linux-kernel - Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier

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Date:   Fri, 16 Mar 2018 15:18:23 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Michal Suchanek <msuchanek@...e.de>
Cc:     linuxppc-dev@...ts.ozlabs.org,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Neuling <mikey@...ling.org>,
        "Bryant G. Ly" <bryantly@...ux.vnet.ibm.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Daniel Axtens <dja@...ens.net>,
        Al Viro <viro@...iv.linux.org.uk>,
        David Gibson <david@...son.dropbear.id.au>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Cédric Le Goater <clg@...d.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Donnellan <andrew.donnellan@....ibm.com>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Joe Perches <joe@...ches.com>,
        Oliver O'Halloran <oohall@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Tobin C. Harding" <me@...in.cc>
Subject: Re: [PATCH RFC rebase 3/9] powerpc/64: Use barrier_nospec in
 syscall entry

On Thu, 15 Mar 2018 20:15:52 +0100
Michal Suchanek <msuchanek@...e.de> wrote:

> On powerpc syscall entry is done in assembly so patch in an explicit
> barrier_nospec.

Same comment as Linus for this -- the barriers are before the branch here,
so is it possible the branch instruction can be speculative while the index
is used to load the syscall table?

Thanks,
Nick

> 
> Signed-off-by: Michal Suchanek <msuchanek@...e.de>
> ---
>  arch/powerpc/kernel/entry_64.S | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/entry_64.S b/arch/powerpc/kernel/entry_64.S
> index 2cb5109a7ea3..7bfc4cf48af2 100644
> --- a/arch/powerpc/kernel/entry_64.S
> +++ b/arch/powerpc/kernel/entry_64.S
> @@ -36,6 +36,7 @@
>  #include <asm/context_tracking.h>
>  #include <asm/tm.h>
>  #include <asm/ppc-opcode.h>
> +#include <asm/barrier.h>
>  #include <asm/export.h>
>  #ifdef CONFIG_PPC_BOOK3S
>  #include <asm/exception-64s.h>
> @@ -159,6 +160,7 @@ system_call:			/* label this so stack traces look sane */
>  	andi.	r11,r10,_TIF_SYSCALL_DOTRACE
>  	bne	.Lsyscall_dotrace		/* does not return */
>  	cmpldi	0,r0,NR_syscalls
> +	barrier_nospec
>  	bge-	.Lsyscall_enosys
>  
>  .Lsyscall:
> @@ -319,6 +321,7 @@ END_FTR_SECTION_IFSET(CPU_FTR_HAS_PPR)
>  	ld	r10,TI_FLAGS(r10)
>  
>  	cmpldi	r0,NR_syscalls
> +	barrier_nospec
>  	blt+	.Lsyscall
>  
>  	/* Return code is already in r3 thanks to do_syscall_trace_enter() */