[PATCH] ext4: verify the depth of extent tree in ext4_find_extent()

[Paolo Pisati]

From: Theodore Ts'o <tytso at mit.edu>

CVE-2018-10877

If there is a corupted file system where the claimed depth of the
extent tree is -1, this can cause a massive buffer overrun leading to
sadness.

This addresses CVE-2018-10877.

https://bugzilla.kernel.org/show_bug.cgi?id=199417

Signed-off-by: Theodore Ts'o <tytso at mit.edu>
Cc: stable at kernel.org
(backported from commit bc890a60247171294acc0bd67d211fa4b88d40ba)
Signed-off-by: Paolo Pisati <paolo.pisati at canonical.com>
---
 fs/ext4/ext4_extents.h | 1 +
 fs/ext4/extents.c      | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/fs/ext4/ext4_extents.h b/fs/ext4/ext4_extents.h
index 5074fe2..9c72cc3 100644
--- a/fs/ext4/ext4_extents.h
+++ b/fs/ext4/ext4_extents.h
@@ -103,6 +103,7 @@ struct ext4_extent_header {
 };
 
 #define EXT4_EXT_MAGIC		cpu_to_le16(0xf30a)
+#define EXT4_MAX_EXTENT_DEPTH 5
 
 #define EXT4_EXTENT_TAIL_OFFSET(hdr) \
 	(sizeof(struct ext4_extent_header) + \
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index 871070d..4b5c5c7 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -847,6 +847,12 @@ ext4_ext_find_extent(struct inode *inode, ext4_lblk_t block,
 
 	eh = ext_inode_hdr(inode);
 	depth = ext_depth(inode);
+	if (depth < 0 || depth > EXT4_MAX_EXTENT_DEPTH) {
+		EXT4_ERROR_INODE(inode, "inode has invalid extent depth: %d",
+				 depth);
+		ret = -EIO;
+		goto err;
+	}
 
 	/* account possible depth increase */
 	if (!path) {
-- 
2.7.4