5.10-stable review patch. If anyone has any objections, please let me know. ------------------ From: Palla Raghunath commit 9b3fcf519d370df94a56f3b272d9eddffffa5be8 upstream. struct dm_ioctl has seven bytes of trailing padding, so the smallest buffer userspace is allowed to ask for, offsetof(struct dm_ioctl, data), is seven bytes shorter than sizeof(struct dm_ioctl). get_result_buffer() sets data_start to sizeof(struct dm_ioctl) without looking at how big the buffer actually is. Ask for the minimum and data_start ends up seven bytes past the end. Callers add it to data_size, and ctl_ioctl() copies that much back to userspace: BUG: KASAN: slab-out-of-bounds in _copy_to_user+0xad/0xd0 Read of size 312 at addr ffff8880169e4c00 by task repro-dm/140 ctl_ioctl+0x5e3/0xcb0 dm_ctl_ioctl+0x25/0x40 allocated 305-byte region [ffff8880169e4c00, ffff8880169e4d31) A DM_LIST_DEVICES with data_size set to 305 is enough to hit it, and the seven bytes that leak are whatever sat next to the allocation. Clamp data_start to the buffer size. Note that this is not a security bug because the out-of-bounds read can only be triggered by root. Reported-by: syzbot+48d935cf48a4a76be346@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=48d935cf48a4a76be346 Fixes: 76c072b48e39 ("dm ioctl: move compat code") Cc: stable@vger.kernel.org Signed-off-by: Palla Raghunath Signed-off-by: Mikulas Patocka Signed-off-by: Greg Kroah-Hartman --- drivers/md/dm-ioctl.c | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) --- a/drivers/md/dm-ioctl.c +++ b/drivers/md/dm-ioctl.c @@ -496,10 +496,16 @@ static void *get_result_buffer(struct dm { param->data_start = align_ptr(param + 1) - (void *) param; - if (param->data_start < param_size) + if (param->data_start < param_size) { *len = param_size - param->data_start; - else + } else { + /* + * The buffer can be as small as offsetof(*param, data), which + * is less than sizeof(*param), so don't run off the end of it. + */ *len = 0; + param->data_start = param_size; + } return ((void *) param) + param->data_start; }