Re: UBSAN: shift-out-of-bounds in exfat_fill_super

[Randy Dunlap]

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