Re: [PATCH v2 4/9] ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

[Jan Kara]

On Tue 27-02-24 17:11:43, Baokun Li wrote:
> We can trigger a slab-out-of-bounds with the following commands:
> 
>     mkfs.ext4 -F /dev/$disk 10G
>     mount /dev/$disk /tmp/test
>     echo 2147483647 > /sys/fs/ext4/$disk/mb_group_prealloc
>     echo test > /tmp/test/file && sync
> 
> ==================================================================
> BUG: KASAN: slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists+0x8a/0x200 [ext4]
> Read of size 8 at addr ffff888121b9d0f0 by task kworker/u2:0/11
> CPU: 0 PID: 11 Comm: kworker/u2:0 Tainted: GL 6.7.0-next-20240118 #521
> Call Trace:
>  dump_stack_lvl+0x2c/0x50
>  kasan_report+0xb6/0xf0
>  ext4_mb_find_good_group_avg_frag_lists+0x8a/0x200 [ext4]
>  ext4_mb_regular_allocator+0x19e9/0x2370 [ext4]
>  ext4_mb_new_blocks+0x88a/0x1370 [ext4]
>  ext4_ext_map_blocks+0x14f7/0x2390 [ext4]
>  ext4_map_blocks+0x569/0xea0 [ext4]
>  ext4_do_writepages+0x10f6/0x1bc0 [ext4]
> [...]
> ==================================================================
> 
> The flow of issue triggering is as follows:
> 
> // Set s_mb_group_prealloc to 2147483647 via sysfs
> ext4_mb_new_blocks
>   ext4_mb_normalize_request
>     ext4_mb_normalize_group_request
>       ac->ac_g_ex.fe_len = EXT4_SB(sb)->s_mb_group_prealloc
>   ext4_mb_regular_allocator
>     ext4_mb_choose_next_group
>       ext4_mb_choose_next_group_best_avail
>         mb_avg_fragment_size_order
>           order = fls(len) - 2 = 29
>         ext4_mb_find_good_group_avg_frag_lists
>           frag_list = &sbi->s_mb_avg_fragment_size[order]
>           if (list_empty(frag_list)) // Trigger SOOB!
> 
> At 4k block size, the length of the s_mb_avg_fragment_size list is 14,
> but an oversized s_mb_group_prealloc is set, causing slab-out-of-bounds
> to be triggered by an attempt to access an element at index 29.
> 
> Add a new attr_id attr_clusters_in_group with values in the range
> [0, sbi->s_clusters_per_group] and declare mb_group_prealloc as
> that type to fix the issue. In addition avoid returning an order
> from mb_avg_fragment_size_order() greater than MB_NUM_ORDERS(sb)
> and reduce some useless loops.
> 
> Fixes: 7e170922f06b ("ext4: Add allocation criteria 1.5 (CR1_5)")
> CC: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>

Looks good. Just one nit below. Otherwise feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

> ---
>  fs/ext4/mballoc.c |  6 ++++++
>  fs/ext4/sysfs.c   | 13 ++++++++++++-
>  2 files changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 85a91a61b761..7ad089df2408 100644
> --- a/fs/ext4/mballoc.c
> +++ b/fs/ext4/mballoc.c
> @@ -831,6 +831,8 @@ static int mb_avg_fragment_size_order(struct super_block *sb, ext4_grpblk_t len)
>  		return 0;
>  	if (order == MB_NUM_ORDERS(sb))
>  		order--;
> +	if (WARN_ON_ONCE(order > MB_NUM_ORDERS(sb)))
> +		order = MB_NUM_ORDERS(sb) - 1;
>  	return order;
>  }
>  
> @@ -1057,6 +1059,10 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
>  			ac->ac_flags |= EXT4_MB_CR_BEST_AVAIL_LEN_OPTIMIZED;
>  			return;
>  		}
> +
> +		/* Skip some unnecessary loops. */
> +		if (unlikely(i > MB_NUM_ORDERS(ac->ac_sb)))
> +			i = MB_NUM_ORDERS(ac->ac_sb);

How can this possibly trigger now? MB_NUM_ORDERS is sb->s_blocksize_bits +
2. 'i' is starting at fls(ac->ac_g_ex.fe_len) and ac_g_ex.fe_len shouldn't
be larger than clusters per group, hence fls() should be less than
sb->s_blocksize_bits? Am I missing something? And if yes, we should rather
make sure 'order' is never absurdly big?

I suspect this code is defensive upto a point of being confusing :)

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR