[PATCH 2/3] f2fs: reject MAIN_AREA boundary underflow

[Cheng Yongkang <teel4res@gmail.com>]

sanity_check_area_boundary() computes

	main_end_blkaddr - segment0_blkaddr

as u64 when main_end_blkaddr < seg_end_blkaddr.  A crafted superblock
can make main_end_blkaddr smaller than segment0_blkaddr, so the
subtraction underflows and cpu_to_le32() truncates the result; the
function persists a wrapped segment_count (0xfffffffd in the
reproduced image) and the next mount rejects the filesystem.  The
same path can also leave the error workqueue dereferencing freed
superblock state:

  BUG: KASAN: slab-use-after-free in f2fs_commit_super

Reject the underflowing geometry before the subtraction.

The underflow was reproduced 3/3 times; the patched kernel rejects
it with "Wrong MAIN_AREA boundary", leaves segment_count unchanged,
and does not trigger the KASAN report.

Fixes: fd694733d523 ("f2fs: cover large section in sanity check of super")
Assisted-by: LLM
Signed-off-by: Cheng Yongkang <teel4res@gmail.com>
---
 fs/f2fs/super.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
index ea2d218..b90831e 100644
--- a/fs/f2fs/super.c
+++ b/fs/f2fs/super.c
@@ -4004,6 +4004,12 @@ static inline bool sanity_check_area_boundary(struct f2fs_sb_info *sbi,
 		return true;
 	}
 
+	if (main_end_blkaddr < segment0_blkaddr) {
+		f2fs_info(sbi, "Wrong MAIN_AREA boundary, start(%u) end(%llu)",
+			  main_blkaddr, main_end_blkaddr);
+		return true;
+	}
+
 	if (main_end_blkaddr > seg_end_blkaddr) {
 		f2fs_info(sbi, "Wrong MAIN_AREA boundary, start(%u) end(%llu) block(%u)",
 			  main_blkaddr, seg_end_blkaddr,
-- 
2.34.1