Re: [RFC] ext4: don't remove already removed extent

[Muhammad Usama Anjum]

Apologies for sending multiple copies of same email. There was some issue
on my side.

On 9/11/23 2:40 PM, Muhammad Usama Anjum wrote:
> Syzbot has hit the following bug on current and all older kernels:
> BUG: KASAN: out-of-bounds in ext4_ext_rm_leaf fs/ext4/extents.c:2736 [inline]
> BUG: KASAN: out-of-bounds in ext4_ext_remove_space+0x2482/0x4d90 fs/ext4/extents.c:2958
> Read of size 18446744073709551508 at addr ffff888073aea078 by task syz-executor420/6443
> 
> On investigation, I've found that eh->eh_entries is zero, ex is
> referring to last entry and EXT_LAST_EXTENT(eh) is referring to first.
> Hence EXT_LAST_EXTENT(eh) - ex becomes negative and causes the wrong
> buffer read.
> 
> element: FFFF8882F8F0D06C       <----- ex
> element: FFFF8882F8F0D060
> element: FFFF8882F8F0D054
> element: FFFF8882F8F0D048
> element: FFFF8882F8F0D03C
> element: FFFF8882F8F0D030
> element: FFFF8882F8F0D024
> element: FFFF8882F8F0D018
> element: FFFF8882F8F0D00C	<------  EXT_FIRST_EXTENT(eh)
> header:  FFFF8882F8F0D000	<------  EXT_LAST_EXTENT(eh) and eh
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: syzbot+6e5f2db05775244c73b7@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://groups.google.com/g/syzkaller-bugs/c/G6zS-LKgDW0/m/63MgF6V7BAAJ
> Fixes: d583fb87a3ff ("ext4: punch out extents")
> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
> ---
> This patch is only fixing the local issue. There may be bigger bug. Why
> is ex set to last entry if the eh->eh_entries is 0. If any ext4
> developer want to look at the bug, please don't hesitate.
> ---
>  fs/ext4/extents.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index e4115d338f101..7b7779b4cb87f 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -2726,7 +2726,7 @@ ext4_ext_rm_leaf(handle_t *handle, struct inode *inode,
>  		 * If the extent was completely released,
>  		 * we need to remove it from the leaf
>  		 */
> -		if (num == 0) {
> +		if (num == 0 && eh->eh_entries) {
>  			if (end != EXT_MAX_BLOCKS - 1) {
>  				/*
>  				 * For hole punching, we need to scoot all the

-- 
BR,
Muhammad Usama Anjum