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

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Message-ID: <20150402123159.GA25151@gmail.com>
Date:	Thu, 2 Apr 2015 14:31:59 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
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


* 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!

  - 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?

Thanks,

	Ingo
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