Date:   Mon, 10 Apr 2017 14:07:56 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>,
        linux-fsdevel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: net: use-after-free in __ns_get_path

On Mon, Apr 10, 2017 at 7:37 AM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
> Hi,
>
> I've got the following error report while fuzzing the kernel with syzkaller.
>
> On commit 39da7c509acff13fc8cb12ec1bb20337c988ed36 (4.11-rc6).
>
> Unfortunately it's not reproducible.
>
> ==================================================================
> BUG: KASAN: use-after-free in __read_once_size
> include/linux/compiler.h:254 [inline] at addr ffff880059ce6590
> BUG: KASAN: use-after-free in atomic_read
> arch/x86/include/asm/atomic.h:26 [inline] at addr ffff880059ce6590
> BUG: KASAN: use-after-free in virt_spin_lock
> arch/x86/include/asm/qspinlock.h:62 [inline] at addr ffff880059ce6590
> BUG: KASAN: use-after-free in queued_spin_lock_slowpath+0xb0a/0xfd0
> kernel/locking/qspinlock.c:421 at addr ffff880059ce6590
> Read of size 4 by task syz-executor6/567
> CPU: 1 PID: 567 Comm: syz-executor6 Not tainted 4.11.0-rc6+ #206
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Call Trace:
>  __dump_stack lib/dump_stack.c:16 [inline]
>  dump_stack+0x292/0x398 lib/dump_stack.c:52
>  kasan_object_err+0x1c/0x70 mm/kasan/report.c:164
>  print_address_description mm/kasan/report.c:202 [inline]
>  kasan_report_error mm/kasan/report.c:291 [inline]
>  kasan_report+0x252/0x510 mm/kasan/report.c:347
>  __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:367
>  __read_once_size include/linux/compiler.h:254 [inline]
>  atomic_read arch/x86/include/asm/atomic.h:26 [inline]
>  virt_spin_lock arch/x86/include/asm/qspinlock.h:62 [inline]
>  queued_spin_lock_slowpath+0xb0a/0xfd0 kernel/locking/qspinlock.c:421
>  queued_spin_lock include/asm-generic/qspinlock.h:103 [inline]
>  do_raw_spin_lock+0x151/0x1e0 kernel/locking/spinlock_debug.c:113
>  __raw_spin_lock include/linux/spinlock_api_smp.h:143 [inline]
>  _raw_spin_lock+0x32/0x40 kernel/locking/spinlock.c:151
>  spin_lock include/linux/spinlock.h:299 [inline]
>  lockref_get_not_dead+0x19/0x80 lib/lockref.c:179
>  __ns_get_path+0x197/0x860 fs/nsfs.c:66

This is almost not related to net, it is almost a pure fs bug.
So let's Cc Al.

I think the following one-line fix should work, since
ns->stashed is protected by RCU on the fast path, but
I am not brave enough to add a new dcache API for it. ;)

diff --git a/fs/nsfs.c b/fs/nsfs.c
index 1656843..ecfd3d1 100644
--- a/fs/nsfs.c
+++ b/fs/nsfs.c
@@ -90,6 +90,7 @@ static void *__ns_get_path(struct path *path, struct
ns_common *ns)
                iput(inode);
                return ERR_PTR(-ENOMEM);
        }
+       dentry->d_flags |= DCACHE_RCUACCESS;
        d_instantiate(dentry, inode);
        dentry->d_fsdata = (void *)ns->ops;
        d = atomic_long_cmpxchg(&ns->stashed, 0, (unsigned long)dentry);



>  open_related_ns+0xda/0x200 fs/nsfs.c:143
>  sock_ioctl+0x39d/0x440 net/socket.c:1001
>  vfs_ioctl fs/ioctl.c:45 [inline]
>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
>  SYSC_ioctl fs/ioctl.c:700 [inline]
>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
>  entry_SYSCALL_64_fastpath+0x1f/0xc2
> RIP: 0033:0x4458d9
> RSP: 002b:00007f1c9259eb58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000016 RCX: 00000000004458d9
> RDX: 000000002090affc RSI: 000000000000894c RDI: 0000000000000016
> RBP: 00000000006e1c50 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000708150
> R13: 0000000000000000 R14: 00007f1c9259f9c0 R15: 00007f1c9259f700
> Object at ffff880059ce6510, in cache dentry size: 288
> Allocated:
> PID = 565
>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:513
>  set_track mm/kasan/kasan.c:525 [inline]
>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:616
>  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:555
>  slab_post_alloc_hook mm/slab.h:456 [inline]
>  slab_alloc_node mm/slub.c:2718 [inline]
>  slab_alloc mm/slub.c:2726 [inline]
>  kmem_cache_alloc+0x1af/0x250 mm/slub.c:2731
>  __d_alloc+0xb3/0xbd0 fs/dcache.c:1571
>  d_alloc_pseudo+0x1d/0x30 fs/dcache.c:1692
>  __ns_get_path+0x3e8/0x860 fs/nsfs.c:88
>  open_related_ns+0xda/0x200 fs/nsfs.c:143
>  sock_ioctl+0x39d/0x440 net/socket.c:1001
>  vfs_ioctl fs/ioctl.c:45 [inline]
>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:685
>  SYSC_ioctl fs/ioctl.c:700 [inline]
>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
>  entry_SYSCALL_64_fastpath+0x1f/0xc2
> Freed:
> PID = 566
>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>  save_stack+0x43/0xd0 mm/kasan/kasan.c:513
>  set_track mm/kasan/kasan.c:525 [inline]
>  kasan_slab_free+0x73/0xc0 mm/kasan/kasan.c:589
>  slab_free_hook mm/slub.c:1357 [inline]
>  slab_free_freelist_hook mm/slub.c:1379 [inline]
>  slab_free mm/slub.c:2961 [inline]
>  kmem_cache_free+0xb2/0x2c0 mm/slub.c:2983
>  __d_free fs/dcache.c:265 [inline]
>  dentry_free+0xd5/0x150 fs/dcache.c:314
>  __dentry_kill+0x485/0x6e0 fs/dcache.c:552
>  dentry_kill fs/dcache.c:579 [inline]
>  dput.part.25+0x5cd/0x7c0 fs/dcache.c:791
>  dput+0x1f/0x30 fs/dcache.c:753
>  __fput+0x527/0x7f0 fs/file_table.c:227
>  ____fput+0x15/0x20 fs/file_table.c:245
>  task_work_run+0x19b/0x270 kernel/task_work.c:116
>  tracehook_notify_resume include/linux/tracehook.h:191 [inline]
>  exit_to_usermode_loop+0x1c2/0x200 arch/x86/entry/common.c:161
>  prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
>  syscall_return_slowpath+0x3d3/0x420 arch/x86/entry/common.c:260
>  entry_SYSCALL_64_fastpath+0xc0/0xc2
> Memory state around the buggy address:
>  ffff880059ce6480: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>  ffff880059ce6500: fc fc fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>ffff880059ce6580: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>                          ^
>  ffff880059ce6600: fb fb fb fb fb fb fc fc fc fc fc fc fc fc fc fc
>  ffff880059ce6680: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ==================================================================

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