linux-kernel - Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed

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Message-ID: <90405c43-daca-48e4-b424-d66d6bf4dd87@citrix.com>
Date: Thu, 11 Apr 2024 11:06:37 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc: linux-kernel@...r.kernel.org,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Thomas Gleixner <tglx@...utronix.de>,
 Alexandre Chartre <alexandre.chartre@...cle.com>,
 Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Sean Christopherson <seanjc@...gle.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Nikolay Borisov <nik.borisov@...e.com>, KP Singh <kpsingh@...nel.org>,
 Waiman Long <longman@...hat.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed

On 11/04/2024 6:40 am, Josh Poimboeuf wrote:
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index 6de50b80702e..80d432d2fe44 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -39,6 +39,28 @@
>  
>  #ifdef CONFIG_X86_64
>  
> +/*
> + * Do either a direct or an indirect call, depending on whether indirect calls
> + * are considered safe.
> + */
> +#define __do_syscall(table, func_direct, nr, regs)			\
> +({									\
> +	unsigned long __rax, __rdi, __rsi;				\
> +									\
> +	asm_inline volatile(						\
> +		ALTERNATIVE("call " __stringify(func_direct) "\n\t",	\
> +			    ANNOTATE_RETPOLINE_SAFE			\
> +			    "call *%[func_ptr]\n\t",			\

This wants to be a plain maybe-thunk'd indirect call, and without the
ANNOTATE_RETPOLINE_SAFE.

Or you're going to get into cases where some combinations of command
line options do unexpected things e.g. retpolining everything except the
syscall dispatch.

~Andrew