Re: [PATCH v2] ext4: fix use-after-free in ext4_ext_shift_extents

[Jan Kara]

On Thu 22-09-22 20:04:34, Baokun Li wrote:
> If the starting position of our insert range happens to be in the hole
> between the two ext4_extent_idx, because the lblk of the ext4_extent in
> the previous ext4_extent_idx is always less than the start, which leads
> to the "extent" variable access across the boundary, the following UAF is
> triggered:
> ==================================================================
> BUG: KASAN: use-after-free in ext4_ext_shift_extents+0x257/0x790
> Read of size 4 at addr ffff88819807a008 by task fallocate/8010
> CPU: 3 PID: 8010 Comm: fallocate Tainted: G            E     5.10.0+ #492
> Call Trace:
>  dump_stack+0x7d/0xa3
>  print_address_description.constprop.0+0x1e/0x220
>  kasan_report.cold+0x67/0x7f
>  ext4_ext_shift_extents+0x257/0x790
>  ext4_insert_range+0x5b6/0x700
>  ext4_fallocate+0x39e/0x3d0
>  vfs_fallocate+0x26f/0x470
>  ksys_fallocate+0x3a/0x70
>  __x64_sys_fallocate+0x4f/0x60
>  do_syscall_64+0x33/0x40
>  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> ==================================================================
> 
> For right shifts, we can divide them into the following situations：
> 
> 1. When the first ee_block of ext4_extent_idx is greater than or equal to
>    start, make right shifts directly from the first ee_block.
>     1) If it is greater than start, we need to continue searching in the
>        previous ext4_extent_idx.
>     2) If it is equal to start, we can exit the loop (iterator=NULL).
> 
> 2. When the first ee_block of ext4_extent_idx is less than start, then
>    traverse from the last extent to find the first extent whose ee_block
>    is less than start.
>     1) If extent is still the last extent after traversal, it means that
>        the last ee_block of ext4_extent_idx is less than start, that is,
>        start is located in the hole between idx and (idx+1), so we can
>        exit the loop directly (break) without right shifts.
>     2) Otherwise, make right shifts at the corresponding position of the
>        found extent, and then exit the loop (iterator=NULL).
> 
> Fixes: 331573febb6a ("ext4: Add support FALLOC_FL_INSERT_RANGE for fallocate")
> Cc: stable@xxxxxxxxxxxxxxx # v4.2+
> Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>

Thanks for the fix! It looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
> V1->V2:
>   Initialize "ret" after the "again:" label to avoid return value mismatch.
>   Refactoring reduces cycles and makes code more readable.
> 
>  fs/ext4/extents.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index c148bb97b527..39c9f87de0be 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -5179,6 +5179,7 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
>  	 * and it is decreased till we reach start.
>  	 */
>  again:
> +	ret = 0;
>  	if (SHIFT == SHIFT_LEFT)
>  		iterator = &start;
>  	else
> @@ -5222,14 +5223,21 @@ ext4_ext_shift_extents(struct inode *inode, handle_t *handle,
>  					ext4_ext_get_actual_len(extent);
>  		} else {
>  			extent = EXT_FIRST_EXTENT(path[depth].p_hdr);
> -			if (le32_to_cpu(extent->ee_block) > 0)
> +			if (le32_to_cpu(extent->ee_block) > start)
>  				*iterator = le32_to_cpu(extent->ee_block) - 1;
> -			else
> -				/* Beginning is reached, end of the loop */
> +			else if (le32_to_cpu(extent->ee_block) == start)
>  				iterator = NULL;
> -			/* Update path extent in case we need to stop */
> -			while (le32_to_cpu(extent->ee_block) < start)
> +			else {
> +				extent = EXT_LAST_EXTENT(path[depth].p_hdr);
> +				while (le32_to_cpu(extent->ee_block) >= start)
> +					extent--;
> +
> +				if (extent == EXT_LAST_EXTENT(path[depth].p_hdr))
> +					break;
> +
>  				extent++;
> +				iterator = NULL;
> +			}
>  			path[depth].p_ext = extent;
>  		}
>  		ret = ext4_ext_shift_path_extents(path, shift, inode,
> -- 
> 2.31.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR