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

[Matthew Wilcox]

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.