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Date: Thu, 29 Mar 2018 23:56:35 +0300
From: Alexander Popov <alex.popov@...ux.com>
To: Boris Lukashev <blukashev@...pervictus.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
kernel-hardening@...ts.openwall.com, Kees Cook <keescook@...omium.org>,
PaX Team <pageexec@...email.hu>, Brad Spengler <spender@...ecurity.net>,
Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
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Richard Sandiford <richard.sandiford@....com>,
Thomas Gleixner <tglx@...utronix.de>, "H . Peter Anvin" <hpa@...or.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>, "Dmitry V . Levin"
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Florian Weimer <fweimer@...hat.com>,
Boris Lukashev <blukashev@...pervictus.com>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel
stack at the end of syscalls
Hello Dave and Boris,
Thanks for your replies!
On 29.03.2018 18:09, Boris Lukashev wrote:
> On Thu, Mar 29, 2018 at 9:51 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>> On 03/28/2018 11:58 PM, Alexander Popov wrote:
>>>> I noticed the 64-bit version saves/restores registers while
>>>> the 32-bit version doesn't. What's the reasoning there?
>>> When erase_kstack() is called from the trampoline stack, it must save and
>>> restore any modified registers, since all registers except RDI are live
>>> (prepared for the userspace).
>>>
>>> When erase_kstack() is called from the thread stack, it can clobber registers
>>> according the function call convention without any harm.
>>
>> Oh, and since there's no 32-bit trampoline stack, we don't need it on
>> 32-bit?
>>
>> If end up reposting this set again,
Hope so. Let's see...
>> could you add a few comments about
>> this around the ERASE_KSTACK macro definitions, or perhaps the call
>> sites? You might even want to call them ERASE_KSTACK_CLOBBER (for
>> 32-bit) and ERASE_KSTACK_NOCLOBBER (for 64-bit) to make this more clear.
>
> Not sure if the macro name differentiation is such a good idea, might
> entice improper use attempts.
> A more detailed explanation of this should probably go into the
> headers and doc/commit log for future implementation on architectures
> which may have their own weird semantics around the trampoline
> stack/not have one.
Ok, I see. Let me give the overview and propose the solution.
The current version has 3 separate ERASE_KSTACK definitions:
1. a simple one in entry_32.S, used only in that file:
+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+.endm
2. another one saving registers in entry_64.S, used only in that file for
erasing from the trampoline stack:
+.macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ PUSH_AND_CLEAR_REGS
+ call erase_kstack
+ POP_REGS
+#endif
+.endm
The call sights are already prepared and documented by Andy Lutomirski:
/*
* We are on the trampoline stack. All regs except RDI are live.
* We can do future final exit work right here.
*/
+ ERASE_KSTACK
3. a simple one in entry_64_compat.S (similar to case 1), used only in that file:
+ .macro ERASE_KSTACK
+#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
+ call erase_kstack
+#endif
+ .endm
+
The call sight is documented as well:
sysret32_from_system_call:
+ /*
+ * We are not going to return to the userspace from the trampoline
+ * stack. So let's erase the thread stack right now.
+ */
+ ERASE_KSTACK
If STACKLEAK is not banned, would you like me to introduce ERASE_KSTACK (for
cases 1 and 3) and ERASE_KSTACK_NOCLOBBER (for case 2) in
arch/x86/entry/calling.h?
Best regards,
Alexander
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