x86/msr: Carry on after a non-"safe" MSR access fails

This demotes an OOPS and likely panic due to a failed non-"safe" MSR
access to a WARN_ONCE() and, for RDMSR, a return value of zero.

To be clear, this type of failure should *not* happen.  This patch
exists to minimize the chance of nasty undebuggable failures
happening when a CONFIG_PARAVIRT=y bug in the non-"safe" MSR helpers
gets fixed.

Tested-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: KVM list <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: xen-devel <Xen-devel@lists.xen.org>
Link: http://lkml.kernel.org/r/26567b216aae70e795938f4b567eace5a0eb90ba.1459605520.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 35 insertions, 2 deletions

diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
index 7a79ee2778b3..25f169c6eb95 100644
--- a/arch/x86/include/asm/msr.h
+++ b/arch/x86/include/asm/msr.h
@@ -84,7 +84,10 @@ static inline unsigned long long native_read_msr(unsigned int msr)
 {
         DECLARE_ARGS(val, low, high);
 
-        asm volatile("rdmsr" : EAX_EDX_RET(val, low, high) : "c" (msr));
+        asm volatile("1: rdmsr\n"
+                     "2:\n"
+                     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_rdmsr_unsafe)
+                     : EAX_EDX_RET(val, low, high) : "c" (msr));
         if (msr_tracepoint_active(__tracepoint_read_msr))
                 do_trace_read_msr(msr, EAX_EDX_VAL(val, low, high), 0);
         return EAX_EDX_VAL(val, low, high);
@@ -111,7 +114,10 @@ static inline unsigned long long native_read_msr_safe(unsigned int msr,
 static inline void native_write_msr(unsigned int msr,
                                     unsigned low, unsigned high)
 {
-        asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
+        asm volatile("1: wrmsr\n"
+                     "2:\n"
+                     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_wrmsr_unsafe)
+                     : : "c" (msr), "a"(low), "d" (high) : "memory");
         if (msr_tracepoint_active(__tracepoint_read_msr))
                 do_trace_write_msr(msr, ((u64)high << 32 | low), 0);
 }
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 061a23758354..fd9eb98c4f58 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -43,6 +43,33 @@ bool ex_handler_ext(const struct exception_table_entry *fixup,
 }
 EXPORT_SYMBOL(ex_handler_ext);
 
+bool ex_handler_rdmsr_unsafe(const struct exception_table_entry *fixup,
+                             struct pt_regs *regs, int trapnr)
+{
+        WARN_ONCE(1, "unchecked MSR access error: RDMSR from 0x%x\n",
+                  (unsigned int)regs->cx);
+
+        /* Pretend that the read succeeded and returned 0. */
+        regs->ip = ex_fixup_addr(fixup);
+        regs->ax = 0;
+        regs->dx = 0;
+        return true;
+}
+EXPORT_SYMBOL(ex_handler_rdmsr_unsafe);
+
+bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
+                             struct pt_regs *regs, int trapnr)
+{
+        WARN_ONCE(1, "unchecked MSR access error: WRMSR to 0x%x (tried to write 0x%08x%08x)\n",
+                  (unsigned int)regs->cx,
+                  (unsigned int)regs->dx, (unsigned int)regs->ax);
+
+        /* Pretend that the write succeeded. */
+        regs->ip = ex_fixup_addr(fixup);
+        return true;
+}
+EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
+
 bool ex_has_fault_handler(unsigned long ip)
 {
         const struct exception_table_entry *e;