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@vger.kernel.org # v4.2+ Signed-off-by: Zhihao Cheng chengzhihao1@huawei.com Signed-off-by: Baokun Li libaokun1@huawei.com

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

Reviewed-by: Jan Kara jack@suse.cz

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