From: Randy Dunlap <rdunlap@infradead.org>
To: Matthew Wilcox <willy@infradead.org>,
syzbot <syzbot+da4fe66aaadd3c2e2d1c@syzkaller.appspotmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
namjae.jeon@samsung.com, sj1557.seo@samsung.com,
syzkaller-bugs@googlegroups.com
Subject: Re: UBSAN: shift-out-of-bounds in exfat_fill_super
Date: Mon, 25 Jan 2021 20:33:54 -0800 [thread overview]
Message-ID: <40b5993e-d99e-b2b9-6568-80e46e2d3cb1@infradead.org> (raw)
In-Reply-To: <20210125183918.GH308988@casper.infradead.org>
On 1/25/21 10:39 AM, Matthew Wilcox wrote:
> On Mon, Jan 25, 2021 at 09:33:14AM -0800, syzbot wrote:
>> UBSAN: shift-out-of-bounds in fs/exfat/super.c:471:28
>> shift exponent 4294967294 is too large for 32-bit type 'int'
>
> This is an integer underflow:
>
> sbi->dentries_per_clu = 1 <<
> (sbi->cluster_size_bits - DENTRY_SIZE_BITS);
>
> I think the problem is that there is no validation of sect_per_clus_bits.
> We should check it is at least DENTRY_SIZE_BITS and probably that it's
> less than ... 16? 64? I don't know what legitimate values are in this
> field, but I would imagine that 255 is completely unacceptable.
Ack all of that. The syzbot boot_sector has sect_per_clus_bits == 3
and sect_size_bits == 0, so sbi->cluster_size_bits is 3, then
UBSAN goes bang on:
sbi->dentries_per_clu = 1 <<
(sbi->cluster_size_bits - DENTRY_SIZE_BITS); // 3 - 5
There is also an unprotected shift at line 480:
if (sbi->num_FAT_sectors << p_boot->sect_size_bits <
sbi->num_clusters * 4) {
that should be protected IMO.
--
~Randy
next prev parent reply other threads:[~2021-01-26 19:54 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-25 17:33 UBSAN: shift-out-of-bounds in exfat_fill_super syzbot
2021-01-25 18:39 ` Matthew Wilcox
2021-01-26 4:33 ` Randy Dunlap [this message]
2021-01-26 5:14 ` Namjae Jeon
2021-01-26 4:40 ` Namjae Jeon
2021-01-26 5:08 ` Namjae Jeon
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