[Xen-changelog] [xen stable-4.12] x86/vtx: Fix fault semantics for early task switch failures

[patchbot@xxxxxxx]

commit 40aaf776a897e459876bd4766a194d30b2ae864a
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Dec 6 12:43:43 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Dec 6 12:43:43 2019 +0100

    x86/vtx: Fix fault semantics for early task switch failures
    
    The VT-x task switch handler adds inst_len to %rip before calling
    hvm_task_switch(), which is problematic in two ways:
    
     1) Early faults (i.e. ones delivered in the context of the old task) get
        delivered with trap semantics, and break restartibility.
    
     2) The addition isn't truncated to 32 bits.  In the corner case of a task
        switch instruction crossing the 4G->0 boundary taking an early fault 
(with
        trap semantics), a VMEntry failure will occur due to %rip being out of
        range.
    
    Instead, pass the instruction length into hvm_task_switch() and write it 
into
    the outgoing TSS only, leaving %rip in its original location.
    
    For now, pass 0 on the SVM side.  This highlights a separate preexisting bug
    which will be addressed in the following patch.
    
    While adjusting call sites, drop the unnecessary uint16_t cast.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
    master commit: 943c74bc0ee5044a826e428a3b2ffbdf9a43628d
    master date: 2019-11-28 17:14:38 +0000
---
 xen/arch/x86/hvm/hvm.c        | 4 ++--
 xen/arch/x86/hvm/svm/svm.c    | 2 +-
 xen/arch/x86/hvm/vmx/vmx.c    | 4 ++--
 xen/include/asm-x86/hvm/hvm.h | 2 +-
 4 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 2128f62243..c87dfe6155 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -2897,7 +2897,7 @@ void hvm_prepare_vm86_tss(struct vcpu *v, uint32_t base, 
uint32_t limit)
 
 void hvm_task_switch(
     uint16_t tss_sel, enum hvm_task_switch_reason taskswitch_reason,
-    int32_t errcode)
+    int32_t errcode, unsigned int insn_len)
 {
     struct vcpu *v = current;
     struct cpu_user_regs *regs = guest_cpu_user_regs();
@@ -2971,7 +2971,7 @@ void hvm_task_switch(
     if ( taskswitch_reason == TSW_iret )
         eflags &= ~X86_EFLAGS_NT;
 
-    tss.eip    = regs->eip;
+    tss.eip    = regs->eip + insn_len;
     tss.eflags = eflags;
     tss.eax    = regs->eax;
     tss.ecx    = regs->ecx;
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index bce2857f31..1ebf3cd7f9 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2890,7 +2890,7 @@ void svm_vmexit_handler(struct cpu_user_regs *regs)
          */
         vmcb->eventinj.bytes = 0;
 
-        hvm_task_switch((uint16_t)vmcb->exitinfo1, reason, errcode);
+        hvm_task_switch(vmcb->exitinfo1, reason, errcode, 0);
         break;
     }
 
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index bae7a6a62b..ad379fb26e 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -4062,8 +4062,8 @@ void vmx_vmexit_handler(struct cpu_user_regs *regs)
             __vmread(IDT_VECTORING_ERROR_CODE, &ecode);
         else
              ecode = -1;
-        regs->rip += inst_len;
-        hvm_task_switch((uint16_t)exit_qualification, reasons[source], ecode);
+
+        hvm_task_switch(exit_qualification, reasons[source], ecode, inst_len);
         break;
     }
     case EXIT_REASON_CPUID:
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index 53ffebb2c5..ad9fd45014 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -302,7 +302,7 @@ void hvm_set_rdtsc_exiting(struct domain *d, bool_t enable);
 enum hvm_task_switch_reason { TSW_jmp, TSW_iret, TSW_call_or_int };
 void hvm_task_switch(
     uint16_t tss_sel, enum hvm_task_switch_reason taskswitch_reason,
-    int32_t errcode);
+    int32_t errcode, unsigned int insn_len);
 
 enum hvm_access_type {
     hvm_access_insn_fetch,
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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