ext4: fix use-after-free in ext4_ext_shift_extents

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+ torvalds#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: 331573f ("ext4: Add support FALLOC_FL_INSERT_RANGE for fallocate")
Cc: [email protected] # v4.2+
Signed-off-by: Zhihao Cheng <[email protected]>
Signed-off-by: Baokun Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Theodore Ts'o <[email protected]>

fs/ext4/extents.c

@@ -5184,6 +5184,7 @@ 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
@@ -5227,14 +5228,21 @@ 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,