[PATCH] ext2fs: fix to avoid invalid memory access

[Wang Shilong <wangshilong1991@gmail.com>]

From: Wang Shilong <wshilong@ddn.com>

Valgrind reported error messages like following:

==129205==  Address 0x1b804b04 is 4 bytes after a block of size 4,096 alloc'd
==129205==    at 0x483980B: malloc (vg_replace_malloc.c:307)
==129205==    by 0x44F973: ext2fs_get_mem (ext2fs.h:1846)
==129205==    by 0x44F973: ext2fs_get_pathname (get_pathname.c:162)
==129205==    by 0x430917: print_pathname (message.c:212)
==129205==    by 0x430FB1: expand_percent_expression (message.c:462)
==129205==    by 0x430FB1: print_e2fsck_message (message.c:544)
==129205==    by 0x430BED: expand_at_expression (message.c:262)
==129205==    by 0x430BED: print_e2fsck_message (message.c:528)
==129205==    by 0x430450: fix_problem (problem.c:2494)
==129205==    by 0x423F8B: e2fsck_process_bad_inode (pass2.c:1929)
==129205==    by 0x425AE8: check_dir_block (pass2.c:1407)
==129205==    by 0x426942: check_dir_block2 (pass2.c:961)
==129205==    by 0x445736: ext2fs_dblist_iterate3.part.0 (dblist.c:254)
==129205==    by 0x423835: e2fsck_pass2 (pass2.c:187)
==129205==    by 0x414B19: e2fsck_run (e2fsck.c:257)

Dir block might be corrupted and cause the next dirent is out
of block size boundary, even though we have the check to avoid
problem, memory check tools like valgrind still complains it.

Patch try to fix the problem by checking if offset exceed max
offset firstly before getting the pointer.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
---
 lib/ext2fs/csum.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/ext2fs/csum.c b/lib/ext2fs/csum.c
index c2550365..643777fd 100644
--- a/lib/ext2fs/csum.c
+++ b/lib/ext2fs/csum.c
@@ -260,22 +260,24 @@ static errcode_t __get_dirent_tail(ext2_filsys fs,
 	void *top;
 	struct ext2_dir_entry_tail *t;
 	unsigned int rec_len;
+	unsigned int max_len;
 	errcode_t retval = 0;
 	__u16 (*translate)(__u16) = (need_swab ? disk_to_host16 : do_nothing16);
 
 	d = dirent;
 	top = EXT2_DIRENT_TAIL(dirent, fs->blocksize);
 
+	max_len = (char *)top - (char *)dirent;
 	rec_len = translate(d->rec_len);
 	while ((void *) d < top) {
 		if ((rec_len < 8) || (rec_len & 0x03))
 			return EXT2_ET_DIR_CORRUPTED;
+		if ((char *)d - (char *)dirent + rec_len > max_len)
+			return EXT2_ET_DIR_CORRUPTED;
 		d = (struct ext2_dir_entry *)(((char *)d) + rec_len);
 		rec_len = translate(d->rec_len);
 	}
 
-	if ((char *)d > ((char *)dirent + fs->blocksize))
-			return EXT2_ET_DIR_CORRUPTED;
 	if (d != top)
 		return EXT2_ET_DIR_NO_SPACE_FOR_CSUM;
 
-- 
2.25.4