Re: WARNING in wp_page_copy

[Magnus Karlsson]

On Mon, Dec 16, 2019 at 4:10 PM Magnus Karlsson
<magnus.karlsson@xxxxxxxxx> wrote:
>
> On Mon, Dec 16, 2019 at 4:00 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Dec 14, 2019 at 08:20:07AM -0800, syzbot wrote:
> > > syzbot has found a reproducer for the following crash on:
> > >
> > > HEAD commit:    1d1997db Revert "nfp: abm: fix memory leak in nfp_abm_u32_..
> > > git tree:       net-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=1029f851e00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=cef1fd5032faee91
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=9301f2f33873407d5b33
> > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=119d9fb1e00000
> > >
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+9301f2f33873407d5b33@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > Bjorn / Magnus, given xsk below, PTAL, thanks!
>
> Thanks. I will take a look at it right away.
>
> /Magnus

After looking through the syzcaller report, I have the following
hypothesis that would dearly need some comments from MM-savy people
out there. Syzcaller creates, using mmap, a memory area that is
write-only and supplies this to a getsockopt call (in this case
XDP_STATISTICS, but probably does not matter really) as the area where
it wants the values to be stored. When the getsockopt implementation
gets to copy_to_user() to write out the values to user space, it
encounters a page fault when accessing this write-only page. When
servicing this, it gets to the following piece of code that triggers
the warning that syzcaller reports:

static inline bool cow_user_page(struct page *dst, struct page *src,
                                 struct vm_fault *vmf)
{
....
snip
....
       /*
         * This really shouldn't fail, because the page is there
         * in the page tables. But it might just be unreadable,
         * in which case we just give up and fill the result with
         * zeroes.
         */
        if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) {
                /*
                 * Give a warn in case there can be some obscure
                 * use-case
                 */
                WARN_ON_ONCE(1);
                clear_page(kaddr);
        }

Before the following commit, it used to look like this:

commit 83d116c53058d505ddef051e90ab27f57015b025
Author: Jia He <justin.he@xxxxxxx>
Date:   Fri Oct 11 22:09:39 2019 +0800

    mm: fix double page fault on arm64 if PTE_AF is cleared

static inline bool cow_user_page(struct page *dst, struct page *src,
                                 struct vm_fault *vmf)
{
....
snip
....
                /*
                 * This really shouldn't fail, because the page is there
                 * in the page tables. But it might just be unreadable,
                 * in which case we just give up and fill the result with
                 * zeroes.
                 */
                if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
                        clear_page(kaddr);

So without a warning. My hypothesis is that if we create a page in the
same way as syzcaller then any getsockopt that does a copy_to_user()
(pretty much all of them I guess) will get this warning. I have not
tried this, so I might be wrong. If this is true, then the question is
what to do about it. One possible fix would be just to remove the
warning to get the same behavior as before. But it was probably put
there for a reason. Maybe some MM person could please comment on the
best way forward.


/Magnus

> > > ------------[ cut here ]------------
> > > WARNING: CPU: 0 PID: 9104 at mm/memory.c:2229 cow_user_page mm/memory.c:2229
> > > [inline]
> > > WARNING: CPU: 0 PID: 9104 at mm/memory.c:2229 wp_page_copy+0x10b7/0x1560
> > > mm/memory.c:2414
> > > Kernel panic - not syncing: panic_on_warn set ...
> > > CPU: 0 PID: 9104 Comm: syz-executor.0 Not tainted 5.5.0-rc1-syzkaller #0
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > Google 01/01/2011
> > > Call Trace:
> > >  __dump_stack lib/dump_stack.c:77 [inline]
> > >  dump_stack+0x197/0x210 lib/dump_stack.c:118
> > >  panic+0x2e3/0x75c kernel/panic.c:221
> > >  __warn.cold+0x2f/0x3e kernel/panic.c:582
> > >  report_bug+0x289/0x300 lib/bug.c:195
> > >  fixup_bug arch/x86/kernel/traps.c:174 [inline]
> > >  fixup_bug arch/x86/kernel/traps.c:169 [inline]
> > >  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
> > >  do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
> > >  invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
> > > RIP: 0010:cow_user_page mm/memory.c:2229 [inline]
> > > RIP: 0010:wp_page_copy+0x10b7/0x1560 mm/memory.c:2414
> > > Code: 4c 89 f7 ba 00 10 00 00 48 81 e6 00 f0 ff ff e8 0f e6 22 06 31 ff 41
> > > 89 c7 89 c6 e8 23 03 d3 ff 45 85 ff 74 0f e8 99 01 d3 ff <0f> 0b 4c 89 f7 e8
> > > 3f d8 22 06 e8 8a 01 d3 ff 65 4c 8b 34 25 c0 1e
> > > RSP: 0018:ffffc90002267668 EFLAGS: 00010293
> > > RAX: ffff8880a04c6140 RBX: ffffc90002267918 RCX: ffffffff81a22a0d
> > > RDX: 0000000000000000 RSI: ffffffff81a22a17 RDI: 0000000000000005
> > > RBP: ffffc900022677a8 R08: ffff8880a04c6140 R09: 0000000000000000
> > > R10: ffffed101125cfff R11: ffff8880892e7fff R12: ffff88809e403108
> > > R13: ffffea000224b9c0 R14: ffff8880892e7000 R15: 0000000000001000
> > >  do_wp_page+0x543/0x1540 mm/memory.c:2724
> > >  handle_pte_fault mm/memory.c:3961 [inline]
> > >  __handle_mm_fault+0x327b/0x3da0 mm/memory.c:4075
> > >  handle_mm_fault+0x3b2/0xa50 mm/memory.c:4112
> > >  do_user_addr_fault arch/x86/mm/fault.c:1441 [inline]
> > >  __do_page_fault+0x536/0xd80 arch/x86/mm/fault.c:1506
> > >  do_page_fault+0x38/0x590 arch/x86/mm/fault.c:1530
> > >  page_fault+0x39/0x40 arch/x86/entry/entry_64.S:1203
> > > RIP: 0010:copy_user_generic_unrolled+0x89/0xc0
> > > arch/x86/lib/copy_user_64.S:91
> > > Code: 38 4c 89 47 20 4c 89 4f 28 4c 89 57 30 4c 89 5f 38 48 8d 76 40 48 8d
> > > 7f 40 ff c9 75 b6 89 d1 83 e2 07 c1 e9 03 74 12 4c 8b 06 <4c> 89 07 48 8d 76
> > > 08 48 8d 7f 08 ff c9 75 ee 21 d2 74 10 89 d1 8a
> > > RSP: 0018:ffffc90002267bb8 EFLAGS: 00010206
> > > RAX: 0000000000000001 RBX: 0000000000000018 RCX: 0000000000000003
> > > RDX: 0000000000000000 RSI: ffffc90002267c58 RDI: 0000000020001300
> > > RBP: ffffc90002267bf0 R08: 0000000000000000 R09: fffff5200044cf8e
> > > R10: fffff5200044cf8d R11: ffffc90002267c6f R12: 0000000020001300
> > > R13: ffffc90002267c58 R14: 0000000020001318 R15: 00007ffffffff000
> > >  copy_to_user include/linux/uaccess.h:152 [inline]
> > >  xsk_getsockopt+0x575/0x6c0 net/xdp/xsk.c:898
> > >  __sys_getsockopt+0x16d/0x310 net/socket.c:2174
> > >  __do_sys_getsockopt net/socket.c:2189 [inline]
> > >  __se_sys_getsockopt net/socket.c:2186 [inline]
> > >  __x64_sys_getsockopt+0xbe/0x150 net/socket.c:2186
> > >  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> > >  entry_SYSCALL_64_after_hwframe+0x49/0xbe
> > > RIP: 0033:0x45a909
> > > Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 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
> > > 0f 83 7b b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
> > > RSP: 002b:00007f0ec9e9ec78 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
> > > RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 000000000045a909
> > > RDX: 0000000000000007 RSI: 000000000000011b RDI: 000000000000000a
> > > RBP: 000000000075bf20 R08: 0000000020000100 R09: 0000000000000000
> > > R10: 0000000020001300 R11: 0000000000000246 R12: 00007f0ec9e9f6d4
> > > R13: 00000000004c1ab5 R14: 00000000004d5f60 R15: 00000000ffffffff
> > > Kernel Offset: disabled
> > > Rebooting in 86400 seconds..
> > >