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

[Baokun Li]

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@xxxxxxxxxxxxx>
>
> > 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> > ---
> >   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
.