Re: general protection fault in kernfs_kill_sb

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

On 2018/04/02 2:01, syzbot wrote:
> Hello,
> 
> syzbot hit the following crash on bpf-next commit
> 7828f20e3779e4e85e55371e0e43f5006a15fb41 (Sat Mar 31 00:17:57 2018 +0000)
> Merge branch 'bpf-cgroup-bind-connect'
> syzbot dashboard link: https://syzkaller.appspot.com/bug?extid=151de3f2be6b40ac8026
> 
> So far this crash happened 3 times on bpf-next.
> C reproducer: https://syzkaller.appspot.com/x/repro.c?id=4857382450495488
> syzkaller reproducer: https://syzkaller.appspot.com/x/repro.syz?id=4644052230209536
> Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5798498637185024
> Kernel config: https://syzkaller.appspot.com/x/.config?id=5909223872832634926
> compiler: gcc (GCC) 7.1.1 20170620

Al, I think this is another example of crash triggered by
commit 9ee332d99e4d5a97 ("sget(): handle failures of register_shrinker()").

----------------------------------------
[   23.407545] FAULT_INJECTION: forcing a failure.
[   23.407545] name failslab, interval 1, probability 0, space 0, times 1
[   23.414735] CPU: 1 PID: 4471 Comm: syzkaller129261 Not tainted 4.16.0-rc6+ #43
[   23.433147] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
[   23.442491] Call Trace:
[   23.445074]  dump_stack+0x194/0x24d
[   23.448689]  ? arch_local_irq_restore+0x53/0x53
[   23.453347]  ? find_held_lock+0x35/0x1d0
[   23.457401]  should_fail+0x8c0/0xa40
[   23.461100]  ? __list_lru_init+0x352/0x750
[   23.465331]  ? fault_create_debugfs_attr+0x1f0/0x1f0
[   23.470453]  ? find_held_lock+0x35/0x1d0
[   23.474503]  ? __lock_is_held+0xb6/0x140
[   23.478556]  ? check_same_owner+0x320/0x320
[   23.482870]  ? rcu_note_context_switch+0x710/0x710
[   23.487785]  ? find_held_lock+0x35/0x1d0
[   23.491931]  should_failslab+0xec/0x120
[   23.495895]  __kmalloc+0x63/0x760
[   23.499332]  ? lock_downgrade+0x980/0x980
[   23.503469]  ? _raw_spin_unlock+0x22/0x30
[   23.507605]  ? register_shrinker+0x10e/0x2d0
[   23.511999]  ? trace_event_raw_event_module_request+0x320/0x320
[   23.518044]  register_shrinker+0x10e/0x2d0
[   23.522265]  ? __bpf_trace_mm_vmscan_wakeup_kswapd+0x40/0x40
[   23.528051]  ? memcpy+0x45/0x50
[   23.531588]  sget_userns+0xbbf/0xe40
[   23.535296]  ? kernfs_sop_show_path+0x190/0x190
[   23.539959]  ? kernfs_sop_show_options+0x180/0x180
[   23.544876]  ? destroy_unused_super.part.6+0xd0/0xd0
[   23.549972]  ? check_same_owner+0x320/0x320
[   23.554281]  ? rcu_pm_notify+0xc0/0xc0
[   23.558161]  ? rcu_read_lock_sched_held+0x108/0x120
[   23.563168]  ? kmem_cache_alloc_trace+0x459/0x740
[   23.567997]  ? lock_downgrade+0x980/0x980
[   23.572142]  kernfs_mount_ns+0x13d/0x8b0
[   23.576192]  ? kernfs_super_ns+0x70/0x70
[   23.580244]  sysfs_mount+0xc2/0x1c0
----------------------------------------

That commit assumes that calling kill_sb() from deactivate_locked_super(s)
without corresponding fill_super() is safe. We have so far crashed with
rpc_mount() and kernfs_mount_ns(). Is that really safe?

Also, I think

----------------------------------------
struct dentry *kernfs_mount_ns(struct file_system_type *fs_type, int flags,
                               struct kernfs_root *root, unsigned long magic,
                               bool *new_sb_created, const void *ns)
{
(...snipped...)
	if (!sb->s_root) {
		struct kernfs_super_info *info = kernfs_info(sb);

		error = kernfs_fill_super(sb, magic);
		if (error) {
			deactivate_locked_super(sb); // <= this call
			return ERR_PTR(error);
		}
		sb->s_flags |= SB_ACTIVE;

		mutex_lock(&kernfs_mutex);
		list_add(&info->node, &root->supers);
		mutex_unlock(&kernfs_mutex);
	}
(...snipped...)
}
----------------------------------------

is not safe, for list_del() is called via kill_sb() without
corresponding list_add().

----------------------------------------
void kernfs_kill_sb(struct super_block *sb)
{
	struct kernfs_super_info *info = kernfs_info(sb);

	mutex_lock(&kernfs_mutex);
	list_del(&info->node); // <= NULL pointer dereference
	mutex_unlock(&kernfs_mutex);

	/*
	 * Remove the superblock from fs_supers/s_instances
	 * so we can't find it, before freeing kernfs_super_info.
	 */
	kill_anon_super(sb);
	kfree(info);
}
----------------------------------------

> 
> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> Reported-by: syzbot+151de3f2be6b40ac8026@syzkaller.appspotmail.com
> It will help syzbot understand when the bug is fixed. See footer for details.
> If you forward the report, please keep this part and the footer.
> 
> kasan: GPF could be caused by NULL-ptr deref or user memory access
>  should_failslab+0xec/0x120 mm/failslab.c:32
>  slab_pre_alloc_hook mm/slab.h:422 [inline]
>  slab_alloc mm/slab.c:3365 [inline]
>  __do_kmalloc mm/slab.c:3703 [inline]
>  __kmalloc+0x63/0x760 mm/slab.c:3714
> general protection fault: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
>    (ftrace buffer empty)
>  kmalloc include/linux/slab.h:517 [inline]
>  kzalloc include/linux/slab.h:701 [inline]
>  register_shrinker+0x10e/0x2d0 mm/vmscan.c:268
> Modules linked in:
> CPU: 1 PID: 4471 Comm: syzkaller129261 Not tainted 4.16.0-rc6+ #43
>  sget_userns+0xbbf/0xe40 fs/super.c:520
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:__list_del_entry_valid+0x7e/0x150 lib/list_debug.c:51
> RSP: 0018:ffff8801ae017658 EFLAGS: 00010246
> RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: 0000000000000000 RSI: ffff8801d97a6e98 RDI: ffff8801d97a6ea0
> RBP: ffff8801ae017670 R08: ffffffff81d39d22 R09: 0000000000000004
> R10: ffff8801ae017670 R11: 0000000000000000 R12: 0000000000000000
> R13: ffff8801d91dec00 R14: ffff8801ae017700 R15: ffff8801d97a6e98
> FS:  0000000001569880(0000) GS:ffff8801db100000(0000) knlGS:0000000000000000
>  kernfs_mount_ns+0x13d/0x8b0 fs/kernfs/mount.c:320
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000006d0188 CR3: 00000001da40c005 CR4: 00000000001606e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>  sysfs_mount+0xc2/0x1c0 fs/sysfs/mount.c:36
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  __list_del_entry include/linux/list.h:117 [inline]
>  list_del include/linux/list.h:125 [inline]
>  kernfs_kill_sb+0x9e/0x330 fs/kernfs/mount.c:361
>  mount_fs+0x66/0x2d0 fs/super.c:1222
>  vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
>  sysfs_kill_sb+0x22/0x40 fs/sysfs/mount.c:50
>  vfs_kern_mount fs/namespace.c:2509 [inline]
>  do_new_mount fs/namespace.c:2512 [inline]
>  do_mount+0xea4/0x2bb0 fs/namespace.c:2842
>  deactivate_locked_super+0x88/0xd0 fs/super.c:312
>  sget_userns+0xbda/0xe40 fs/super.c:522
>  SYSC_mount fs/namespace.c:3058 [inline]
>  SyS_mount+0xab/0x120 fs/namespace.c:3035
>  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
>  kernfs_mount_ns+0x13d/0x8b0 fs/kernfs/mount.c:320
>  sysfs_mount+0xc2/0x1c0 fs/sysfs/mount.c:36
>  mount_fs+0x66/0x2d0 fs/super.c:1222
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
>  vfs_kern_mount.part.26+0xc6/0x4a0 fs/namespace.c:1037
> RIP: 0033:0x442609
> RSP: 002b:00007fff40a278e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000442609
> RDX: 0000000020000140 RSI: 0000000020000040 RDI: 0000000020000000
> RBP: 00007fff40a28190 R08: 00000000200002c0 R09: 0000000300000000
>  vfs_kern_mount fs/namespace.c:2509 [inline]
>  do_new_mount fs/namespace.c:2512 [inline]
>  do_mount+0xea4/0x2bb0 fs/namespace.c:2842
> R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffffff
> R13: 0000000000000003 R14: 0000000000001380 R15: 00007fff40a27a28
>  SYSC_mount fs/namespace.c:3058 [inline]
>  SyS_mount+0xab/0x120 fs/namespace.c:3035
>  do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287
>  entry_SYSCALL_64_after_hwframe+0x42/0xb7
> RIP: 0033:0x442609
> RSP: 002b:00007fff40a278e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000442609
> RDX: 0000000020000140 RSI: 0000000020000040 RDI: 0000000020000000
> RBP: 00007fff40a28190 R08: 00000000200002c0 R09: 0000000300000000
> R10: 0000000000000000 R11: 0000000000000246 R12: ffffffffffffffff
> R13: 0000000000000003 R14: 0000000000001380 R15: 00007fff40a27a28
> Code: 00 00 00 00 ad de 49 39 c4 74 66 48 b8 00 02 00 00 00 00 ad de 48 89 da 48 39 c3 74 65 48 c1 ea 03 48 b8 00 00 00 00 00 fc ff df <80> 3c 02 00 75 7b 48 8b 13 48 39 f2 75 57 49 8d 7c 24 08 48 b8
> RIP: __list_del_entry_valid+0x7e/0x150 lib/list_debug.c:51 RSP: ffff8801ae017658
> ---[ end trace b14d521943ecadbd ]---
> 
> 
> ---
> This bug is generated by a dumb bot. It may contain errors.
> See https://goo.gl/tpsmEJ for details.
> Direct all questions to syzkaller@googlegroups.com.
> 
> syzbot will keep track of this bug report.
> If you forgot to add the Reported-by tag, once the fix for this bug is merged
> into any tree, please reply to this email with:
> #syz fix: exact-commit-title
> If you want to test a patch for this bug, please reply with:
> #syz test: git://repo/address.git branch
> and provide the patch inline or as an attachment.
> To mark this as a duplicate of another syzbot report, please reply with:
> #syz dup: exact-subject-of-another-report
> If it's a one-off invalid bug report, please reply with:
> #syz invalid
> Note: if the crash happens again, it will cause creation of a new bug report.
> Note: all commands must start from beginning of the line in the email body.
>