *ppos + count against size_buf.

vme_user_write()/vme_user_read() only clamp count to the VME window size
(image_size = vme_get_size(resource)), which VME_SET_SLAVE sets from the
user-supplied slave.size -- validated against the VME address space (up
to VME_A32_MAX = 4 GiB), not against PCI_BUF_SIZE.  When the window
exceeds 128 KiB, a write()/read() copies past the kern_buf allocation.

Clamp count against size_buf in both helpers, with an early return when
*ppos is already at/after the buffer end.  *ppos is >= 0 here (the caller
rejects negative offsets), so size_buf - *ppos cannot wrap.  This mirrors
the existing clamp in the MASTER-path helpers resource_to_user() /
resource_from_user(), and matches the read()/write() convention of a
short transfer at end-of-buffer.

Found by static analysis (CodeQL taint tracking + CBMC bounded model
checking) and confirmed dynamically under KASAN with the vme_fake bridge:

  BUG: KASAN: slab-out-of-bounds in _copy_from_user+0x2d/0x80
  Write of size 262144 at addr ffff888004100000 by task trigger/68
    _copy_from_user+0x2d/0x80
    vme_user_write+0x13e/0x240 [vme_user]
    vfs_write+0x1b8/0x7a0
    ksys_write+0xb8/0x150

Fixes: f00a86d ("Staging: vme: add VME userspace driver")
Cc: stable <stable@kernel.org>
Signed-off-by: Michael Tautschnig <tautschn@amazon.com>
Link: https://patch.msgid.link/20260618114709.72499-1-tautschn@amazon.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: tautschnig

drivers/staging/vme_user/vme_user.c

@@ -156,6 +156,17 @@ static ssize_t buffer_to_user(unsigned int minor, char __user *buf,
 {
 	void *image_ptr;
 
+	/*
+	 * The slave window (image_size) can exceed the fixed kern_buf
+	 * (size_buf == PCI_BUF_SIZE), so bound the copy to kern_buf.
+	 * *ppos is >= 0 here (checked by the caller), so the
+	 * subtraction below cannot wrap.
+	 */
+	if (*ppos >= image[minor].size_buf)
+		return 0;
+	if (count > image[minor].size_buf - *ppos)
+		count = image[minor].size_buf - *ppos;
+
 	image_ptr = image[minor].kern_buf + *ppos;
 	if (copy_to_user(buf, image_ptr, (unsigned long)count))
 		return -EFAULT;
@@ -168,6 +179,17 @@ static ssize_t buffer_from_user(unsigned int minor, const char __user *buf,
 {
 	void *image_ptr;
 
+	/*
+	 * The slave window (image_size) can exceed the fixed kern_buf
+	 * (size_buf == PCI_BUF_SIZE), so bound the copy to kern_buf.
+	 * *ppos is >= 0 here (checked by the caller), so the
+	 * subtraction below cannot wrap.
+	 */
+	if (*ppos >= image[minor].size_buf)
+		return 0;
+	if (count > image[minor].size_buf - *ppos)
+		count = image[minor].size_buf - *ppos;
+
 	image_ptr = image[minor].kern_buf + *ppos;
 	if (copy_from_user(image_ptr, buf, (unsigned long)count))
 		return -EFAULT;