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

Ojaswin Mujoo
2024-02-20T05:39:55Z

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