Re: [PATCH v2] block/ioctl: prefer different overflow check Running syzkaller with the newly reintroduced signed integer overflow sanitizer shows this report:

[Justin Stitt]

On Mon, May 6, 2024 at 8:48 PM Justin Stitt <justinstitt@xxxxxxxxxx> wrote:
>

Agh. Sorry about the noise, the first line of my patch body got eaten
by the subject line because a new line was missing in mutt.

FIXED in [v3] for real this time.

> [   62.982337] ------------[ cut here ]------------
> [   62.985692] cgroup: Invalid name
> [   62.986211] UBSAN: signed-integer-overflow in ../block/ioctl.c:36:46
> [   62.989370] 9pnet_fd: p9_fd_create_tcp (7343): problem connecting socket to 127.0.0.1
> [   62.992992] 9223372036854775807 + 4095 cannot be represented in type 'long long'
> [   62.997827] 9pnet_fd: p9_fd_create_tcp (7345): problem connecting socket to 127.0.0.1
> [   62.999369] random: crng reseeded on system resumption
> [   63.000634] GUP no longer grows the stack in syz-executor.2 (7353): 20002000-20003000 (20001000)
> [   63.000668] CPU: 0 PID: 7353 Comm: syz-executor.2 Not tainted 6.8.0-rc2-00035-gb3ef86b5a957 #1
> [   63.000677] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> [   63.000682] Call Trace:
> [   63.000686]  <TASK>
> [   63.000731]  dump_stack_lvl+0x93/0xd0
> [   63.000919]  __get_user_pages+0x903/0xd30
> [   63.001030]  __gup_longterm_locked+0x153e/0x1ba0
> [   63.001041]  ? _raw_read_unlock_irqrestore+0x17/0x50
> [   63.001072]  ? try_get_folio+0x29c/0x2d0
> [   63.001083]  internal_get_user_pages_fast+0x1119/0x1530
> [   63.001109]  iov_iter_extract_pages+0x23b/0x580
> [   63.001206]  bio_iov_iter_get_pages+0x4de/0x1220
> [   63.001235]  iomap_dio_bio_iter+0x9b6/0x1410
> [   63.001297]  __iomap_dio_rw+0xab4/0x1810
> [   63.001316]  iomap_dio_rw+0x45/0xa0
> [   63.001328]  ext4_file_write_iter+0xdde/0x1390
> [   63.001372]  vfs_write+0x599/0xbd0
> [   63.001394]  ksys_write+0xc8/0x190
> [   63.001403]  do_syscall_64+0xd4/0x1b0
> [   63.001421]  ? arch_exit_to_user_mode_prepare+0x3a/0x60
> [   63.001479]  entry_SYSCALL_64_after_hwframe+0x6f/0x77
> [   63.001535] RIP: 0033:0x7f7fd3ebf539
> [   63.001551] 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 b8 ff ff ff f7 d8 64 89 01 48
> [   63.001562] RSP: 002b:00007f7fd32570c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> [   63.001584] RAX: ffffffffffffffda RBX: 00007f7fd3ff3f80 RCX: 00007f7fd3ebf539
> [   63.001590] RDX: 4db6d1e4f7e43360 RSI: 0000000020000000 RDI: 0000000000000004
> [   63.001595] RBP: 00007f7fd3f1e496 R08: 0000000000000000 R09: 0000000000000000
> [   63.001599] R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> [   63.001604] R13: 0000000000000006 R14: 00007f7fd3ff3f80 R15: 00007ffd415ad2b8
> ...
> [   63.018142] ---[ end trace ]---
>
> 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").
>
> Let's rework this overflow checking logic to not actually perform an
> overflow during the check itself, thus avoiding the UBSAN splat.
>
> [1]: https://github.com/llvm/llvm-project/pull/82432
>
> Signed-off-by: Justin Stitt <justinstitt@xxxxxxxxxx>
> ---
> Changes in v2:
> - don't use check_add_overflow as I accidentally was writing to p.start
>   and the alternative of using a dummy (unused) variable does not seem great.
> - Link to v1: https://lore.kernel.org/r/20240506-b4-sio-block-ioctl-v1-1-da535cc020dc@xxxxxxxxxx
> ---
>  block/ioctl.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/block/ioctl.c b/block/ioctl.c
> index f505f9c341eb..2639ce9df385 100644
> --- a/block/ioctl.c
> +++ b/block/ioctl.c
> @@ -33,7 +33,7 @@ static int blkpg_do_ioctl(struct block_device *bdev,
>         if (op == BLKPG_DEL_PARTITION)
>                 return bdev_del_partition(disk, p.pno);
>
> -       if (p.start < 0 || p.length <= 0 || p.start + p.length < 0)
> +       if (p.start < 0 || p.length <= 0 || LLONG_MAX - p.length < p.start)
>                 return -EINVAL;
>         /* Check that the partition is aligned to the block size */
>         if (!IS_ALIGNED(p.start | p.length, bdev_logical_block_size(bdev)))
>
> ---
> base-commit: 0106679839f7c69632b3b9833c3268c316c0a9fc
> change-id: 20240506-b4-sio-block-ioctl-78efd742fff4
>
> Best regards,
> --
> Justin Stitt <justinstitt@xxxxxxxxxx>
>

[v3]: https://lore.kernel.org/all/20240507-b4-sio-block-ioctl-v3-1-ba0c2b32275e@xxxxxxxxxx/

Thanks
Justin