From: Eric Biggers <ebigg...@kernel.org>
Date: Sun, 1 Sep 2019 09:32:39 -0500
> From: Eric Biggers <ebigg...@google.com>
>
> syzbot reported:
>
> BUG: KMSAN: uninit-value in capi_write+0x791/0xa90
> drivers/isdn/capi/capi.c:700
> CPU: 0 PID: 10025 Comm: syz-executor379 Not tainted 4.20.0-rc7+ #2
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> Call Trace:
> __dump_stack lib/dump_stack.c:77 [inline]
> dump_stack+0x173/0x1d0 lib/dump_stack.c:113
> kmsan_report+0x12e/0x2a0 mm/kmsan/kmsan.c:613
> __msan_warning+0x82/0xf0 mm/kmsan/kmsan_instr.c:313
> capi_write+0x791/0xa90 drivers/isdn/capi/capi.c:700
> do_loop_readv_writev fs/read_write.c:703 [inline]
> do_iter_write+0x83e/0xd80 fs/read_write.c:961
> vfs_writev fs/read_write.c:1004 [inline]
> do_writev+0x397/0x840 fs/read_write.c:1039
> __do_sys_writev fs/read_write.c:1112 [inline]
> __se_sys_writev+0x9b/0xb0 fs/read_write.c:1109
> __x64_sys_writev+0x4a/0x70 fs/read_write.c:1109
> do_syscall_64+0xbc/0xf0 arch/x86/entry/common.c:291
> entry_SYSCALL_64_after_hwframe+0x63/0xe7
> [...]
>
> The problem is that capi_write() is reading past the end of the message.
> Fix it by checking the message's length in the needed places.
>
> Reported-and-tested-by: syzbot+0849c524d9c634f5a...@syzkaller.appspotmail.com
> Signed-off-by: Eric Biggers <ebigg...@google.com>
> ---
> drivers/isdn/capi/capi.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/isdn/capi/capi.c b/drivers/isdn/capi/capi.c
> index 3c3ad42f22bf..a016d8c3c410 100644
> --- a/drivers/isdn/capi/capi.c
> +++ b/drivers/isdn/capi/capi.c
> @@ -688,6 +688,9 @@ capi_write(struct file *file, const char __user *buf,
> size_t count, loff_t *ppos
> if (!cdev->ap.applid)
> return -ENODEV;
>
> + if (count < CAPIMSG_BASELEN)
> + return -EINVAL;
> +
> skb = alloc_skb(count, GFP_USER);
> if (!skb)
> return -ENOMEM;
This is fine.
> @@ -698,7 +701,8 @@ capi_write(struct file *file, const char __user *buf,
> size_t count, loff_t *ppos
> }
> mlen = CAPIMSG_LEN(skb->data);
> if (CAPIMSG_CMD(skb->data) == CAPI_DATA_B3_REQ) {
> - if ((size_t)(mlen + CAPIMSG_DATALEN(skb->data)) != count) {
> + if (count < 18 ||
...
> + if (count < 12) {
These magic constants, on the other hand, are not.
