[PATCH] ext4: validate p_idx bounds in ext4_ext_correct_indexes

[tejas bharambe]

>From 980406b391b29715589842a4ae4391a87afaaef9 Mon Sep 17 00:00:00 2001
From: Tejas Bharambe <tejas.bharambe@xxxxxxxxxxx>
Date: Tue, 3 Mar 2026 23:14:34 -0800
Subject: [PATCH] ext4: validate p_idx bounds in ext4_ext_correct_indexes

ext4_ext_correct_indexes() walks up the extent tree correcting
index entries when the first extent in a leaf is modified. Before
accessing path[k].p_idx->ei_block, there is no validation that
p_idx falls within the valid range of index entries for that
level.

If the on-disk extent header contains a corrupted or crafted
eh_entries value, p_idx can point past the end of the allocated
buffer, causing a slab-out-of-bounds read.

Fix this by validating path[k].p_idx against EXT_LAST_INDEX() at
both access sites: before the while loop and inside it. Return
-EFSCORRUPTED if the index pointer is out of range, consistent
with how other bounds violations are handled in the ext4 extent
tree code.

Reported-by: syzbot+04c4e65cab786a2e5b7e@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=04c4e65cab786a2e5b7e
Signed-off-by: Tejas Bharambe <tejas.bharambe@xxxxxxxxxxx>
---
 fs/ext4/extents.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index ae3804f365..f204285c71 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1736,6 +1736,13 @@ static int ext4_ext_correct_indexes(handle_t *handle, struct inode *inode,
        err = ext4_ext_get_access(handle, inode, path + k);
        if (err)
                return err;
+       if (unlikely(path[k].p_idx > EXT_LAST_INDEX(path[k].p_hdr))) {
+               EXT4_ERROR_INODE(inode,
+                       "path[%d].p_idx %p > EXT_LAST_INDEX %p",
+                       k, path[k].p_idx,
+                       EXT_LAST_INDEX(path[k].p_hdr));
+               return -EFSCORRUPTED;
+       }
        path[k].p_idx->ei_block = border;
        err = ext4_ext_dirty(handle, inode, path + k);
        if (err)
@@ -1748,6 +1755,14 @@ static int ext4_ext_correct_indexes(handle_t *handle, struct inode *inode,
                err = ext4_ext_get_access(handle, inode, path + k);
                if (err)
                        goto clean;
+               if (unlikely(path[k].p_idx > EXT_LAST_INDEX(path[k].p_hdr))) {
+                       EXT4_ERROR_INODE(inode,
+                               "path[%d].p_idx %p > EXT_LAST_INDEX %p",
+                               k, path[k].p_idx,
+                               EXT_LAST_INDEX(path[k].p_hdr));
+                       err = -EFSCORRUPTED;
+                       goto clean;
+               }
                path[k].p_idx->ei_block = border;
                err = ext4_ext_dirty(handle, inode, path + k);
                if (err)
-- 
2.53.0