linux-kernel - Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

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Date:   Tue, 23 Jun 2020 16:53:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <Thomas.Lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <JGross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP
 from userspace)

On Tue, Jun 23, 2020 at 03:59:16PM +0200, Peter Zijlstra wrote:

> So basically when your exception frame points to your own IST, you die.
> That sounds like something we should have in generic IST code.

Something like this... #DF already dies and NMI is 'magic'

---
 arch/x86/entry/common.c         |  7 +++++++
 arch/x86/include/asm/idtentry.h | 12 +++++++++++-
 2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index af0d57ed5e69..e38e4f34c90c 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -742,6 +742,13 @@ noinstr void idtentry_exit_nmi(struct pt_regs *regs, bool restore)
 	__nmi_exit();
 }
 
+noinstr void idtentry_validate_ist(struct pt_regs *regs)
+{
+	if ((regs->sp & ~(EXCEPTION_STKSZ-1)) ==
+	    (_RET_IP_ & ~(EXCEPTION_STKSZ-1)))
+		die("IST stack recursion", regs, 0);
+}
+
 #ifdef CONFIG_XEN_PV
 #ifndef CONFIG_PREEMPTION
 /*
diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 4e399f120ff8..974c1a4eacbb 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -19,6 +19,8 @@ void idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit);
 bool idtentry_enter_nmi(struct pt_regs *regs);
 void idtentry_exit_nmi(struct pt_regs *regs, bool irq_state);
 
+void idtentry_validate_ist(struct pt_regs *regs);
+
 /**
  * DECLARE_IDTENTRY - Declare functions for simple IDT entry points
  *		      No error code pushed by hardware
@@ -322,7 +324,15 @@ static __always_inline void __##func(struct pt_regs *regs)
  * Maps to DEFINE_IDTENTRY_RAW
  */
 #define DEFINE_IDTENTRY_IST(func)					\
-	DEFINE_IDTENTRY_RAW(func)
+static __always_inline void __##func(struct pt_regs *regs);		\
+									\
+__visible noinstr void func(struct pt_regs *regs)			\
+{									\
+	idtentry_validate_ist(regs);					\
+	__##func(regs);							\
+}									\
+									\
+static __always_inline void __##func(struct pt_regs *regs)
 
 /**
  * DEFINE_IDTENTRY_NOIST - Emit code for NOIST entry points which