[syzbot] [jffs2?] kernel BUG in jffs2_start_garbage_collect_thread

[syzbot] [jffs2?] kernel BUG in jffs2_start_garbage_collect_thread

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    fe46a7dd189e Merge tag 'sound-6.9-rc1' of git://git.kernel..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=16f4efc5180000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4d90a36f0cab495a
dashboard link: https://syzkaller.appspot.com/bug?extid=61a9d95630970eece39d
compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ea8f4b180000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1754c105180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/72ab73815344/disk-fe46a7dd.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/2d6d6b0d7071/vmlinux-fe46a7dd.xz
kernel image: https://storage.googleapis.com/syzbot-assets/48e275e5478b/bzImage-fe46a7dd.xz

The issue was bisected to:

commit 9c8ad7a2ff0bfe58f019ec0abc1fb965114dde7d
Author: David Howells <dhowells@redhat.com>
Date:   Thu May 16 11:52:27 2019 +0000

    uapi, x86: Fix the syscall numbering of the mount API syscalls [ver #2]

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=168fca9d180000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=158fca9d180000
console output: https://syzkaller.appspot.com/x/log.txt?x=118fca9d180000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+61a9d95630970eece39d@syzkaller.appspotmail.com
Fixes: 9c8ad7a2ff0b ("uapi, x86: Fix the syscall numbering of the mount API syscalls [ver #2]")

------------[ cut here ]------------
kernel BUG at fs/jffs2/background.c:40!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN PTI
CPU: 0 PID: 5060 Comm: syz-executor108 Not tainted 6.8.0-syzkaller-08951-gfe46a7dd189e #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
RIP: 0010:jffs2_start_garbage_collect_thread+0x1f5/0x200 fs/jffs2/background.c:40
Code: 03 ff e9 1b ff ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c 64 ff ff ff 48 89 df e8 76 78 03 ff e9 57 ff ff ff e8 9c 77 a3 fe 90 <0f> 0b 66 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc9000422fd00 EFLAGS: 00010293
RAX: ffffffff82f17cb4 RBX: ffff8880240a6018 RCX: ffff8880242b5a00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880240a6000
RBP: 0000000000000000 R08: ffffffff82f1c1c9 R09: 1ffff92000845f94
R10: dffffc0000000000 R11: fffff52000845f95 R12: ffff8880240a6000
R13: dffffc0000000000 R14: ffff8880240a6000 R15: ffff88802e7774f8
FS:  000055559415a380(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f54dacad0d0 CR3: 000000007aeec000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 jffs2_do_remount_fs+0x15b/0x1d0 fs/jffs2/fs.c:415
 reconfigure_super+0x445/0x880 fs/super.c:1071
 vfs_cmd_reconfigure fs/fsopen.c:267 [inline]
 vfs_fsconfig_locked fs/fsopen.c:296 [inline]
 __do_sys_fsconfig fs/fsopen.c:476 [inline]
 __se_sys_fsconfig+0xab5/0xec0 fs/fsopen.c:349
 do_syscall_64+0xfb/0x240
 entry_SYSCALL_64_after_hwframe+0x6d/0x75
RIP: 0033:0x7f54dac35cf9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 c1 17 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc9713c8d8 EFLAGS: 00000246 ORIG_RAX: 00000000000001af
RAX: ffffffffffffffda RBX: 0030656c69662f2e RCX: 00007f54dac35cf9
RDX: 0000000000000000 RSI: 0000000000000007 RDI: 0000000000000003
RBP: 0000000000010305 R08: 0000000000000000 R09: 0000000000000006
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffc9713c8ec
R13: 431bde82d7b634db R14: 0000000000000001 R15: 0000000000000001
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:jffs2_start_garbage_collect_thread+0x1f5/0x200 fs/jffs2/background.c:40
Code: 03 ff e9 1b ff ff ff 89 d9 80 e1 07 80 c1 03 38 c1 0f 8c 64 ff ff ff 48 89 df e8 76 78 03 ff e9 57 ff ff ff e8 9c 77 a3 fe 90 <0f> 0b 66 0f 1f 84 00 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90
RSP: 0018:ffffc9000422fd00 EFLAGS: 00010293
RAX: ffffffff82f17cb4 RBX: ffff8880240a6018 RCX: ffff8880242b5a00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff8880240a6000
RBP: 0000000000000000 R08: ffffffff82f1c1c9 R09: 1ffff92000845f94
R10: dffffc0000000000 R11: fffff52000845f95 R12: ffff8880240a6000
R13: dffffc0000000000 R14: ffff8880240a6000 R15: ffff88802e7774f8
FS:  000055559415a380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc9713c7d8 CR3: 000000007aeec000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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

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Subject: #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
Author: tristmd@gmail.com

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

>From 9c3f65e66c3a938b75d165bec0686e05db473070 Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Thu, 30 Apr 2026 23:57:44 +0000
Subject: [PATCH] jffs2: fix GC thread BUG_ON during reconfigure via fspick

jffs2_do_remount_fs() uses fc->sb_flags to decide whether to start
the garbage collection thread.  However, when called via fspick(2)
followed by fsconfig(FSCONFIG_CMD_RECONFIGURE), fc->sb_flags does
not reflect the current mount state -- it only contains flags being
explicitly changed (as indicated by fc->sb_flags_mask).

When fspick() is called with flags=0 on a read-only mount,
fc->sb_flags has SB_RDONLY clear (since SB_RDONLY is not in
sb_flags_mask).  This causes jffs2_start_garbage_collect_thread()
to be called even though the filesystem remains read-only.  On the
second reconfigure, BUG_ON(c->gc_task) fires because the thread
from the first call is still running.

Fix this by computing the effective read-only state using both
fc->sb_flags and fc->sb_flags_mask.  Also unconditionally call
jffs2_stop_garbage_collect_thread() before potentially restarting
it, which is safe when gc_task is NULL and prevents the BUG_ON.

Reported-by: syzbot+61a9d95630970eece39d@syzkaller.appspotmail.com
Fixes: ec10a24f10c8f ("vfs: Convert jffs2 to use the new mount API")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/jffs2/fs.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/fs/jffs2/fs.c b/fs/jffs2/fs.c
index 6ada8369a7622..33574312b7abe 100644
--- a/fs/jffs2/fs.c
+++ b/fs/jffs2/fs.c
@@ -396,28 +396,28 @@ void jffs2_dirty_inode(struct inode *inode, int flags)
 int jffs2_do_remount_fs(struct super_block *sb, struct fs_context *fc)
 {
 	struct jffs2_sb_info *c = JFFS2_SB_INFO(sb);
+	bool new_ro;
 
 	if (c->flags & JFFS2_SB_FLAG_RO && !sb_rdonly(sb))
 		return -EROFS;
 
-	/* We stop if it was running, then restart if it needs to.
-	   This also catches the case where it was stopped and this
-	   is just a remount to restart it.
-	   Flush the writebuffer, if necessary, else we loose it */
+	new_ro = (fc->sb_flags_mask & SB_RDONLY) ?
+		 (fc->sb_flags & SB_RDONLY) : sb_rdonly(sb);
+
+	jffs2_stop_garbage_collect_thread(c);
+
 	if (!sb_rdonly(sb)) {
-		jffs2_stop_garbage_collect_thread(c);
 		mutex_lock(&c->alloc_sem);
 		jffs2_flush_wbuf_pad(c);
 		mutex_unlock(&c->alloc_sem);
 	}
 
-	if (!(fc->sb_flags & SB_RDONLY))
+	if (!new_ro)
 		jffs2_start_garbage_collect_thread(c);
 
 	fc->sb_flags |= SB_NOATIME;
 	return 0;
 }
-
 /* jffs2_new_inode: allocate a new inode and inocache, add it to the hash,
    fill in the raw_inode while you're at it. */
 struct inode *jffs2_new_inode (struct inode *dir_i, umode_t mode, struct jffs2_raw_inode *ri)
-- 
2.47.3

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

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For archival purposes, forwarding an incoming command email to
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Subject: #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
Author: tristmd@gmail.com

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

>From fa9693d02a6d2e2fda72504085400c761d5eec1f Mon Sep 17 00:00:00 2001
From: Tristan Madani <tristan@talencesecurity.com>
Date: Sat, 18 Apr 2026 13:39:03 +0000
Subject: [PATCH] hfsplus: zero-initialize buffer in hfs_bnode_read

hfs_bnode_read() can return early without initializing the output
buffer when the offset is invalid or the requested length is
corrected to zero by check_and_correct_requested_length().  Callers
such as hfs_bnode_read_u16() 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 the buffer is always in a known state regardless of which
early-return path is taken.

Reported-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Tested-by: syzbot+217eb327242d08197efb@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=217eb327242d08197efb
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
---
 fs/hfsplus/bnode.c | 2 ++
 1 file changed, 2 insertions(+)

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