linux-kernel - RE: UBSAN: shift-out-of-bounds in exfat_fill

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Date:   Tue, 26 Jan 2021 13:40:08 +0900
From:   "Namjae Jeon" <namjae.jeon@...sung.com>
To:     "'Matthew Wilcox'" <willy@...radead.org>
Cc:     <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <sj1557.seo@...sung.com>, <syzkaller-bugs@...glegroups.com>,
        "'syzbot'" <syzbot+da4fe66aaadd3c2e2d1c@...kaller.appspotmail.com>
Subject: RE: UBSAN: shift-out-of-bounds in exfat_fill_super

> 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.
exfat specification describe sect_per_clus_bits field of boot sector could be at most 32 and
at least 0. And sect_size_bits can also affect this calculation, It also needs validation.
I will fix it.
Thanks!