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 12:37:35 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Borislav Petkov <bp@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH urgent v2] x86, asm: Disable opportunistic SYSRET if
 regs->flags has TF set


* Denys Vlasenko <dvlasenk@...hat.com> wrote:

> On 04/02/2015 11:07 AM, Ingo Molnar wrote:
> > 
> > * Andy Lutomirski <luto@...nel.org> 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
> > 
> > So I merged this as it's an obvious bugfix, but in hindsight I'm 
> > really uneasy about the whole opportunistic SYSRET concept: it appears 
> > that the chance that %rcx matches return-%rip is astronomical - this 
> > is why this bug wasn't noticed live so far.
> > 
> > So should we really be doing this?
> 
> Andy does this not for the off-chance that userspace's RCX is equal 
> to return address and R11 == RFLAGS. The chances of that are 
> astronomically small.
> 
> This code path triggers when ptrace/audit/seccomp is active. Instead 
> of torturing ourselves trying to not divert into IRET return, now 
> code is steered that way. But then immediately before actual IRET, 
> we check again: "do we really need IRET?" IOW "did ptrace really 
> touch pt_regs->ss? ->flags? ->rip? ->rcx?" which in vast majority of 
> cases will not be true.

I keep forgetting about that, my test systems have the audit muck 
turned off ;-)

Fair enough - and it's sensible to share the IRET path between 
interrupts and complex-return system calls, even though the check
is unnecessary overhead for the pure interrupt return path...

Thanks,

	Ingo
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