linux-kernel - Re: [tip:x86/asm] x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 26 Feb 2015 18:33:47 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, Li Zefan <lizefan@...wei.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP

On Thu, Feb 26, 2015 at 6:21 PM, Wang Nan <wangnan0@...wei.com> wrote:
> On 2015/2/26 23:12, Andy Lutomirski wrote:
>> On Thu, Feb 26, 2015 at 5:12 AM, tip-bot for Wang Nan <tipbot@...or.com> wrote:
>>> Commit-ID:  b4d8327024637cb2a1f7910dcb5d0ad7a096f473
>>> Gitweb:     http://git.kernel.org/tip/b4d8327024637cb2a1f7910dcb5d0ad7a096f473
>>> Author:     Wang Nan <wangnan0@...wei.com>
>>> AuthorDate: Thu, 26 Feb 2015 13:49:39 +0800
>>> Committer:  Ingo Molnar <mingo@...nel.org>
>>> CommitDate: Thu, 26 Feb 2015 12:29:20 +0100
>>>
>>> x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP
>>>
>>> Before this patch early_trap_init() installs DEBUG_STACK for
>>> X86_TRAP_BP and X86_TRAP_DB. However, DEBUG_STACK doesn't work
>>> correctly until cpu_init() <-- trap_init().
>>>
>>> This patch passes 0 to set_intr_gate_ist() and
>>> set_system_intr_gate_ist() instead of DEBUG_STACK to let it use
>>> same stack as kernel, and installs DEBUG_STACK for them in
>>> trap_init().
>>>
>>
>> Sorry, I'm late to the party.  This patch, while technically correct
>> AFAICT, is really ugly, because the whole point is that it *doesn't*
>> use ist.  In other words:
>>
>>> +       set_intr_gate_ist(X86_TRAP_DB, &debug, 0);
>>
>> That should be set_intr_gate(X86_TRAP_DB, &debug);
>>
>
> I considered this, but found that set_intr_gate() and set_intr_gate_ist(..., 0)
> behaviors differently: set_intr_gate() -> _trace_set_gate() requires 'trace_##addr'
> to be installed to trace_idt_table, while set_intr_gate_ist(..., 0) puts 'addr'.
> into trace_idt_table() through _set_gate().
> Therefore, if we want to use set_intr_gate() we need to provide a trace_debug entry
> for it, which will be overwritten in trap_init() so it is in fact a useless symbol.

Yuck.  Then IMO we should have a separate helper for this.  Better
yet, we should rename set_intr_gate, because this distinction could
easily be overlooked.

>
> Anyway, I'd like to post a v3 patch follow your advise. Please keep an eye on it.
>
> Thank you!
>
>>>         /* int3 can be called from all */
>>> -       set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
>>> +       set_system_intr_gate_ist(X86_TRAP_BP, &int3, 0);
>>
>> Similarly, this should be set_system_gate.
>>
>>>  #ifdef CONFIG_X86_32
>>>         set_intr_gate(X86_TRAP_PF, page_fault);
>>>  #endif
>>> @@ -1005,6 +1013,15 @@ void __init trap_init(void)
>>>          */
>>>         cpu_init();
>>>
>>> +       /*
>>> +        * X86_TRAP_DB and X86_TRAP_BP have been set
>>> +        * in early_trap_init(). However, DEBUG_STACK works only after
>>> +        * cpu_init() loads TSS. See comments in early_trap_init().
>>
>> It's not that DEBUG_STACK only works after the TSS is loaded; it's
>> that the IST mechanism only works after TSS is loaded.
>>
>> --Andy
>>
>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/