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Reply-To: alex.popov@linux.com
Subject: Re: [PATCH RFC v10 2/6] x86/entry: Add STACKLEAK erasing the kernel
 stack at the end of syscalls
To: Dave Hansen <dave.hansen@linux.intel.com>,
 kernel-hardening@lists.openwall.com, Kees Cook <keescook@chromium.org>,
 PaX Team <pageexec@freemail.hu>, Brad Spengler <spender@grsecurity.net>,
 Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@kernel.org>,
 Tycho Andersen <tycho@tycho.ws>, Laura Abbott <labbott@redhat.com>,
 Mark Rutland <mark.rutland@arm.com>,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>, Borislav Petkov <bp@alien8.de>,
 Richard Sandiford <richard.sandiford@arm.com>,
 Thomas Gleixner <tglx@linutronix.de>, "H . Peter Anvin" <hpa@zytor.com>,
 Peter Zijlstra <a.p.zijlstra@chello.nl>, "Dmitry V . Levin"
 <ldv@altlinux.org>, Emese Revfy <re.emese@gmail.com>,
 Jonathan Corbet <corbet@lwn.net>, Andrey Ryabinin <aryabinin@virtuozzo.com>,
 "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
 Thomas Garnier <thgarnie@google.com>,
 Andrew Morton <akpm@linux-foundation.org>,
 Alexei Starovoitov <ast@kernel.org>, Josef Bacik <jbacik@fb.com>,
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 Ding Tianhong <dingtianhong@huawei.com>, David Woodhouse
 <dwmw@amazon.co.uk>, Josh Poimboeuf <jpoimboe@redhat.com>,
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References: <1522267032-6603-1-git-send-email-alex.popov@linux.com>
 <1522267032-6603-3-git-send-email-alex.popov@linux.com>
 <77fab6ea-0bbf-d68b-a38e-3e22d1ae6093@linux.intel.com>
From: Alexander Popov <alex.popov@linux.com>
Message-ID: <16a56461-6501-7eb9-e1a4-761c9eff9386@linux.com>
Date: Thu, 29 Mar 2018 09:58:32 +0300
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On 29.03.2018 01:55, Dave Hansen wrote:
> On 03/28/2018 12:57 PM, Alexander Popov wrote:
>> +.macro ERASE_KSTACK
>> +#ifdef CONFIG_GCC_PLUGIN_STACKLEAK
>> +	PUSH_AND_CLEAR_REGS
>> +	call erase_kstack
>> +	POP_REGS
>> +#endif
>> +.endm
> 
> Thanks again for the hard work to trim down the complexity of the
> assembly. 

Hello Dave, thanks!

> 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.

Best regards,
Alexander