lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
Open Source and information security mailing list archives
| ||
|
Date: Fri, 21 Aug 2020 10:15:02 -0400 From: Brian Gerst <brgerst@...il.com> To: Paolo Bonzini <pbonzini@...hat.com> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, kvm list <kvm@...r.kernel.org>, "the arch/x86 maintainers" <x86@...nel.org>, Sean Christopherson <sean.j.christopherson@...el.com>, Dave Hansen <dave.hansen@...el.com>, Chang Seok Bae <chang.seok.bae@...el.com>, Peter Zijlstra <peterz@...radead.org>, Sasha Levin <sashal@...nel.org>, Tom Lendacky <thomas.lendacky@....com>, Andy Lutomirski <luto@...nel.org> Subject: Re: [PATCH v2] x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM On Fri, Aug 21, 2020 at 6:56 AM Paolo Bonzini <pbonzini@...hat.com> wrote: > > From: Sean Christopherson <sean.j.christopherson@...el.com> > > Don't use RDPID in the paranoid entry flow, as it can consume a KVM > guest's MSR_TSC_AUX value if an NMI arrives during KVM's run loop. > > In general, the kernel does not need TSC_AUX because it can just use > __this_cpu_read(cpu_number) to read the current processor id. It can > also just block preemption and thread migration at its will, therefore > it has no need for the atomic rdtsc+vgetcpu provided by RDTSCP. For this > reason, as a performance optimization, KVM loads the guest's TSC_AUX when > a CPU first enters its run loop. On AMD's SVM, it doesn't restore the > host's value until the CPU exits the run loop; VMX is even more aggressive > and defers restoring the host's value until the CPU returns to userspace. > > This optimization obviously relies on the kernel not consuming TSC_AUX, > which falls apart if an NMI arrives during the run loop and uses RDPID. > Removing it would be painful, as both SVM and VMX would need to context > switch the MSR on every VM-Enter (for a cost of 2x WRMSR), whereas using > LSL instead RDPID is a minor blip. > > Both SAVE_AND_SET_GSBASE and GET_PERCPU_BASE are only used in paranoid entry, > therefore the patch can just remove the RDPID alternative. > > Fixes: eaad981291ee3 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro") > Cc: Dave Hansen <dave.hansen@...el.com> > Cc: Chang Seok Bae <chang.seok.bae@...el.com> > Cc: Peter Zijlstra <peterz@...radead.org> > Cc: Sasha Levin <sashal@...nel.org> > Cc: Paolo Bonzini <pbonzini@...hat.com> > Cc: kvm@...r.kernel.org > Reported-by: Tom Lendacky <thomas.lendacky@....com> > Debugged-by: Tom Lendacky <thomas.lendacky@....com> > Suggested-by: Andy Lutomirski <luto@...nel.org> > Suggested-by: Peter Zijlstra <peterz@...radead.org> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com> > Signed-off-by: Paolo Bonzini <pbonzini@...hat.com> > --- > arch/x86/entry/calling.h | 10 ++++++---- > 1 file changed, 6 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h > index 98e4d8886f11..ae9b0d4615b3 100644 > --- a/arch/x86/entry/calling.h > +++ b/arch/x86/entry/calling.h > @@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with > * Fetch the per-CPU GSBASE value for this processor and put it in @reg. > * We normally use %gs for accessing per-CPU data, but we are setting up > * %gs here and obviously can not use %gs itself to access per-CPU data. > + * > + * Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and > + * may not restore the host's value until the CPU returns to userspace. > + * Thus the kernel would consume a guest's TSC_AUX if an NMI arrives > + * while running KVM's run loop. > */ > .macro GET_PERCPU_BASE reg:req > - ALTERNATIVE \ > - "LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \ > - "RDPID \reg", \ > - X86_FEATURE_RDPID > + LOAD_CPU_AND_NODE_SEG_LIMIT \reg > andq $VDSO_CPUNODE_MASK, \reg > movq __per_cpu_offset(, \reg, 8), \reg > .endm LOAD_CPU_AND_NODE_SEG_LIMIT can be merged into this, as its only purpose was to work around using CPP macros in an alternative. -- Brian Gerst
Powered by blists - more mailing lists