Re: [PATCH v2] fs/ext4: fix integer overflow in s_log_groups_per_flex

["Theodore Ts'o" <tytso@mit.edu>]

On Wed, Feb 24, 2021 at 03:58:00PM +0600, Sabyrzhan Tasbolatov wrote:
> syzbot found UBSAN: shift-out-of-bounds in ext4_mb_init [1], when
> 1 << sbi->s_es->s_log_groups_per_flex is bigger than UINT_MAX,
> where sbi->s_mb_prefetch is unsigned integer type.
> 
> 32 is the maximum allowed power of s_log_groups_per_flex. Following if
> check will also trigger UBSAN shift-out-of-bound:
> 
> if (1 << sbi->s_es->s_log_groups_per_flex >= UINT_MAX) {
> 
> So I'm checking it against the raw number, perhaps there is another way
> to calculate UINT_MAX max power. Also use min_t as to make sure it's
> uint type.
> 
> [1] UBSAN: shift-out-of-bounds in fs/ext4/mballoc.c:2713:24
> shift exponent 60 is too large for 32-bit type 'int'
> Call Trace:
>  __dump_stack lib/dump_stack.c:79 [inline]
>  dump_stack+0x137/0x1be lib/dump_stack.c:120
>  ubsan_epilogue lib/ubsan.c:148 [inline]
>  __ubsan_handle_shift_out_of_bounds+0x432/0x4d0 lib/ubsan.c:395
>  ext4_mb_init_backend fs/ext4/mballoc.c:2713 [inline]
>  ext4_mb_init+0x19bc/0x19f0 fs/ext4/mballoc.c:2898
>  ext4_fill_super+0xc2ec/0xfbe0 fs/ext4/super.c:4983
> 
> Reported-by: syzbot+a8b4b0c60155e87e9484@syzkaller.appspotmail.com
> Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>

Applied, thanks.

					- Ted