Date:	Thu, 2 Apr 2015 07:14:25 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH urgent v2] x86, asm: Disable opportunistic SYSRET if
 regs->flags has TF set

On Thu, Apr 2, 2015 at 6:37 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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...


Maybe we could reintroduce TIF_IRET for this purpose instead of
(ab)using TIF_NOTIFY_RESUME.  Then we would only do the opportunistic
check for those cases (ptrace, audit, exec, sigreturn, etc.), and skip
it for interrupts.

--
Brian Gerst
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