[PATCH v2] f2fs: fix to do sanity check on segment/section count

[Chao Yu]

As syzbot reported:

BUG: KASAN: slab-out-of-bounds in init_min_max_mtime fs/f2fs/segment.c:4710 [inline]
BUG: KASAN: slab-out-of-bounds in f2fs_build_segment_manager+0x9302/0xa6d0 fs/f2fs/segment.c:4792
Read of size 8 at addr ffff8880a1b934a8 by task syz-executor682/6878

CPU: 1 PID: 6878 Comm: syz-executor682 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x198/0x1fd lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0xae/0x497 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 init_min_max_mtime fs/f2fs/segment.c:4710 [inline]
 f2fs_build_segment_manager+0x9302/0xa6d0 fs/f2fs/segment.c:4792
 f2fs_fill_super+0x381a/0x6e80 fs/f2fs/super.c:3633
 mount_bdev+0x32e/0x3f0 fs/super.c:1417
 legacy_get_tree+0x105/0x220 fs/fs_context.c:592
 vfs_get_tree+0x89/0x2f0 fs/super.c:1547
 do_new_mount fs/namespace.c:2875 [inline]
 path_mount+0x1387/0x20a0 fs/namespace.c:3192
 do_mount fs/namespace.c:3205 [inline]
 __do_sys_mount fs/namespace.c:3413 [inline]
 __se_sys_mount fs/namespace.c:3390 [inline]
 __x64_sys_mount+0x27f/0x300 fs/namespace.c:3390
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

The root cause is: if segs_per_sec is larger than one, and segment count
in last section is less than segs_per_sec, we will suffer out-of-boundary
memory access on sit_i->sentries[] in init_min_max_mtime().

Fix this by adding sanity check among segment count, section count and
segs_per_sec value in sanity_check_raw_super().

Reported-by: syzbot+481a3ffab50fed41dcc0@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
---
v2:
- add reported-by tag.
 fs/f2fs/super.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index a24980aa425d..79d5e0e2225a 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -2834,6 +2834,12 @@ static int sanity_check_raw_super(struct f2fs_sb_info *sbi,
 		return -EFSCORRUPTED;
 	}
 
+	if (segment_count_main != total_sections * segs_per_sec) {
+		f2fs_info(sbi, "Invalid segment/section count (%u != %u * %u)",
+			  segment_count_main, total_sections, segs_per_sec);
+		return -EFSCORRUPTED;
+	}
+
 	if ((segment_count / segs_per_sec) < total_sections) {
 		f2fs_info(sbi, "Small segment_count (%u < %u * %u)",
 			  segment_count, segs_per_sec, total_sections);
-- 
2.26.2