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Subject: [PATCH] ext4: add defensive checks for extent header corruption
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
multiple locations. Testing revealed crashes in both ext4_ext_map_blocks()
and ext4_ext_correct_indexes() where extent headers or paths contain
invalid data.

The crashes occur when:
1. In ext4_ext_map_blocks(): After ext4_find_extent() returns, the extent
   header at path[depth] can be corrupted
2. In ext4_ext_correct_indexes(): The path structure itself may be invalid
   when accessed at path[depth]

These issues are more easily triggered after commit 665575cff098 ("filemap:
move prefaulting out of hot write path") which changed timing in write
paths, though the underlying vulnerability appears to be pre-existing.

Add defensive validation checks:
- In ext4_ext_map_blocks(): Validate extent header magic after getting path
- In ext4_ext_correct_indexes(): Check path validity before dereferencing

These checks prevent crashes but don't address the root cause of how
these structures become corrupted. Further investigation is needed for
a complete fix.

Reported-by: syzbot+9db318d6167044609878@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=9db318d6167044609878
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 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 ca5499e9412b..04d2328ee1d4 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -1708,7 +1708,8 @@ 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;
 
@@ -4200,19 +4201,26 @@ 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);
-
+	depth = ext_depth(inode);
+	if (depth == 0)
+		eh = ext_inode_hdr(inode);
+	else
+		eh = path[depth].p_hdr;
+	if (!eh || le16_to_cpu(eh->eh_magic) != EXT4_EXT_MAGIC) {
+		EXT4_ERROR_INODE(inode, "invalid extent header at depth %d", depth);
+		err = -EFSCORRUPTED;
+		goto out;
+	}
 	/* find extent for this block */
 	path = ext4_find_extent(inode, map->m_lblk, NULL, flags);
 	if (IS_ERR(path)) {
 		err = PTR_ERR(path);
 		goto out;
 	}
-
-	depth = ext_depth(inode);
-
 	/*
 	 * consistent leaf must not be empty;
 	 * this situation is possible, though, _during_ tree modification;
-- 
2.43.0