Re: [PATCH 5/7] ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

[Baokun Li <libaokun1@huawei.com>]

On 2024/2/20 13:39, Ojaswin Mujoo wrote:
> On Fri, Jan 26, 2024 at 04:57:14PM +0800, Baokun Li wrote:
>
> Hey Baokun,
>
> Good catch! I've added some minor comments below. Other than that feel
> free to add
>
> Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
>
>> 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.
>>
>> Therefore it is not allowed to set s_mb_group_prealloc to a value greater
>> than s_clusters_per_group via sysfs, and to avoid returning an order from
>> mb_avg_fragment_size_order() that is greater than MB_NUM_ORDERS(sb).
>>
>> Fixes: 7e170922f06b ("ext4: Add allocation criteria 1.5 (CR1_5)")
>> CC: stable@vger.kernel.org
>> Signed-off-by: Baokun Li <libaokun1@huawei.com>
>> ---
>>   fs/ext4/mballoc.c | 2 ++
>>   fs/ext4/sysfs.c   | 9 ++++++++-
>>   2 files changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
>> index f44f668e407f..1ea6491b6b00 100644
>> --- a/fs/ext4/mballoc.c
>> +++ b/fs/ext4/mballoc.c
>> @@ -832,6 +832,8 @@ 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;
>>   }
> So along with this change, I think it'll also be good to add an extra
> check in ext4_mb_choose_next_group_best_avail() as:
>
>    if (1 << min_order < ac->ac_o_ex.fe_len)
>      min_order = fls(ac->ac_o_ex.fe_len);
>   
> + if (order >= MB_NUM_ORDERS(ac->ac_sb))
> +   order = MB_NUM_ORDERS(ac->ac_sb) - 1;
> +
>    for (i = order; i >= min_order; i--) {
>      int frag_order;
>      /*
>
>
> The reason for this is that otherwise when order is large eg 29,
> we would unnecessarily loop from i=29 to i=13 while always
> looking at the same avg_fragment_list[13].
>
> Regards,
> ojaswin


Yeah, good point! This will cut down on some unnecessary loops.

I'll add this extra check in the next version.

Thanks for having a look!

-- 
With Best Regards,
Baokun Li
.