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

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Message-ID: <551D3D2A.2040802@redhat.com>
Date:	Thu, 02 Apr 2015 14:59:22 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
CC:	Brian Gerst <brgerst@...il.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 04/02/2015 02:31 PM, Ingo Molnar wrote:
> 
> * Denys Vlasenko <dvlasenk@...hat.com> wrote:
> 
>> On 04/02/2015 01:14 PM, Brian Gerst wrote:
>>>>>> 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.
>>
>> The very first check in the existing code, pt_regs->cx == 
>> pt_regs->ip, will fail for interrupt returns.
>>
>> You hardly can save anything by placing a (ti->flags & 
>> TIF_TRY_SYSRET) check in front of it, it's almost as expensive.
> 
> Well, what I was thinking of was to have a pure irq (well, async 
> context) return path, not shared with the weird-syscall-IRET return 
> path at all ...
> 
> It would be open coded, not obfuscated via macros.
> 
> That way AFAICS the upsides are:
> 
>   - it's easier to read (and maintain) what goes on in which case.
>     '*intr*' labels would truly identify interrupt return related 
>     processing, for a change!

Re labels: I fully agree they need cleanup (mass rename).

Something along the lines of

int_ret_from_sys_call -> return_from_syscall
int_with_check   -> sysret_check_workmask_in_edi
int_careful      -> sysret_check_NEED_RESCHED
int_very_careful -> sysret_check_SYSCALL_EXIT
int_signal       -> sysret_check_DO_NOTIFY_MASK
int_restore_rest -> sysret_next_check

ret_from_intr    -> return_from_intr
retint_with_reschedule -> intr_check_WORK_MASK
retint_check     -> intr_check_workmask_in_edi
retint_careful   -> intr_check_NEED_RESCHED
retint_signal    -> intr_check_DO_NOTIFY_MASK

retint_swapgs    -> return_from_syscall_or_intr
irq_return_via_sysret -> return_via_sysret

retint_kernel    -> intr_check_preempt
restore_args     -> restore_c_regs
irq_return       -> return_via_iret


and then your proposal can be rephrased as "let's stop
merging sysret and intr code paths at retint_swapgs".

Makes sense. It would entail some code duplication,
but the code will be easier to maintain.

>   - we can optimize in a more directed fashion - like here
> 
> ... while the downsides are:
> 
>   - more code
>   - a (small) chance of a fix going to one path while not the other.
> 
> How much extra code would it be?

A screenful or two.
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