Re: [syzbot] upstream test error: KFENCE: use-after-free in kvm_fastop_exception

[Marco Elver <elver@google.com>]

On Fri, 17 Sept 2021 at 12:01, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Sat, 4 Sept 2021 at 20:58, syzbot
> <syzbot+d08efd12a2905a344291@syzkaller.appspotmail.com> wrote:
> >
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    835d31d319d9 Merge tag 'media/v5.15-1' of git://git.kernel..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=1189fe49300000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=d1a7a34dc082816f
> > dashboard link: https://syzkaller.appspot.com/bug?extid=d08efd12a2905a344291
> > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.1
> >
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+d08efd12a2905a344291@syzkaller.appspotmail.com
> >
> > ==================================================================
> > BUG: KFENCE: use-after-free read in kvm_fastop_exception+0xf6d/0x105b
> >
> > Use-after-free read at 0xffff88823bc0c020 (in kfence-#5):
> >  kvm_fastop_exception+0xf6d/0x105b
>
> There is probably some bug in d_lookup, but there is also something
> wrong with the unwinder. It prints an unrelated kvm_fastop_exception
> frame instead of __d_lookup and interestingly a very similar thing
> happens on arm64 with HWASAN and a similar bug in d_lookup. The
> corresponding report is:
> https://syzkaller.appspot.com/bug?extid=488ddf8087564d6de6e2
>
> BUG: KASAN: invalid-access in __entry_tramp_text_end+0xddc/0xd000
> CPU: 0 PID: 22 Comm: kdevtmpfs Not tainted
> 5.14.0-syzkaller-11152-g78e709522d2c #0
> Hardware name: linux,dummy-virt (DT)
> Call trace:
>  dump_backtrace+0x0/0x1ac arch/arm64/kernel/stacktrace.c:76
>  show_stack+0x18/0x24 arch/arm64/kernel/stacktrace.c:215
>  __dump_stack lib/dump_stack.c:88 [inline]
>  dump_stack_lvl+0x68/0x84 lib/dump_stack.c:106
>  print_address_description+0x7c/0x2b4 mm/kasan/report.c:256
>  __kasan_report mm/kasan/report.c:442 [inline]
>  kasan_report+0x134/0x380 mm/kasan/report.c:459
>  __do_kernel_fault+0x128/0x1bc arch/arm64/mm/fault.c:317
>  do_bad_area arch/arm64/mm/fault.c:466 [inline]
>  do_tag_check_fault+0x74/0x90 arch/arm64/mm/fault.c:737
>  do_mem_abort+0x44/0xb4 arch/arm64/mm/fault.c:813
>  el1_abort+0x40/0x60 arch/arm64/kernel/entry-common.c:357
>  el1h_64_sync_handler+0xb0/0xd0 arch/arm64/kernel/entry-common.c:408
>  el1h_64_sync+0x78/0x7c arch/arm64/kernel/entry.S:567
>  __entry_tramp_text_end+0xddc/0xd000
>  d_lookup+0x44/0x70 fs/dcache.c:2370
>  lookup_dcache+0x24/0x84 fs/namei.c:1520
>  __lookup_hash+0x24/0xd0 fs/namei.c:1543
>  kern_path_locked+0x90/0x10c fs/namei.c:2567
>  handle_remove+0x38/0x284 drivers/base/devtmpfs.c:312
>  handle drivers/base/devtmpfs.c:382 [inline]
>  devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
>  devtmpfsd+0x8c/0xd0 drivers/base/devtmpfs.c:437
>  kthread+0x150/0x15c kernel/kthread.c:319
>  ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:756
>
> Here kernel unwinder prints __entry_tramp_text_end instead of __d_lookup.
>
> I've looked in more detail into the arm64 case:
> d_lookup contains a static call to __d_lookup as expected:
>
> ffff8000102e0780 <d_lookup>:
> ...
> ffff8000102e07c0: 97ffffa4 bl ffff8000102e0650 <__d_lookup>
> ...
> ffff8000102e07e8: d65f03c0 ret
>
> and these symbols don't overlap or something:
>
> $ aarch64-linux-gnu-nm -nS vmlinux | egrep -C 1 " (t|T)
> (__entry_tramp_text|__d_lookup)"
> ffff8000102e01f0 0000000000000458 T d_alloc_parallel
> ffff8000102e0650 0000000000000128 T __d_lookup
> ffff8000102e0780 000000000000006c T d_lookup
> --
> ffff8000117a1f88 T __hibernate_exit_text_end
> ffff8000117a2000 T __entry_tramp_text_start
> ffff8000117a2000 00000000000007c8 T tramp_vectors
> --
> ffff8000117a27f0 0000000000000024 T tramp_exit_compat
> ffff8000117a3000 T __entry_tramp_text_end
> ffff8000117b0000 D _etext
>
> So it looks like in both cases the top fault frame is just wrong. But
> I would assume it's extracted by arch-dependent code, so it's
> suspicious that it affects both x86 and arm64...
>
> Any ideas what's happening?

My suspicion for the x86 case is that kvm_fastop_exception is related
to instruction emulation and the fault occurs in an emulated
instruction?

But I can't explain the arm64 case.

> >  d_lookup+0xd8/0x170 fs/dcache.c:2370
> >  lookup_dcache+0x1e/0x130 fs/namei.c:1520
> >  __lookup_hash+0x29/0x180 fs/namei.c:1543
> >  kern_path_locked+0x17e/0x320 fs/namei.c:2567
> >  handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
> >  handle drivers/base/devtmpfs.c:382 [inline]
> >  devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
> >  devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
> >  kthread+0x3e5/0x4d0 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> >
> > kfence-#5 [0xffff88823bc0c000-0xffff88823bc0cfff, size=4096, cache=names_cache] allocated by task 22:
> >  getname_kernel+0x4e/0x370 fs/namei.c:226
> >  kern_path_locked+0x71/0x320 fs/namei.c:2558
> >  handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
> >  handle drivers/base/devtmpfs.c:382 [inline]
> >  devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
> >  devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
> >  kthread+0x3e5/0x4d0 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> >
> > freed by task 22:
> >  putname.part.0+0xe1/0x120 fs/namei.c:270
> >  putname include/linux/err.h:41 [inline]
> >  filename_parentat fs/namei.c:2547 [inline]
> >  kern_path_locked+0xc2/0x320 fs/namei.c:2558
> >  handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
> >  handle drivers/base/devtmpfs.c:382 [inline]
> >  devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
> >  devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
> >  kthread+0x3e5/0x4d0 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> >
> > CPU: 1 PID: 22 Comm: kdevtmpfs Not tainted 5.14.0-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > RIP: 0010:kvm_fastop_exception+0xf6d/0x105b
> > Code: d3 ed e9 14 1b 6d f8 49 8d 0e 48 83 e1 f8 4c 8b 21 41 8d 0e 83 e1 07 c1 e1 03 49 d3 ec e9 6a 28 6d f8 49 8d 4d 00 48 83 e1 f8 <4c> 8b 21 41 8d 4d 00 83 e1 07 c1 e1 03 49 d3 ec e9 5a 32 6d f8 bd
> > RSP: 0018:ffffc90000fe7ae8 EFLAGS: 00010282
> > RAX: 0000000035736376 RBX: ffff88803b141cc0 RCX: ffff88823bc0c020
> > RDX: ffffed100762839f RSI: 0000000000000004 RDI: 0000000000000007
> > RBP: 0000000000000004 R08: 0000000000000000 R09: ffff88803b141cf0
> > R10: ffffed100762839e R11: 0000000000000000 R12: ffff88823bc0c020
> > R13: ffff88823bc0c020 R14: ffff88803b141cf0 R15: dffffc0000000000
> > FS:  0000000000000000(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: ffff88823bc0c020 CR3: 0000000029892000 CR4: 00000000001506e0
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> > Call Trace:
> >  d_lookup+0xd8/0x170 fs/dcache.c:2370
> >  lookup_dcache+0x1e/0x130 fs/namei.c:1520
> >  __lookup_hash+0x29/0x180 fs/namei.c:1543
> >  kern_path_locked+0x17e/0x320 fs/namei.c:2567
> >  handle_remove+0xa2/0x5fe drivers/base/devtmpfs.c:312
> >  handle drivers/base/devtmpfs.c:382 [inline]
> >  devtmpfs_work_loop drivers/base/devtmpfs.c:395 [inline]
> >  devtmpfsd+0x1b9/0x2a3 drivers/base/devtmpfs.c:437
> >  kthread+0x3e5/0x4d0 kernel/kthread.c:319
> >  ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> > ==================================================================
> > ----------------
> > Code disassembly (best guess):
> >    0:   d3 ed                   shr    %cl,%ebp
> >    2:   e9 14 1b 6d f8          jmpq   0xf86d1b1b
> >    7:   49 8d 0e                lea    (%r14),%rcx
> >    a:   48 83 e1 f8             and    $0xfffffffffffffff8,%rcx
> >    e:   4c 8b 21                mov    (%rcx),%r12
> >   11:   41 8d 0e                lea    (%r14),%ecx
> >   14:   83 e1 07                and    $0x7,%ecx
> >   17:   c1 e1 03                shl    $0x3,%ecx
> >   1a:   49 d3 ec                shr    %cl,%r12
> >   1d:   e9 6a 28 6d f8          jmpq   0xf86d288c
> >   22:   49 8d 4d 00             lea    0x0(%r13),%rcx
> >   26:   48 83 e1 f8             and    $0xfffffffffffffff8,%rcx
> > * 2a:   4c 8b 21                mov    (%rcx),%r12 <-- trapping instruction
> >   2d:   41 8d 4d 00             lea    0x0(%r13),%ecx
> >   31:   83 e1 07                and    $0x7,%ecx
> >   34:   c1 e1 03                shl    $0x3,%ecx
> >   37:   49 d3 ec                shr    %cl,%r12
> >   3a:   e9 5a 32 6d f8          jmpq   0xf86d3299
> >   3f:   bd                      .byte 0xbd
> >
> >
> > ---
> > This report is generated by a bot. It may contain errors.
> > See https://goo.gl/tpsmEJ for more information about syzbot.
> > syzbot engineers can be reached at syzkaller@googlegroups.com.
> >
> > syzbot will keep track of this issue. See:
> > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.