Re: [PATCH] tty: vt: saturate scrollback_delta to avoid overflow

[Jiri Slaby <jirislaby@kernel.org>]

On 06. 05. 24, 20:55, Justin Stitt wrote:
> Using the signed overflow sanitizer with syzkaller produces this UBSAN
> report:
> 
> [   31.304043] ------------[ cut here ]------------
> [   31.304048] UBSAN: signed-integer-overflow in ../drivers/tty/vt/vt.c:309:19
> [   31.304055] -2147483648 + -1073741824 cannot be represented in type 'int'
> [   31.304066] CPU: 1 PID: 3894 Comm: syz-executor Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
> [   31.304073] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [   31.304077] Call Trace:
> [   31.304080]  <TASK>
> [   31.304083]  dump_stack_lvl+0x93/0xd0
> [   31.304177]  handle_overflow+0x171/0x1b0
> [   31.304186]  scrollfront+0xcb/0xd0
> [   31.304196]  tioclinux+0x3cc/0x450
> [   31.304205]  tty_ioctl+0x7fc/0xc00

The rest of the stack trace can be trimmed:

> [   31.304212]  ? __pfx_tty_ioctl+0x10/0x10
> [   31.304219]  __se_sys_ioctl+0xe0/0x140
> [   31.304228]  do_syscall_64+0xd7/0x1b0
> [   31.304236]  ? arch_exit_to_user_mode_prepare+0x11/0x60
> [   31.304244]  entry_SYSCALL_64_after_hwframe+0x6f/0x77
> [   31.304254] RIP: 0033:0x7fc3902ae539
> [   31.304263] Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 f1 14 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 8
> [   31.304282] RSP: 002b:00007ffc8a457998 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> [   31.304289] RAX: ffffffffffffffda RBX: 00007fc3903e2f80 RCX: 00007fc3902ae539
> [   31.304293] RDX: 0000000020000040 RSI: 000000000000541c RDI: 0000000000000003
> [   31.304297] RBP: 00007fc39030d496 R08: 0000000000000000 R09: 0000000000000000
> [   31.304300] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> [   31.304304] R13: 0000000000000800 R14: 00007fc3903e2f80 R15: 00007fc3903e2f80
> [   31.304310]  </TASK>
> [   31.304371] ---[ end trace ]---
> 
> This is caused by the scrollback_delta overflowing. Historically, the
> signed integer overflow sanitizer did not work in the kernel due to its
> interaction with `-fwrapv` but this has since been changed [1] in the
> newest version of Clang; It being re-enabled in the kernel with Commit
> 557f8c582a9ba8ab ("ubsan: Reintroduce signed overflow sanitizer").
> 
> Note that it would be difficult to reproduce this bug in a non-fuzzing
> scenario as it requires inputting tons of scroll inputs via keyboard
> before the scheduled console callback has had a chance to update.
> Nonetheless, let's saturate scrollback_delta so it stays clamped to
> integer bounds without wrapping around.

And what is actually broken, given signed overflow is well defined under 
the -fwrapv wings?

> [1]: https://github.com/llvm/llvm-project/pull/82432
> 
> Closes: https://github.com/KSPP/linux/issues/351
> Signed-off-by: Justin Stitt <justinstitt@google.com>
> ---
> Note: I am using Kees' SIO tree as a base:
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/log/?h=dev/v6.8-rc2/signed-overflow-sanitizer
> ---
>   drivers/tty/vt/vt.c | 9 ++++++++-
>   1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/vt.c b/drivers/tty/vt/vt.c
> index 9b5b98dfc8b4..b4768336868e 100644
> --- a/drivers/tty/vt/vt.c
> +++ b/drivers/tty/vt/vt.c
> @@ -308,7 +308,14 @@ static inline void scrolldelta(int lines)
>   	/* FIXME */
>   	/* scrolldelta needs some kind of consistency lock, but the BKL was
>   	   and still is not protecting versus the scheduled back end */
> -	scrollback_delta += lines;
> +
> +	/* saturate scrollback_delta so that it never wraps around */
> +	if (lines > 0 && unlikely(INT_MAX - lines < scrollback_delta))
> +		scrollback_delta = INT_MAX;
> +	else if (lines < 0 && unlikely(INT_MIN - lines > scrollback_delta))
> +		scrollback_delta = INT_MIN;
> +	else
> +		scrollback_delta += lines;

NACK, this is horrid.

Introduce a helper for this in overflow.h if we have none yet.

thanks,
-- 
js
suse labs