Re: fs: use-after-free in path_lookupat

From: Dmitry Vyukov <dvyukov@google.com>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	syzkaller <syzkaller@googlegroups.com>
Subject: Re: fs: use-after-free in path_lookupat
Date: Sun, 5 Mar 2017 12:15:56 +0100	[thread overview]
Message-ID: <CACT4Y+Y23b8QhB8Hmasr5jXtwAwvvY=wVrorgRHzUWcdspURMg@mail.gmail.com> (raw)
In-Reply-To: <20170304193910.GG29622@ZenIV.linux.org.uk>

On Sat, Mar 4, 2017 at 8:39 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Sat, Mar 04, 2017 at 03:59:36PM +0100, Dmitry Vyukov wrote:
>
>> I am getting the following use-after-free reports while running
>> syzkaller fuzzer on 86292b33d4b79ee03e2f43ea0381ef85f077c760 (but also
>> happened on 6dc39c50e4aeb769c8ae06edf2b1a732f3490913 and
>> c82be9d2244aacea9851c86f4fb74694c99cd874).
>
> IOW, it's not fs/namei.c patches from this window...
>
>>  unlazy_walk+0xf2/0x4b0 fs/namei.c:692
>
> Could you post disassembly (e.g. from objdump -d) of your unlazy_walk()?
> For the kernel the trace is from...
>
>> r4 = memfd_create(&(0x7f0000013000)="2f6465762f6877726e6700", 0x0)
>> name_to_handle_at(r4, &(0x7f0000003000-0x6)="2e2f62757300",
>> &(0x7f0000003000-0xd)={0xc, 0x0, "cd21"}, &(0x7f0000002000)=0x0,
>> 0x1000)
>>
>> What's strange is that dirfd passed to name_to_handle_at is memfd
>> handle (sic). And path lookup somehow does not fail early on this.
>> Does it make any sense?
>
> It doesn't, but is that the triggering call of name_to_handle_at(), or do you
> have it called elsewhere?
>
> FWIW, no LOOKUP_ROOT in filename_lookup() flags + NULL root + dfd not
> equal to AT_FDCWD + non-empty name should've ended up in
>                         if (!d_can_lookup(dentry)) {
>                                 fdput(f);
>                                 return ERR_PTR(-ENOTDIR);
>                         }
> in path_init() and it shouldn't have progressed any further.  And in case
> of name_to_handle_at() we have user_path_at(dfd, name, lookup_flags, &path),
> i.e. user_path_at_empty(dfd, name, lookup_flags, &path, NULL), i.e.
> filename_lookup(dfd, getname_flags(name, lookup_flags, NULL), lookup_flags,
> &path, NULL).  IOW, filename_lookup() is called with root equal to NULL,
> dfd and name coming straight from userland and lookup_flags containing
> nothing beyond LOOKUP_EMPTY and LOOKUP_FOLLOW...

I probably messed something. Here is a new report on
0710f3ff91ecc4a715db6e4d0690472b13c4dac6. Line numbers seem to match
now.

BUG: KASAN: use-after-free in debug_spin_lock_before
kernel/locking/spinlock_debug.c:83 [inline] at addr ffff880059c2ace4
BUG: KASAN: use-after-free in do_raw_spin_lock+0x1bb/0x1f0
kernel/locking/spinlock_debug.c:112 at addr ffff880059c2ace4
Read of size 4 by task syz-executor/21853
CPU: 0 PID: 21853 Comm: syz-executor Not tainted 4.10.0+ #293
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+0x2fb/0x3fd lib/dump_stack.c:52
 kasan_object_err+0x1c/0x90 mm/kasan/report.c:166
 print_address_description mm/kasan/report.c:208 [inline]
 kasan_report_error mm/kasan/report.c:292 [inline]
 kasan_report.part.2+0x1b0/0x460 mm/kasan/report.c:314
 kasan_report mm/kasan/report.c:334 [inline]
 __asan_report_load4_noabort+0x29/0x30 mm/kasan/report.c:334
 debug_spin_lock_before kernel/locking/spinlock_debug.c:83 [inline]
 do_raw_spin_lock+0x1bb/0x1f0 kernel/locking/spinlock_debug.c:112
 __raw_spin_lock include/linux/spinlock_api_smp.h:143 [inline]
 _raw_spin_lock+0x3b/0x50 kernel/locking/spinlock.c:151
 spin_lock include/linux/spinlock.h:299 [inline]
 lockref_get_not_dead+0x19/0x80 lib/lockref.c:179
 legitimize_path.isra.36+0x7d/0x1a0 fs/namei.c:640
 unlazy_walk+0xf2/0x4b0 fs/namei.c:692
 complete_walk+0xb2/0x1f0 fs/namei.c:805
 path_lookupat+0x1c1/0x400 fs/namei.c:2275
 filename_lookup+0x282/0x540 fs/namei.c:2301
 user_path_at_empty+0x40/0x50 fs/namei.c:2555
 user_path_at include/linux/namei.h:55 [inline]
 SYSC_name_to_handle_at fs/fhandle.c:106 [inline]
 SyS_name_to_handle_at+0xff/0x720 fs/fhandle.c:92
 entry_SYSCALL_64_fastpath+0x1f/0xc2
RIP: 0033:0x4458d9
RSP: 002b:00007f462e243b58 EFLAGS: 00000286 ORIG_RAX: 000000000000012f
RAX: ffffffffffffffda RBX: 0000000000000051 RCX: 00000000004458d9
RDX: 0000000020002ff3 RSI: 0000000020002ffa RDI: 0000000000000051
RBP: 00000000006e11b0 R08: 0000000000001000 R09: 0000000000000000
R10: 0000000020002000 R11: 0000000000000286 R12: 0000000000708000
R13: 0000000020000000 R14: 0000000000013000 R15: 0000000000000003
Object at ffff880059c2ac60, in cache dentry size: 288
Allocated:
PID = 21878
 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+0xaa/0xd0 mm/kasan/kasan.c:616
 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:555
 kmem_cache_alloc+0x102/0x6e0 mm/slab.c:3572
 __d_alloc+0xb3/0xbb0 fs/dcache.c:1571
 d_alloc_pseudo+0x1d/0x30 fs/dcache.c:1692
 __shmem_file_setup+0x20c/0x5a0 mm/shmem.c:4157
 shmem_file_setup mm/shmem.c:4212 [inline]
 SYSC_memfd_create mm/shmem.c:3672 [inline]
 SyS_memfd_create+0x172/0x2c0 mm/shmem.c:3630
 entry_SYSCALL_64_fastpath+0x1f/0xc2
Freed:
PID = 21878
 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+0x6f/0xb0 mm/kasan/kasan.c:589
 __cache_free mm/slab.c:3514 [inline]
 kmem_cache_free+0x71/0x240 mm/slab.c:3774
 __d_free fs/dcache.c:265 [inline]
 dentry_free+0xd5/0x160 fs/dcache.c:314
 __dentry_kill+0x471/0x6d0 fs/dcache.c:552
 dentry_kill fs/dcache.c:579 [inline]
 dput.part.25+0x5ce/0x7c0 fs/dcache.c:791
 dput+0x1f/0x30 fs/dcache.c:753
 __fput+0x538/0x800 fs/file_table.c:227
 ____fput+0x15/0x20 fs/file_table.c:245
 task_work_run+0x197/0x260 kernel/task_work.c:116
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x23b/0x2a0 arch/x86/entry/common.c:161
 prepare_exit_to_usermode arch/x86/entry/common.c:191 [inline]
 syscall_return_slowpath+0x4d3/0x570 arch/x86/entry/common.c:260
 entry_SYSCALL_64_fastpath+0xc0/0xc2
Memory state around the buggy address:
 ffff880059c2ab80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff880059c2ac00: 00 00 00 00 fc fc fc fc fc fc fc fc fb fb fb fb
>ffff880059c2ac80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                       ^
 ffff880059c2ad00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
 ffff880059c2ad80: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
==================================================================