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
