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

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Josh Poimboeuf <jpoimboe@kernel.org>, x86@kernel.org
Cc: linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Alexandre Chartre <alexandre.chartre@oracle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Sean Christopherson <seanjc@google.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Nikolay Borisov <nik.borisov@suse.com>,
	KP Singh <kpsingh@kernel.org>, Waiman Long <longman@redhat.com>,
	Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 5/7] x86/bugs: Only harden syscalls when needed
Date: Thu, 11 Apr 2024 11:06:37 +0100	[thread overview]
Message-ID: <90405c43-daca-48e4-b424-d66d6bf4dd87@citrix.com> (raw)
In-Reply-To: <97befd7c1e008797734dee05181c49056ff6de57.1712813475.git.jpoimboe@kernel.org>

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