Especially on 32-bit systems, it is possible for the pointer arithmetic
to overflow and cause a userspace pointer to be dereferenced in the
kernel.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
---
 drivers/md/dm-ioctl.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 34fa74c6a70db8aa67aaba3f6a2fc4f38ef736bc..64e8f16d344c47057de5e2d29e3d63202197dca0 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -1396,6 +1396,25 @@ static int next_target(struct dm_target_spec *last, uint32_t next, void *end,
 {
 	static_assert(_Alignof(struct dm_target_spec) <= 8,
 		      "struct dm_target_spec has excessive alignment requirements");
+	static_assert(offsetof(struct dm_ioctl, data) >= sizeof(struct dm_target_spec),
+		      "struct dm_target_spec too big");
+
+	/*
+	 * Number of bytes remaining, starting with last. This is always
+	 * sizeof(struct dm_target_spec) or more, as otherwise *last was
+	 * out of bounds already.
+	 */
+	size_t remaining = (char *)end - (char *)last;
+
+	/*
+	 * There must be room for both the next target spec and the
+	 * NUL-terminator of the target itself.
+	 */
+	if (remaining - sizeof(struct dm_target_spec) <= next) {
+		DMERR("Target spec extends beyond end of parameters");
+		return -EINVAL;
+	}
+
 	if (next % 8) {
 		DMERR("Next target spec (offset %u) is not 8-byte aligned", next);
 		return -EINVAL;
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab