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Subject: [PATCH] ext4: fix use-after-free in extent header access
Author: kartikey406@gmail.com

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

syzbot reported use-after-free bugs when accessing extent headers in
ext4_ext_insert_extent() and ext4_ext_correct_indexes(). These occur
when the extent path structure becomes invalid during operations.

The crashes show two patterns:
1. In ext4_ext_map_blocks(), the extent header can be corrupted after
   ext4_find_extent() returns, particularly during concurrent writes
   to the same file.
2. In ext4_ext_correct_indexes(), accessing path[depth] causes a
   use-after-free, indicating the path structure itself is corrupted.

This is partially exposed by commit 665575cff098 ("filemap: move
prefaulting out of hot write path") which changed timing windows in
the write path, making these races more likely to occur.

Fix this by adding validation checks:
- In ext4_ext_map_blocks(): validate the extent header after getting
  the path from ext4_find_extent()
- In ext4_ext_correct_indexes(): validate the path pointer before
  dereferencing and check extent header magic

While these checks are defensive and don't address the root cause of
path corruption, they prevent kernel crashes from invalid memory access.
A more comprehensive fix to path lifetime management may be needed in
the future.

Reported-by: syzbot+9db318d6167044609878@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9db318d6167044609878
Fixes: 665575cff098 ("filemap: move prefaulting out of hot write path")
Cc: stable@vger.kernel.org
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/ext4/extents.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index ca5499e9412b..903578d5f68d 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1708,7 +1708,9 @@ static int ext4_ext_correct_indexes(handle_t *handle, struct inode *inode,
 	struct ext4_extent *ex;
 	__le32 border;
 	int k, err = 0;
-
+	if (!path || depth < 0 || depth > EXT4_MAX_EXTENT_DEPTH) {
+		return -EFSCORRUPTED;
+	}
 	eh = path[depth].p_hdr;
 	ex = path[depth].p_ext;
 
@@ -4200,6 +4202,7 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	unsigned int allocated_clusters = 0;
 	struct ext4_allocation_request ar;
 	ext4_lblk_t cluster_offset;
+	struct ext4_extent_header *eh;
 
 	ext_debug(inode, "blocks %u/%u requested\n", map->m_lblk, map->m_len);
 	trace_ext4_ext_map_blocks_enter(inode, map->m_lblk, map->m_len, flags);
@@ -4212,7 +4215,12 @@ int ext4_ext_map_blocks(handle_t *handle, struct inode *inode,
 	}
 
 	depth = ext_depth(inode);
-
+	eh = path[depth].p_hdr;
+	if (!eh || le16_to_cpu(eh->eh_magic) != EXT4_EXT_MAGIC) {
+		EXT4_ERROR_INODE(inode, "invalid extent header after find_extent");
+		err = -EFSCORRUPTED;
+		goto out;
+	}
 	/*
 	 * consistent leaf must not be empty;
 	 * this situation is possible, though, _during_ tree modification;
-- 
2.43.0