Re: [PATCH] exfat: fix shift-out-of-bounds in exfat_fill_super()

[Randy Dunlap <rdunlap@infradead.org>]

On 1/28/21 11:12 PM, Namjae Jeon wrote:
> syzbot reported a warning which could cause shift-out-of-bounds issue.
> 
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x183/0x22e lib/dump_stack.c:120
>  ubsan_epilogue lib/ubsan.c:148 [inline]
>  __ubsan_handle_shift_out_of_bounds+0x432/0x4d0 lib/ubsan.c:395
>  exfat_read_boot_sector fs/exfat/super.c:471 [inline]
>  __exfat_fill_super fs/exfat/super.c:556 [inline]
>  exfat_fill_super+0x2acb/0x2d00 fs/exfat/super.c:624
>  get_tree_bdev+0x406/0x630 fs/super.c:1291
>  vfs_get_tree+0x86/0x270 fs/super.c:1496
>  do_new_mount fs/namespace.c:2881 [inline]
>  path_mount+0x1937/0x2c50 fs/namespace.c:3211
>  do_mount fs/namespace.c:3224 [inline]
>  __do_sys_mount fs/namespace.c:3432 [inline]
>  __se_sys_mount+0x2f9/0x3b0 fs/namespace.c:3409
>  do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> 
> exfat specification describe sect_per_clus_bits field of boot sector
> could be at most 16 and at least 0. And sect_size_bits can also
> affect this calculation, It also needs validation.
> This patch add validation for sect_per_clus_bits and sect_size_bits
> field of boot sector.
> 
> Fixes: 719c1e182916 ("exfat: add super block operations")
> Cc: stable@vger.kernel.org # v5.9+
> Reported-by: syzbot+da4fe66aaadd3c2e2d1c@syzkaller.appspotmail.com
> Reviewed-by: Sungjong Seo <sj1557.seo@samsung.com>
> Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>

Tested-by: Randy Dunlap <rdunlap@infradead.org>

[  172.721719] exFAT-fs (loop0): bogus sector size bits : 0

[  172.721736] exFAT-fs (loop0): failed to read boot sector
[  172.721744] exFAT-fs (loop0): failed to recognize exfat type

Thanks.

> ---
>  fs/exfat/exfat_raw.h |  4 ++++
>  fs/exfat/super.c     | 31 ++++++++++++++++++++++++++-----
>  2 files changed, 30 insertions(+), 5 deletions(-)


-- 
~Randy