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

From: Denys Vlasenko
Date: Thu Apr 02 2015 - 08:24:46 EST

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.
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