Messages in this thread | | | Date | Wed, 1 Oct 2014 00:21:55 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [PATCH 0/2] x86_64,entry: Clear NT on entry and speed up switch_to |
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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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