linux-kernel - Re: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

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Message-ID: <CALCETrVodX3DkoaNygYDA+U5neBoCk51m+dNUkkY7p8-9Ces_A@mail.gmail.com>
Date:	Wed, 3 Aug 2016 09:42:29 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Borislav Petkov <bp@...en8.de>
Cc:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/entry: Clarify the RF saving/restoring situation with SYSCALL/SYSRET

On Wed, Aug 3, 2016 at 3:19 AM, Borislav Petkov <bp@...en8.de> wrote:
> From: Borislav Petkov <bp@...e.de>
>
> Clarify why exactly RF cannot be restored properly by SYSRET to avoid
> confusion.
>
> No functionality change.
>
> Signed-off-by: Borislav Petkov <bp@...e.de>
> Cc: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/entry/entry_64.S | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 8956eae04c25..80ad6d0fe38b 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -288,11 +288,15 @@ return_from_SYSCALL_64:
>         jne     opportunistic_sysret_failed
>
>         /*
> -        * SYSRET can't restore RF.  SYSRET can restore TF, but unlike IRET,
> -        * restoring TF results in a trap from userspace immediately after
> -        * SYSRET.  This would cause an infinite loop whenever #DB happens
> -        * with register state that satisfies the opportunistic SYSRET
> -        * conditions.  For example, single-stepping this user code:
> +        * SYSCALL clears RF when it saves rFLAGS in R11 so SYSRET cannot

I would change "so" and "and" -- the CPU designers could have make
SYSRET restore RF, but they chose not to.

Other than that substitution:

Acked-by: Andy Lutomirski <luto@...nel.org>