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

From: Dmitry Vyukov
Date: Fri Sep 17 2021 - 08:43:55 EST

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On Fri, 17 Sept 2021 at 13:04, Marco Elver <elver@xxxxxxxxxx> wrote:
> > On Sat, 4 Sept 2021 at 20:58, syzbot
> > <syzbot+d08efd12a2905a344291@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > ==================================================================
> > > 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?

Why would the kernel emulate a plain MOV?
2a: 4c 8b 21 mov (%rcx),%r12

And it would also mean a broken unwind because the emulated
instruction is in __d_lookup, so it should be in the stack trace.

> 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@xxxxxxxxxxxxxxxx.
> > >
> > > syzbot will keep track of this issue. See:
> > > https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

Next message: Sudeep Holla: "Re: linux-next: build warning after merge of the scmi tree"
Previous message: Jonas Dreßler: "Re: [PATCH] Bluetooth: btusb: Lower passive lescan interval on Marvell 88W8897"
In reply to: Marco Elver: "Re: [syzbot] upstream test error: KFENCE: use-after-free in kvm_fastop_exception"
Next in thread: Sean Christopherson: "Re: [syzbot] upstream test error: KFENCE: use-after-free in kvm_fastop_exception"
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