[PATCH v2 06/16] device-mapper: Avoid double-fetch of version

[Demi Marie Obenour <demi@invisiblethingslab.com>]

The version is fetched once in check_version(), which then does some
validation and then overwrites the version in userspace with the API
version supported by the kernel.  copy_params() then fetches the version
from userspace *again*, and this time no validation is done.  The result
is that the kernel's version number is completely controllable by
userspace, provided that userspace can win a race condition.

Fix this flaw by not copying the version back to the kernel the second
time.  This is not exploitable as the version is not further used in the
kernel.  However, it could become a problem if future patches start
relying on the version field.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
---
 drivers/md/dm-ioctl.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/md/dm-ioctl.c b/drivers/md/dm-ioctl.c
index 491ef55b9e8662c3b02a2162b8c93ee086c078a1..20f452b6c61c1c4d20259fd0fc5443977e4454a0 100644
--- a/drivers/md/dm-ioctl.c
+++ b/drivers/md/dm-ioctl.c
@@ -1873,12 +1873,13 @@ static ioctl_fn lookup_ioctl(unsigned int cmd, int *ioctl_flags)
  * As well as checking the version compatibility this always
  * copies the kernel interface version out.
  */
-static int check_version(unsigned int cmd, struct dm_ioctl __user *user)
+static int check_version(unsigned int cmd, struct dm_ioctl __user *user,
+			 struct dm_ioctl *kernel_params)
 {
-	uint32_t version[3];
 	int r = 0;
+	uint32_t *version = kernel_params->version;
 
-	if (copy_from_user(version, user->version, sizeof(version)))
+	if (copy_from_user(version, user->version, sizeof(user->version)))
 		return -EFAULT;
 
 	if ((version[0] != DM_VERSION_MAJOR) ||
@@ -1922,7 +1923,10 @@ static int copy_params(struct dm_ioctl __user *user, struct dm_ioctl *param_kern
 	const size_t minimum_data_size = offsetof(struct dm_ioctl, data);
 	unsigned int noio_flag;
 
-	if (copy_from_user(param_kernel, user, minimum_data_size))
+	/* Version has been copied from userspace already, avoid TOCTOU */
+	if (copy_from_user((char *)param_kernel + sizeof(param_kernel->version),
+			   (char __user *)user + sizeof(param_kernel->version),
+			   minimum_data_size - sizeof(param_kernel->version)))
 		return -EFAULT;
 
 	if (param_kernel->data_size < minimum_data_size) {
@@ -2034,7 +2038,7 @@ static int ctl_ioctl(struct file *file, uint command, struct dm_ioctl __user *us
 	 * Check the interface version passed in.  This also
 	 * writes out the kernel's interface version.
 	 */
-	r = check_version(cmd, user);
+	r = check_version(cmd, user, &param_kernel);
 	if (r)
 		return r;
 
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab