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

[Namjae Jeon]

> > 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
                                                                                                 typo ^^16
> 0. And sect_size_bits can also affect this calculation, It also needs validation.
> I will fix it.
> Thanks!