Re: [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch_to

From: Andy Lutomirski
Date: Tue Sep 30 2014 - 18:30:32 EST

On Tue, Sep 30, 2014 at 3:21 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Tue, 30 Sep 2014, Andy Lutomirski wrote:
>> Anish Bhatt noticed that user programs can set RFLAGS.NT before
>> syscall or sysenter, and the kernel entry code doesn't filter out
>> NT. This causes kernel C code and, depending on thread flags, the
>> exit slow path to run with NT set.
>> The former is a little bit scary (imagine calling into EFI with NT
>> set), and the latter will fail with #GP and send a spurious SIGSEGV.
>> One answer would be "don't do that". But the kernel can do better
>> here.
>> These patches, which I'm not completely thrilled by, filter NT on
>> all kernel entries. For syscall (both bitnesses), this is free.
>> For sysenter, it costs 15 cycles or so. As a consolation prize, we
>> can speed up context switches by avoiding saving and restoring flags.
> That's a nice reason not to do any of the other ugly variants.

We could do something hideous:

Don't filter NT in sysexit or on context switch. Instead, handle it
in bad_iret.

Up side: all common cases are maximally fast.

Down side: Ugly. And malicious processes can leak NT, causing return
to a different process to fault, thereby adding a thousand or two
cycles (or possibly a lot more if the fault hits in the middle of
espfix64. Egads.)

This is not intended to be a serious suggestion...

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