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

[Thomas Gleixner]

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.

Thanks,

	tglx
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