Forwarded: Re: [syzbot] KMSAN: uninit-value in hfs_bnode_read

[syzbot <syzbot+217eb327242d08197efb@syzkaller.appspotmail.com>]

For archival purposes, forwarding an incoming command email to
linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com.

***

Subject: Re: [syzbot] KMSAN: uninit-value in hfs_bnode_read
Author: tristmd@gmail.com

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

>From 9844dc36acf6c4c44348a5ea5ece3367423b0519 Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Tue, 5 May 2026 11:08:43 +0000
Subject: [PATCH v2 2/2] hfs/hfsplus: zero-initialize buffer in hfs_bnode_read

hfs_bnode_read() can return early without writing to the output buffer
when is_bnode_offset_valid() fails or when check_and_correct_requested_
length() corrects the length to zero.  Callers such as hfs_bnode_read_
u16() and hfs_bnode_read_u8() pass stack-allocated buffers and use the
result unconditionally, leading to KMSAN uninit-value reports.

Rather than initializing at each individual call site, zero the buffer
at the start of hfs_bnode_read() before any validation checks.  This
ensures all callers in both hfs and hfsplus get a deterministic zero
value regardless of which early-return path is taken.

Reported-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=217eb327242d08197efb
Tested-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Fixes: a431930c9bac ("hfs: fix slab-out-of-bounds in hfs_bnode_read()")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/hfs/bnode.c     | 2 ++
 fs/hfsplus/bnode.c | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/fs/hfs/bnode.c b/fs/hfs/bnode.c
index 9571f33b91085..25cef62fbba6d 100644
--- a/fs/hfs/bnode.c
+++ b/fs/hfs/bnode.c
@@ -64,6 +64,8 @@ void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32 off, u32 len)
 	u32 bytes_read;
 	u32 bytes_to_read;
 
+	memset(buf, 0, len);
+
 	if (!is_bnode_offset_valid(node, off))
 		return;
 
diff --git a/fs/hfsplus/bnode.c b/fs/hfsplus/bnode.c
index f8b5a8ae58ff5..14d1af2c7ba93 100644
--- a/fs/hfsplus/bnode.c
+++ b/fs/hfsplus/bnode.c
@@ -25,6 +25,8 @@ void hfs_bnode_read(struct hfs_bnode *node, void *buf, u32 off, u32 len)
 	struct page **pagep;
 	u32 l;
 
+	memset(buf, 0, len);
+
 	if (!is_bnode_offset_valid(node, off))
 		return;
 
-- 
2.47.3