linux-kernel - Re: [PATCH urgent v2] x86, asm: Disable opportunistic SYSRET if regs->flags has TF set

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Date:	Thu, 2 Apr 2015 08:21:07 +0200
From:	Borislav Petkov <bp@...e.de>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	Ingo Molnar <mingo@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH urgent v2] x86, asm: Disable opportunistic SYSRET if
 regs->flags has TF set

On Wed, Apr 01, 2015 at 02:26:34PM -0700, Andy Lutomirski wrote:
> When I wrote the opportunistic SYSRET code, I missed an important
> difference between SYSRET and IRET.  Both instructions are capable
> of setting EFLAGS.TF, but they behave differently when doing so.
> IRET will not issue a #DB trap after execution when it sets TF This
> is critical -- otherwise you'd never be able to make forward
> progress when returning to userspace.  SYSRET, on the other hand,
> will trap with #DB immediately after returning to CPL3, and the next
> instruction will never execute.
> 
> This breaks anything that opportunistically SYSRETs to a user
> context with TF set.  For example, running this code with TF set and
> a SIGTRAP handler loaded never gets past post_nop.
> 
> 	extern unsigned char post_nop[];
> 	asm volatile ("pushfq\n\t"
> 		      "popq %%r11\n\t"
> 		      "nop\n\t"
> 		      "post_nop:"
> 		      : : "c" (post_nop) : "r11");
> 
> In my defense, I can't find this documented in the AMD or Intel
> manual.
> 
> Fix it by using IRET to restore TF.
> 
> Fixes: 2a23c6b8a9c4 x86_64, entry: Use sysret to return to userspace when possible
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
> 
> This affects 4.0-rc as well as -tip.  A full test case lives here:
> 
> https://git.kernel.org/cgit/linux/kernel/git/luto/misc-tests.git/
> 
> It's called single_step_syscall_64.
> 
> On Intel systems, the 32-bit version of that test fails for unrelated
> reasons, but that's not a regression, and fixing it will be much more
> intrusive.
> 
> Changes from v1:
>  - Remove mention of testl from changelog.
>  - Improve comment per Denys' suggestion.
> 
> arch/x86/kernel/entry_64.S | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 750c6efcb718..537716380959 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -715,7 +715,21 @@ retint_swapgs:		/* return to user-space */
>  	cmpq %r11,EFLAGS(%rsp)		/* R11 == RFLAGS */
>  	jne opportunistic_sysret_failed
>  
> -	testq $X86_EFLAGS_RF,%r11	/* sysret can't restore RF */
> +	/*
> +	 * 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:
> +	 *
> +	 *           movq $stuck_here,%rcx
> +	 *           pushfq
> +	 *           popq %r11
> +	 *   stuck_here:
> +	 *
> +	 * would never get past stuck_here.
> +	 */
> +	testq $(X86_EFLAGS_RF|X86_EFLAGS_TF),%r11
>  	jnz opportunistic_sysret_failed
>  
>  	/* nothing to check for RSP */

Acked-by: Borislav Petkov <bp@...e.de>

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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