Linux Virtualization - Jul 2014 - [PATCH] x86, paravirt: BUG_ON on {rd,wr}msr exceptions

When CONFIG_PARAVIRT is enabled, the kernel is ignoring exceptions on
the {rd,wr}msr instructions. This makes serious issues (either on the
guest kernel, or on the host) be silently ignored, and is different from
the native MSR code (which does not ignore the exceptions).

As paravirt.h already includes linux/bug.h, I don't see what was the
original issue preventing BUG_ON from being used.

Change rdmsr(), wrmsr(), and rdmsrl() to BUG_ON() on errors.

Signed-off-by: Eduardo Habkost <ehabkost at redhat.com>
---
* Build-tested using allyesconfig, with no build errors.

* Tested by being able to detect the following host bug:
  https://bugzilla.redhat.com/show_bug.cgi?id=1025868
  (#GP exception on wrmsr(0x410, 0xfffffffffffffbff) during MCE
  initialization)
---
 arch/x86/include/asm/paravirt.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index cd6e161..7d42ca4 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -133,24 +133,27 @@ static inline int paravirt_write_msr(unsigned msr,
unsigned low, unsigned high)
 	return PVOP_CALL3(int, pv_cpu_ops.write_msr, msr, low, high);
 }
 
-/* These should all do BUG_ON(_err), but our headers are too tangled. */
 #define rdmsr(msr, val1, val2)			\
 do {						\
 	int _err;				\
 	u64 _l = paravirt_read_msr(msr, &_err);	\
+	BUG_ON(_err);				\
 	val1 = (u32)_l;				\
 	val2 = _l >> 32;			\
 } while (0)
 
-#define wrmsr(msr, val1, val2)			\
-do {						\
-	paravirt_write_msr(msr, val1, val2);	\
+#define wrmsr(msr, val1, val2)				\
+do {							\
+	int _err;					\
+	_err = paravirt_write_msr(msr, val1, val2);	\
+	BUG_ON(_err);					\
 } while (0)
 
 #define rdmsrl(msr, val)			\
 do {						\
 	int _err;				\
 	val = paravirt_read_msr(msr, &_err);	\
+	BUG_ON(_err);				\
 } while (0)
 
 #define wrmsrl(msr, val)	wrmsr(msr, (u32)((u64)(val)), ((u64)(val))>>32)
-- 
1.9.3

On 07/28/2014 12:04 PM, Eduardo Habkost wrote:
> When CONFIG_PARAVIRT is enabled, the kernel is ignoring exceptions on
> the {rd,wr}msr instructions. This makes serious issues (either on the
> guest kernel, or on the host) be silently ignored, and is different from
> the native MSR code (which does not ignore the exceptions).
> 
> As paravirt.h already includes linux/bug.h, I don't see what was the
> original issue preventing BUG_ON from being used.
> 
> Change rdmsr(), wrmsr(), and rdmsrl() to BUG_ON() on errors.
How much does this bloat the kernel?

#include <stdparavirtrant.h>

	-hpa

On Mon, Jul 28, 2014 at 12:18:10PM -0700, H. Peter Anvin
wrote:
> On 07/28/2014 12:04 PM, Eduardo Habkost wrote:
> > When CONFIG_PARAVIRT is enabled, the kernel is ignoring exceptions on
> > the {rd,wr}msr instructions. This makes serious issues (either on the
> > guest kernel, or on the host) be silently ignored, and is different
from
> > the native MSR code (which does not ignore the exceptions).
> > 
> > As paravirt.h already includes linux/bug.h, I don't see what was
the
> > original issue preventing BUG_ON from being used.
> > 
> > Change rdmsr(), wrmsr(), and rdmsrl() to BUG_ON() on errors.
> 
> How much does this bloat the kernel?
It seems to add 8 bytes to each {wr,rd}msr() call (4 extra instructions:
test, jmp, ud2, jmp).

allyesconfig, paravirt enabled, before:

  text    data     bss     dec     hex filename
108368312       23500872        55705600        187574784       b2e2a00 vmlinux

allyesconfig, paravirt enabled, after:

   text    data     bss     dec     hex filename
108384438       23500904        55717888        187603230       b2e991e vmlinux

allyesconfig vmlinux is 28446 bytes larger.

An alternative is to add read_msr_unsafe() & write_msr_unsafe() fields
to pv_cpu_ops, pointing to native_read_msr() & native_write_msr().

-- 
Eduardo