[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 <dhow...@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+61a9d9...@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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***

Subject: #syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
Author: tri...@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 <tri...@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+61a9d9...@syzkaller.appspotmail.com
Fixes: ec10a24f10c8f ("vfs: Convert jffs2 to use the new mount API")
Cc: sta...@vger.kernel.org
Signed-off-by: Tristan Madani <tri...@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