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

[Namjae Jeon]

> 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.
Right. I will also add validation for fat_length as well as sect_size_bits before this.

Thanks!
> 
> 
> --
> ~Randy