Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled

From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>, X86 ML <x86@kernel.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Balbir Singh <bsingharora@gmail.com>,
	Borislav Petkov <bp@alien8.de>,
	Cyrill Gorcunov <gorcunov@gmail.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Eugene Syromiatnikov <esyr@redhat.com>,
	Florian Weimer <fweimer@redhat.com>,
	"H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Kees Cook <keescook@chromium.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>,
	Weijiang Yang <weijiang.yang@intel.com>
Subject: Re: [PATCH v12 8/8] x86: Disallow vsyscall emulation when CET is enabled
Date: Wed, 23 Sep 2020 14:29:25 -0700	[thread overview]
Message-ID: <20200923212925.GC15101@linux.intel.com> (raw)
In-Reply-To: <CALCETrWssUxxfhPPJZgPOmpaQcf4o9qCe1j-P7yiPyZVV+O8ZQ@mail.gmail.com>

On Mon, Sep 21, 2020 at 04:48:25PM -0700, Andy Lutomirski wrote:
> On Mon, Sep 21, 2020 at 3:37 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c
> > b/arch/x86/entry/vsyscall/vsyscall_64.c
> > index 44c33103a955..0131c9f7f9c5 100644
> > --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> > +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> > @@ -38,6 +38,9 @@
> >  #include <asm/fixmap.h>
> >  #include <asm/traps.h>
> >  #include <asm/paravirt.h>
> > +#include <asm/fpu/xstate.h>
> > +#include <asm/fpu/types.h>
> > +#include <asm/fpu/internal.h>
> >
> >  #define CREATE_TRACE_POINTS
> >  #include "vsyscall_trace.h"
> > @@ -286,6 +289,32 @@ bool emulate_vsyscall(unsigned long error_code,
> >         /* Emulate a ret instruction. */
> >         regs->ip = caller;
> >         regs->sp += 8;
> > +
> > +       if (current->thread.cet.shstk_size ||
> > +           current->thread.cet.ibt_enabled) {
> > +               u64 r;
> > +
> > +               fpregs_lock();
> > +               if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > +                       __fpregs_load_activate();
> 
> Wouldn't this be nicer if you operated on the memory image, not the registers?
> 
> > +
> > +#ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
> > +               /* Fixup branch tracking */
> > +               if (current->thread.cet.ibt_enabled) {
> > +                       rdmsrl(MSR_IA32_U_CET, r);
> > +                       wrmsrl(MSR_IA32_U_CET, r & ~CET_WAIT_ENDBR);
> > +               }
> > +#endif
> 
> Seems reasonable on first glance.
> 
> > +
> > +#ifdef CONFIG_X86_INTEL_SHADOW_STACK_USER
> > +               /* Unwind shadow stack. */
> > +               if (current->thread.cet.shstk_size) {
> > +                       rdmsrl(MSR_IA32_PL3_SSP, r);
> > +                       wrmsrl(MSR_IA32_PL3_SSP, r + 8);
> > +               }
> > +#endif
> 
> What happens if the result is noncanonical?  A quick skim of the SDM
> didn't find anything.  This latter issue goes away if you operate on
> the memory image, though -- writing a bogus value is just fine, since
> the FP restore will handle it.

#GP, the SSP MSRs do canonical checks.