[lvc-project] [PATCH v2] jffs2: fix use-after-free in jffs2_garbage_collect_thread()
Dmitriy Chumachenko
Dmitry.Chumachenko at cyberprotect.ru
Wed Apr 8 13:31:27 MSK 2026
During fuzz testing, the following issue was discovered.
BUG: KASAN: use-after-free in __lock_acquire+0x3f22/0x53c0 kernel/locking/lockdep.c:4825
Read of size 8 at addr ffff888053cfa098 by task jffs2_gcd_mtd0/11093
CPU: 1 PID: 11093 Comm: jffs2_gcd_mtd0 Not tainted 5.10.232-syzkaller #0
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x107/0x167 lib/dump_stack.c:118
print_address_description.constprop.0+0x1c/0x220 mm/kasan/report.c:377
__kasan_report mm/kasan/report.c:537 [inline]
kasan_report.cold+0x1f/0x37 mm/kasan/report.c:554
__lock_acquire+0x3f22/0x53c0 kernel/locking/lockdep.c:4825
lock_acquire kernel/locking/lockdep.c:5566 [inline]
lock_acquire+0x197/0x480 kernel/locking/lockdep.c:5531
__raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
_raw_spin_lock_irqsave+0x36/0x60 kernel/locking/spinlock.c:159
complete+0x13/0x60 kernel/sched/completion.c:32
complete_and_exit+0x20/0x40 kernel/exit.c:943
jffs2_garbage_collect_thread+0x554/0x750 fs/jffs2/background.c:164
kthread+0x3a9/0x490 kernel/kthread.c:328
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
Allocated by task 11091:
kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
kasan_set_track mm/kasan/common.c:56 [inline]
__kasan_kmalloc.constprop.0+0xc9/0xd0 mm/kasan/common.c:461
kmalloc include/linux/slab.h:552 [inline]
kzalloc include/linux/slab.h:664 [inline]
jffs2_init_fs_context+0x41/0xd0 fs/jffs2/super.c:314
alloc_fs_context+0x4f9/0x840 fs/fs_context.c:267
do_new_mount fs/namespace.c:2896 [inline]
path_mount+0xb99/0x2140 fs/namespace.c:3247
do_mount fs/namespace.c:3260 [inline]
__do_sys_mount fs/namespace.c:3468 [inline]
__se_sys_mount fs/namespace.c:3445 [inline]
__x64_sys_mount+0x283/0x300 fs/namespace.c:3445
do_syscall_64+0x30/0x40 arch/x86/entry/common.c:46
entry_SYSCALL_64_after_hwframe+0x67/0xd1
Freed by task 28546:
kasan_save_stack+0x1b/0x40 mm/kasan/common.c:48
kasan_set_track+0x1c/0x30 mm/kasan/common.c:56
kasan_set_free_info+0x1b/0x30 mm/kasan/generic.c:355
__kasan_slab_free+0x112/0x170 mm/kasan/common.c:422
slab_free_hook mm/slub.c:1542 [inline]
slab_free_freelist_hook+0xb8/0x1b0 mm/slub.c:1576
slab_free mm/slub.c:3149 [inline]
kfree+0xd9/0x360 mm/slub.c:4125
deactivate_locked_super+0x96/0x170 fs/super.c:335
deactivate_super+0xb2/0xd0 fs/super.c:366
cleanup_mnt+0x3a3/0x530 fs/namespace.c:1118
task_work_run+0xdf/0x1a0 kernel/task_work.c:185
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_user_mode_loop kernel/entry/common.c:172 [inline]
exit_to_user_mode_prepare+0x1de/0x1f0 kernel/entry/common.c:199
syscall_exit_to_user_mode+0x38/0x1e0 kernel/entry/common.c:274
In jffs2_garbage_collect_thread() gc_task is set to NULL and then
kthread_complete_and_exit() calls complete() on gc_thread_exit. These
operations are not atomic: stop path can see gc_task == NULL, skip
wait_for_completion(), and the caller frees jffs2_sb_info while the GC
thread still accesses gc_thread_exit in complete().
Moreover, spin_unlock() itself accesses c after complete() has woken the
stop path:
jffs2_kill_sb jffs2_garbage_collect_thread
jffs2_stop_garbage_collect_thread
spin_lock
send_sig(SIGKILL)
wait = 1
spin_unlock
goto die
spin_lock
c->gc_task = NULL
spin_unlock
kthread_complete_and_exit()
complete(&c->gc_thread_exit)
wait_for_completion()
kfree(c)
Fix by adding a gc_thread_started flag that is set when the GC thread is
successfully started. Use this flag instead of gc_task to decide whether
to wait. The flag is never cleared by the GC thread, so
wait_for_completion() is always called when start() succeeded, regardless
of the current value of gc_task.
Found by Linux Verification Center (linuxtesting.org) with Syzkaller.
Fixes: e2d48b1a98bb ("[JFFS2] Fix cleanup in case of GC-Task not started")
Signed-off-by: Dmitriy Chumachenko <Dmitry.Chumachenko at cyberprotect.ru>
---
v1->v2: Use gc_thread_started flag as a reliable indicator instead
of gc_task. Drop complete() under erase_completion_lock
(thanks, Zhihao Cheng).
fs/jffs2/background.c | 7 ++++---
fs/jffs2/jffs2_fs_sb.h | 1 +
2 files changed, 5 insertions(+), 3 deletions(-)
diff --git a/fs/jffs2/background.c b/fs/jffs2/background.c
index bb0ee1a59e71..5e5ed9053326 100644
--- a/fs/jffs2/background.c
+++ b/fs/jffs2/background.c
@@ -52,6 +52,7 @@ int jffs2_start_garbage_collect_thread(struct jffs2_sb_info *c)
/* Wait for it... */
jffs2_dbg(1, "Garbage collect thread is pid %d\n", tsk->pid);
wait_for_completion(&c->gc_thread_start);
+ c->gc_thread_started = true;
ret = tsk->pid;
}
@@ -60,16 +61,16 @@ int jffs2_start_garbage_collect_thread(struct jffs2_sb_info *c)
void jffs2_stop_garbage_collect_thread(struct jffs2_sb_info *c)
{
- int wait = 0;
spin_lock(&c->erase_completion_lock);
if (c->gc_task) {
jffs2_dbg(1, "Killing GC task %d\n", c->gc_task->pid);
send_sig(SIGKILL, c->gc_task, 1);
- wait = 1;
}
spin_unlock(&c->erase_completion_lock);
- if (wait)
+ if (c->gc_thread_started) {
wait_for_completion(&c->gc_thread_exit);
+ c->gc_thread_started = false;
+ }
}
static int jffs2_garbage_collect_thread(void *_c)
diff --git a/fs/jffs2/jffs2_fs_sb.h b/fs/jffs2/jffs2_fs_sb.h
index 5a7091746f68..4c833e0ff03c 100644
--- a/fs/jffs2/jffs2_fs_sb.h
+++ b/fs/jffs2/jffs2_fs_sb.h
@@ -55,6 +55,7 @@ struct jffs2_sb_info {
unsigned int flags;
struct task_struct *gc_task; /* GC task struct */
+ bool gc_thread_started; /* GC thread was successfully started */
struct completion gc_thread_start; /* GC thread start completion */
struct completion gc_thread_exit; /* GC thread exit completion port */
--
2.49.0
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