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From: Jan Beulich <jbeulich@suse.com>
Subject: x86/PV: writes of %fs and %gs base MSRs require canonical addresses

Commit c42494acb2 ("x86: fix FS/GS base handling when using the
fsgsbase feature") replaced the use of wrmsr_safe() on these paths
without recognizing that wr{f,g}sbase() use just wrmsrl() and that the
WR{F,G}SBASE instructions also raise #GP for non-canonical input.

Similarly arch_set_info_guest() needs to prevent non-canonical
addresses from getting stored into state later to be loaded by context
switch code. For consistency also check stack pointers and LDT base.
DR0..3, otoh, already get properly checked in set_debugreg() (albeit
we discard the error there).

The SHADOW_GS_BASE check isn't strictly necessary, but I think we
better avoid trying the WRMSR if we know it's going to fail.

This is XSA-193.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -890,7 +890,13 @@ int arch_set_info_guest(
     {
         if ( !compat )
         {
-            if ( !is_canonical_address(c.nat->user_regs.eip) ||
+            if ( !is_canonical_address(c.nat->user_regs.rip) ||
+                 !is_canonical_address(c.nat->user_regs.rsp) ||
+                 !is_canonical_address(c.nat->kernel_sp) ||
+                 (c.nat->ldt_ents && !is_canonical_address(c.nat->ldt_base)) ||
+                 !is_canonical_address(c.nat->fs_base) ||
+                 !is_canonical_address(c.nat->gs_base_kernel) ||
+                 !is_canonical_address(c.nat->gs_base_user) ||
                  !is_canonical_address(c.nat->event_callback_eip) ||
                  !is_canonical_address(c.nat->syscall_callback_eip) ||
                  !is_canonical_address(c.nat->failsafe_callback_eip) )
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2723,19 +2723,22 @@ static int emulate_privileged_op(struct
         switch ( regs->_ecx )
         {
         case MSR_FS_BASE:
-            if ( is_pv_32bit_domain(currd) )
+            if ( is_pv_32bit_domain(currd) ||
+                 !is_canonical_address(msr_content) )
                 goto fail;
             wrfsbase(msr_content);
             v->arch.pv_vcpu.fs_base = msr_content;
             break;
         case MSR_GS_BASE:
-            if ( is_pv_32bit_domain(currd) )
+            if ( is_pv_32bit_domain(currd) ||
+                 !is_canonical_address(msr_content) )
                 goto fail;
             wrgsbase(msr_content);
             v->arch.pv_vcpu.gs_base_kernel = msr_content;
             break;
         case MSR_SHADOW_GS_BASE:
-            if ( is_pv_32bit_domain(currd) )
+            if ( is_pv_32bit_domain(currd) ||
+                 !is_canonical_address(msr_content) )
                 goto fail;
             if ( wrmsr_safe(MSR_SHADOW_GS_BASE, msr_content) )
                 goto fail;