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>
---
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
