Forwarded: Re: [syzbot] KASAN: slab-out-of-bounds in hfs_bnode_read

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Subject: Re: [syzbot] KASAN: slab-out-of-bounds in hfs_bnode_read
Author: tristmd@xxxxxxxxx

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master

>From f6b9bdbe52aac7659475c0474819f7b6c837761f Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
Date: Tue, 5 May 2026 11:05:24 +0000
Subject: [PATCH v2 1/2] hfs/hfsplus: fix u32 overflow in
 check_and_correct_requested_length

check_and_correct_requested_length() compares (off + len) against
node_size using u32 arithmetic.  When the caller passes a large len
value (e.g. from an underflowed subtraction in hfs_brec_remove()),
off + len can wrap past 2^32 and produce a small result, causing the
bounds check to pass when it should fail.

For example, with off=14 and len=0xFFFFFFF2 (underflowed from
data_off - keyoffset - size in hfs_brec_remove), off + len wraps to 6,
which is less than a typical node_size of 512, so the check passes and
the subsequent memmove reads ~4GB past the node buffer.

Fix this by widening the addition to u64 before comparing against
node_size.  This prevents the u32 wrap while keeping the logic
straightforward.

Reported-by: syzbot+6df204b70bf3261691c5@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=6df204b70bf3261691c5
Tested-by: syzbot+6df204b70bf3261691c5@xxxxxxxxxxxxxxxxxxxxxxxxx
Reported-by: syzbot+e76bf3d19b85350571ac@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=e76bf3d19b85350571ac
Tested-by: syzbot+e76bf3d19b85350571ac@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: a431930c9bac ("hfs: fix slab-out-of-bounds in hfs_bnode_read()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tristan Madani <tristan@xxxxxxxxxxxxxxxxxxx>
---
 fs/hfs/bnode.c          | 2 +-
 fs/hfsplus/hfsplus_fs.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/hfs/bnode.c b/fs/hfs/bnode.c
index 13d58c51fc46b..9571f33b91085 100644
--- a/fs/hfs/bnode.c
+++ b/fs/hfs/bnode.c
@@ -41,7 +41,7 @@ u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
 
 	node_size = node->tree->node_size;
 
-	if ((off + len) > node_size) {
+	if ((u64)off + len > node_size) {
 		u32 new_len = node_size - off;
 
 		pr_err("requested length has been corrected: "
diff --git a/fs/hfsplus/hfsplus_fs.h b/fs/hfsplus/hfsplus_fs.h
index 3545b8dbf11c5..0e4268de9e60e 100644
--- a/fs/hfsplus/hfsplus_fs.h
+++ b/fs/hfsplus/hfsplus_fs.h
@@ -600,7 +600,7 @@ u32 check_and_correct_requested_length(struct hfs_bnode *node, u32 off, u32 len)
 
 	node_size = node->tree->node_size;
 
-	if ((off + len) > node_size) {
+	if ((u64)off + len > node_size) {
 		u32 new_len = node_size - off;
 
 		pr_err("requested length has been corrected: "
-- 
2.47.3