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SubjectRe: [PATCH] fs/ext4: fix integer overflow in s_log_groups_per_flex
On Wed 03-02-21 19:43:51, 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>
> ---
> fs/ext4/mballoc.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 99bf091fee10..e1e7ffbba1a6 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -2709,8 +2709,15 @@ static int ext4_mb_init_backend(struct super_block *sb)
> }
>
> if (ext4_has_feature_flex_bg(sb)) {
> - /* a single flex group is supposed to be read by a single IO */
> - sbi->s_mb_prefetch = min(1 << sbi->s_es->s_log_groups_per_flex,
> + /* a single flex group is supposed to be read by a single IO.
> + * 2 ^ s_log_groups_per_flex != UINT_MAX as s_mb_prefetch is
> + * unsigned integer, so the maximum shift is 32.
> + */
> + if (sbi->s_es->s_log_groups_per_flex > 32) {
^^ >= 32?

Otherwise the patch looks good.

Honza


> + ext4_msg(sb, KERN_ERR, "too many log groups per flexible block group");
> + goto err_freesgi;
> + }
> + sbi->s_mb_prefetch = min_t(uint, 1 << sbi->s_es->s_log_groups_per_flex,
> BLK_MAX_SEGMENT_SIZE >> (sb->s_blocksize_bits - 9));
> sbi->s_mb_prefetch *= 8; /* 8 prefetch IOs in flight at most */
> } else {
> --
> 2.25.1
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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