[Xen-devel] [PATCH for-4.11] x86/traps: Fix error handling of the pv %dr7 shadow state

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s "x86/pv: Introduce and use x86emul_write_dr()" fixed a bug with IO shadow
handling, in that it remained stale and visible until %dr7.L/G got set again.

However, it neglected the -EPERM return inbetween these two hunks, introducing
a different bug in which a write to %dr7 which tries to set IO breakpoints
without %cr4.DE being set clobbers the IO state, rather than leaves it alone.

Instead, move the zeroing slightly later, which guarentees that the shadow
gets written exactly once, on a successful update to %dr7.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>

Although minor, this is a regression from Xen 4.10, so should be considered
for 4.11 at this point.  Given that PV debugging was basically completely
broken before the XSA-263 investigation work, the risk of further issues is
very small.
---
 xen/arch/x86/traps.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 8a99174..e79ca88 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2123,9 +2123,6 @@ long set_debugreg(struct vcpu *v, unsigned int reg, 
unsigned long value)
         if ( value & DR_GENERAL_DETECT )
             return -EPERM;
 
-        /* Zero the IO shadow before recalculating the real %dr7 */
-        v->arch.debugreg[5] = 0;
-
         /* DR7.{G,L}E = 0 => debugging disabled for this domain. */
         if ( value & DR7_ACTIVE_MASK )
         {
@@ -2154,6 +2151,10 @@ long set_debugreg(struct vcpu *v, unsigned int reg, 
unsigned long value)
                  !(v->arch.debugreg[7] & DR7_ACTIVE_MASK) )
                 activate_debugregs(v);
         }
+        else
+            /* Zero the emulated controls if %dr7 isn't active. */
+            v->arch.debugreg[5] = 0;
+
         if ( v == curr )
             write_debugreg(7, value);
         break;
-- 
2.1.4


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