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@vger.kernel.org Signed-off-by: Baokun Li libaokun1@huawei.com
Looks good. Just one nit below. Otherwise feel free to add:
Reviewed-by: Jan Kara jack@suse.cz
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;} @@ -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