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Date: Fri, 9 Nov 2018 17:21:56 -0600
From: Kees Cook <keescook@...omium.org>
To: Alexander Popov <alex.popov@...ux.com>
Cc: Andy Lutomirski <luto@...capital.net>, Jann Horn <jannh@...gle.com>,
Joerg Roedel <joro@...tes.org>, Andy Lutomirski <luto@...nel.org>, Ingo Molnar <mingo@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>, LKP <lkp@...org>, kbuild test robot <lkp@...el.com>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, kernel list <linux-kernel@...r.kernel.org>,
Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: afaef01c00 ("x86/entry: Add STACKLEAK erasing the kernel stack
.."): double fault: 0000 [#1]
On Fri, Nov 9, 2018 at 5:09 PM, Alexander Popov <alex.popov@...ux.com> wrote:
>
> On 09.11.2018 23:46, Andy Lutomirski wrote:
>>> On Nov 9, 2018, at 12:06 PM, Jann Horn <jannh@...gle.com> wrote:
>>>
>>> +Andy, Thomas, Ingo
>>>
>>>> On Fri, Nov 9, 2018 at 2:24 PM kernel test robot <lkp@...el.com> wrote:
>>>> 0day kernel testing robot got the below dmesg and the first bad commit is
>>>>
>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
>>>>
>>>> commit afaef01c001537fa97a25092d7f54d764dc7d8c1
>>>> Author: Alexander Popov <alex.popov@...ux.com>
>>>> AuthorDate: Fri Aug 17 01:16:58 2018 +0300
>>>> Commit: Kees Cook <keescook@...omium.org>
>>>> CommitDate: Tue Sep 4 10:35:47 2018 -0700
>>>>
>>>> x86/entry: Add STACKLEAK erasing the kernel stack at the end of syscalls
>>> [...]
>>>> [ 127.808225] double fault: 0000 [#1]
>>>> [ 127.808695] CPU: 0 PID: 414 Comm: trinity-main Tainted: G T 4.19.0-rc2-00001-gafaef01 #1
>>>> [ 127.809799] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
>>>> [ 127.810760] RIP: 0010:ftrace_ops_test+0x27/0xa0
>>>> [ 127.811289] Code: eb 9a 90 41 54 55 49 89 f4 53 48 89 d3 48 89 fd 48 81 ec b0 00 00 00 65 48 8b 04 25 28 00 00 00 48 89 84 24 a8 00 00 00 31 c0 <e8> 54 df ff ff 48 85 db 74 57 e8 4a df ff ff 48 8b 85 d0 00 00 00
>>>> [ 127.813385] RSP: 0018:fffffe0000001fb8 EFLAGS: 00010046
>>> [...]
>>>> [ 127.819762] CR2: fffffe0000001fa8 CR3: 000000001579a000 CR4: 00000000000006b0
>>> [...]
>>>> [ 127.822234] Call Trace:
>>>> [ 127.822530] <ENTRY_TRAMPOLINE>
>>>> [ 127.822914] ? __ia32_sys_rseq+0x2f0/0x2f0
>>>> [ 127.823395] ftrace_ops_list_func+0xa5/0x1b0
>>>> [ 127.823922] ftrace_call+0x5/0x34
>>>> [ 127.824318] ? stackleak_erase+0x5/0xf0
>>>> [ 127.824789] ? stackleak_erase+0x43/0xf0
>>>> [ 127.825260] stackleak_erase+0x5/0xf0
>>>> [ 127.825699] syscall_return_via_sysret+0x61/0x81
>>>> [ 127.826238] WARNING: stack recursion on stack type 4
>>>> [ 127.826243] WARNING: can't dereference registers at (____ptrval____) for ip syscall_return_via_sysret+0x61/0x81
>>>> [ 127.826246] </ENTRY_TRAMPOLINE>
>>>> [ 127.828342] ---[ end trace e9f96d3f45575499 ]---
>>>> [ 127.828911] RIP: 0010:ftrace_ops_test+0x27/0xa0
>>>
>>> CR2: fffffe0000001fa8, RSP: 0018:fffffe0000001fb8; this is a pagefault
>>> on the stack. fffffe0000000000 is CPU_ENTRY_AREA_RO_IDT;
>>> fffffe0000001000 is CPU_ENTRY_AREA_PER_CPU; so fffffe0000002000 is the
>>> page with the entry stack for cpu 0, and you overflowed from that into
>>> the readonly gdt at fffffe0000001000, which doubles as a guard page
>>> for the entry stack:
>>>
>>> struct cpu_entry_area {
>>> char gdt[PAGE_SIZE];
>>>
>>> /*
>>> * The GDT is just below entry_stack and thus serves (on x86_64) as
>>> * a a read-only guard page.
>>> */
>>> struct entry_stack_page entry_stack_page;
>>> [...]
>>> };
>>>
>>> In other words: You're calling C code on the entry trampoline stack;
>>> this C code can call into ftrace; and the entry trampoline stack isn't
>>> big enough for ftrace shenanigans. I think you probably shouldn't be
>>> calling C code on the entry stack, but maybe one of the X86 folks has
>>> a different opinion?
>>
>> My opinion was that, on x86_32, the entry stack ought to be fairly large so
>> that NMIs could execute on the entry stack. I don’t remember what the code
>> actually does, though.
>>
>> But stackleak_erase should probably not run on the entry stack. That seems
>> like it’s just asking for trouble.
>
> Hello Jann and Andy,
>
>
> The stackleak_erase() function is called on the trampoline stack at the end of
> syscall, it erases the used part of the kernel thread stack after the syscall is
> handled.
>
>
> I've reproduced such a double fault with function tracing for stackleak_erase():
>
> # mount -t tracefs nodev /sys/kernel/tracing
> # echo 'p:myprobe stackleak_erase' > /sys/kernel/debug/tracing/kprobe_events
> # echo 1 > /sys/kernel/debug/tracing/events/kprobes/myprobe/enable
>
>
> I think we should simply not allow function tracing for stackleak_*() functions:
>
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 7343b3a..0906f6d 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -18,6 +18,7 @@ obj-$(CONFIG_MULTIUSER) += groups.o
> ifdef CONFIG_FUNCTION_TRACER
> # Do not trace internal ftrace files
> CFLAGS_REMOVE_irq_work.o = $(CC_FLAGS_FTRACE)
> +CFLAGS_REMOVE_stackleak.o = $(CC_FLAGS_FTRACE)
> endif
Yeah, that's what I was suspecting on IRC. This looks like the right
fix. Can you send that to me as a "regular" patch with changelog, etc,
and I'll send it up to Linus.
Reviewed-by: Kees Cook <keescook@...omium.org>
Thanks for everyone's attention on this! I've been travelling this
week, so I've been a little slow. :)
-Kees
>
>
> With this patch setting kprobe event for stackleak_erase() is not allowed. This
> is the corresponding dmesg output:
> [ 75.660478] trace_kprobe: Could not probe notrace function stackleak_erase
>
>
> If you agree, I'll prepare the patch for LKML.
>
> Best regards,
> Alexander
--
Kees Cook
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