On Mon, Jan 25, 2021 at 09:33:14AM -0800, syzbot wrote:

quoted

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