[Xen-devel] [PATCH] x86/hvm: Don't let hvm_set_efer() raise #GP behind the emulators back

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s 49de10f3c "x86/hvm: Don't raise #GP behind the emulators back for MSR
accesses" missed an edge case.

hvm_set_efer() raises #GP itself, so deliberately avoided the goto gp_fault
path in hvm_msr_write_intercept().

With the above change, guest updates to MSR_EFER which end up faulting raises
hvm_msr_write_intercept() returning X86EMUL_EXCEPTION.  The second #GP gets
combined to #DF and handed back to the guest.

Update hvm_set_efer() to avoid raising #GP, requiring its callers to do so.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
CC: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
---
 xen/arch/x86/hvm/hvm.c           | 2 --
 xen/arch/x86/hvm/svm/nestedsvm.c | 4 ++++
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 5372a9a..9eeb0a2 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -1989,7 +1989,6 @@ int hvm_set_efer(uint64_t value)
         printk(XENLOG_G_WARNING
                "%pv: Invalid EFER update: %#"PRIx64" -> %#"PRIx64" - %s\n",
                v, v->arch.hvm_vcpu.guest_efer, value, errstr);
-        hvm_inject_hw_exception(TRAP_gp_fault, 0);
         return X86EMUL_EXCEPTION;
     }
 
@@ -1998,7 +1997,6 @@ int hvm_set_efer(uint64_t value)
     {
         gdprintk(XENLOG_WARNING,
                  "Trying to change EFER.LME with paging enabled\n");
-        hvm_inject_hw_exception(TRAP_gp_fault, 0);
         return X86EMUL_EXCEPTION;
     }
 
diff --git a/xen/arch/x86/hvm/svm/nestedsvm.c b/xen/arch/x86/hvm/svm/nestedsvm.c
index ca11b96..5979b56 100644
--- a/xen/arch/x86/hvm/svm/nestedsvm.c
+++ b/xen/arch/x86/hvm/svm/nestedsvm.c
@@ -278,6 +278,8 @@ static int nsvm_vcpu_hostrestore(struct vcpu *v, struct 
cpu_user_regs *regs)
     /* EFER */
     v->arch.hvm_vcpu.guest_efer = n1vmcb->_efer;
     rc = hvm_set_efer(n1vmcb->_efer);
+    if ( rc == X86EMUL_EXCEPTION )
+        hvm_inject_hw_exception(TRAP_gp_fault, 0);
     if (rc != X86EMUL_OKAY)
         gdprintk(XENLOG_ERR, "hvm_set_efer failed, rc: %u\n", rc);
 
@@ -538,6 +540,8 @@ static int nsvm_vmcb_prepare4vmrun(struct vcpu *v, struct 
cpu_user_regs *regs)
     /* EFER */
     v->arch.hvm_vcpu.guest_efer = ns_vmcb->_efer;
     rc = hvm_set_efer(ns_vmcb->_efer);
+    if ( rc == X86EMUL_EXCEPTION )
+        hvm_inject_hw_exception(TRAP_gp_fault, 0);
     if (rc != X86EMUL_OKAY)
         gdprintk(XENLOG_ERR, "hvm_set_efer failed, rc: %u\n", rc);
 
-- 
2.1.4


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