Re: [PATCH 6.6/6.9 v2 2/2] ext4: fix slab-out-of-bounds in ext4_mb_find_good_group_avg_frag_lists()

[Baokun Li <libaokun@huaweicloud.com>]

On 2024/6/19 20:19, libaokun@huaweicloud.com wrote:
> From: Baokun Li <libaokun1@huawei.com>
>
> [ Upstream commit 13df4d44a3aaabe61cd01d277b6ee23ead2a5206 ]
>
> 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@vger.kernel.org
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> Reviewed-by: Jan Kara <jack@suse.cz>
> Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Link: https://lore.kernel.org/r/20240319113325.3110393-5-libaokun1@huawei.com
> Signed-off-by: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> Signed-off-by: Baokun Li <libaokun1@huawei.com>
> ---
>   fs/ext4/mballoc.c |  4 ++++
>   fs/ext4/sysfs.c   | 15 ++++++++++++++-
>   2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/fs/ext4/mballoc.c b/fs/ext4/mballoc.c
> index 714f83632e3f..66b5a68b0254 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;
>   }
>   
> @@ -1008,6 +1010,8 @@ static void ext4_mb_choose_next_group_best_avail(struct ext4_allocation_context
>   	 * goal length.
>   	 */
>   	order = fls(ac->ac_g_ex.fe_len) - 1;
> +	if (WARN_ON_ONCE(order - 1 > MB_NUM_ORDERS(ac->ac_sb)))
> +		order = MB_NUM_ORDERS(ac->ac_sb);
>   	min_order = order - sbi->s_mb_best_avail_max_trim_order;
>   	if (min_order < 0)
>   		min_order = 0;
> diff --git a/fs/ext4/sysfs.c b/fs/ext4/sysfs.c
> index ca820620b974..d65dccb44ed5 100644
> --- a/fs/ext4/sysfs.c
> +++ b/fs/ext4/sysfs.c
> @@ -29,6 +29,7 @@ typedef enum {
>   	attr_trigger_test_error,
>   	attr_first_error_time,
>   	attr_last_error_time,
> +	attr_clusters_in_group,
>   	attr_feature,
>   	attr_pointer_ui,
>   	attr_pointer_ul,
> @@ -207,13 +208,14 @@ EXT4_ATTR_FUNC(sra_exceeded_retry_limit, 0444);
>   
>   EXT4_ATTR_OFFSET(inode_readahead_blks, 0644, inode_readahead,
>   		 ext4_sb_info, s_inode_readahead_blks);
> +EXT4_ATTR_OFFSET(mb_group_prealloc, 0644, clusters_in_group,
> +		 ext4_sb_info, s_mb_group_prealloc);
>   EXT4_RW_ATTR_SBI_UI(inode_goal, s_inode_goal);
>   EXT4_RW_ATTR_SBI_UI(mb_stats, s_mb_stats);
>   EXT4_RW_ATTR_SBI_UI(mb_max_to_scan, s_mb_max_to_scan);
>   EXT4_RW_ATTR_SBI_UI(mb_min_to_scan, s_mb_min_to_scan);
>   EXT4_RW_ATTR_SBI_UI(mb_order2_req, s_mb_order2_reqs);
>   EXT4_RW_ATTR_SBI_UI(mb_stream_req, s_mb_stream_request);
> -EXT4_RW_ATTR_SBI_UI(mb_group_prealloc, s_mb_group_prealloc);
>   EXT4_RW_ATTR_SBI_UI(mb_max_linear_groups, s_mb_max_linear_groups);
>   EXT4_RW_ATTR_SBI_UI(extent_max_zeroout_kb, s_extent_max_zeroout_kb);
>   EXT4_ATTR(trigger_fs_error, 0200, trigger_test_error);
> @@ -392,6 +394,7 @@ static ssize_t ext4_attr_show(struct kobject *kobj,
>   				(unsigned long long)
>   			percpu_counter_sum(&sbi->s_sra_exceeded_retry_limit));
>   	case attr_inode_readahead:
> +	case attr_clusters_in_group:
>   	case attr_pointer_ui:
>   		if (!ptr)
>   			return 0;
> @@ -469,6 +472,16 @@ static ssize_t ext4_attr_store(struct kobject *kobj,
>   		else
>   			*((unsigned int *) ptr) = t;
>   		return len;
> +	case attr_clusters_in_group:

Hi Greg,

> +		if (!ptr)
> +			return 0;

I've found that the commit that eventually gets merged in doesn't have 
this judgment.

6.6: 677ff4589f15 ("ext4: fix slab-out-of-bounds in 
ext4_mb_find_good_group_avg_frag_lists()")
6.9: b829687ae122 ("ext4: fix slab-out-of-bounds in 
ext4_mb_find_good_group_avg_frag_lists()")

This may result in a null pointer dereference.


History:
https://lore.kernel.org/all/0d620010-c6b4-4f80-a835-451813f957e3@huawei.com/
https://lore.kernel.org/all/20240619121952.3508695-2-libaokun@huaweicloud.com/
https://lore.kernel.org/all/20240625085542.189183696@linuxfoundation.org/

Regards,
Baokun
> +		ret = kstrtouint(skip_spaces(buf), 0, &t);
> +		if (ret)
> +			return ret;
> +		if (t > sbi->s_clusters_per_group)
> +			return -EINVAL;
> +		*((unsigned int *) ptr) = t;
> +		return len;
>   	case attr_pointer_ul:
>   		if (!ptr)
>   			return 0;