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From:   Jann Horn <jannh@google.com>
Date:   Fri, 9 Nov 2018 21:06:27 +0100
Message-ID: <CAG48ez2xSLFGjMYSYJTLwEnZoZJuXUfdoCp6_vGrywBU4SQNkQ@mail.gmail.com>
Subject: Re: afaef01c00 ("x86/entry: Add STACKLEAK erasing the kernel stack
 .."): double fault: 0000 [#1]
To:     alex.popov@linux.com, Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>
Cc:     lkp@01.org, lkp@intel.com,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-doc@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>
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+Andy, Thomas, Ingo

On Fri, Nov 9, 2018 at 2:24 PM kernel test robot <lkp@intel.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@linux.com>
> AuthorDate: Fri Aug 17 01:16:58 2018 +0300
> Commit:     Kees Cook <keescook@chromium.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?