Subject: xen/hypercall: Don't use BUG() for parameter checking in hypercall_create_continuation()
From: Andrew Cooper <[email-masked]>

Since c/s 1d429034 "hypercall: update vcpu_op to take an unsigned vcpuid",
which incorrectly swapped 'i' for 'u' in the parameter type list, guests have
been able to hit the BUG() in next_args()'s default case.

Correct these back to 'i'.

In addition, make adjustments to prevent this class of issue from occurring in
the future - crashing Xen is not an appropriate form of parameter checking.

Capitalise NEXT_ARG() to catch all uses, to highlight that it is a macro doing
non-function-like things behind the scenes, and undef it when appropriate.
Implement a bad_fmt: block which prints an error, asserts unreachable, and
crashes the guest.

On the ARM side, drop all parameter checking of p.  It is asymmetric with the
x86 side, and akin to expecting memcpy() or sprintf() to check their src/fmt
parameter before use.  A caller passing "" or something other than a string
literal will be obvious during code review.

This is XSA-296.

Signed-off-by: Andrew Cooper <[email-masked]>
Acked-by: Julien Grall <[email-masked]>

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index d9e796dcbe..f6678d2227 100644
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -336,14 +336,15 @@ void sync_vcpu_execstate(struct vcpu *v)
     /* Nothing to do -- no lazy switching */
 }
 
-#define next_arg(fmt, args) ({                                              \
+#define NEXT_ARG(fmt, args)                                                 \
+({                                                                          \
     unsigned long __arg;                                                    \
     switch ( *(fmt)++ )                                                     \
     {                                                                       \
     case 'i': __arg = (unsigned long)va_arg(args, unsigned int);  break;    \
     case 'l': __arg = (unsigned long)va_arg(args, unsigned long); break;    \
     case 'h': __arg = (unsigned long)va_arg(args, void *);        break;    \
-    default:  __arg = 0; BUG();                                             \
+    default:  goto bad_fmt;                                                 \
     }                                                                       \
     __arg;                                                                  \
 })
@@ -373,9 +374,6 @@ unsigned long hypercall_create_continuation(
     unsigned int i;
     va_list args;
 
-    /* All hypercalls take at least one argument */
-    BUG_ON( !p || *p == '\0' );
-
     va_start(args, format);
 
     if ( mcs->flags & MCSF_in_multicall )
@@ -383,7 +381,7 @@ unsigned long hypercall_create_continuation(
         __set_bit(_MCSF_call_preempted, &mcs->flags);
 
         for ( i = 0; *p != '\0'; i++ )
-            mcs->call.args[i] = next_arg(p, args);
+            mcs->call.args[i] = NEXT_ARG(p, args);
 
         /* Return value gets written back to mcs->call.result */
         rc = mcs->call.result;
@@ -402,7 +400,7 @@ unsigned long hypercall_create_continuation(
 
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
 
                 switch ( i )
                 {
@@ -425,7 +423,7 @@ unsigned long hypercall_create_continuation(
 
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
 
                 switch ( i )
                 {
@@ -446,8 +444,16 @@ unsigned long hypercall_create_continuation(
     va_end(args);
 
     return rc;
+
+ bad_fmt:
+    gprintk(XENLOG_ERR, "Bad hypercall continuation format '%c'\n", *p);
+    ASSERT_UNREACHABLE();
+    domain_crash(current->domain);
+    return 0;
 }
 
+#undef NEXT_ARG
+
 void startup_cpu_idle_loop(void)
 {
     struct vcpu *v = current;
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index ddeb68f967..3946ea38fd 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -2403,14 +2403,15 @@ void sync_vcpu_execstate(struct vcpu *v)
     flush_tlb_mask(v->vcpu_dirty_cpumask);
 }
 
-#define next_arg(fmt, args) ({                                              \
+#define NEXT_ARG(fmt, args)                                                 \
+({                                                                          \
     unsigned long __arg;                                                    \
     switch ( *(fmt)++ )                                                     \
     {                                                                       \
     case 'i': __arg = (unsigned long)va_arg(args, unsigned int);  break;    \
     case 'l': __arg = (unsigned long)va_arg(args, unsigned long); break;    \
     case 'h': __arg = (unsigned long)va_arg(args, void *);        break;    \
-    default:  __arg = 0; BUG();                                             \
+    default:  goto bad_fmt;                                                 \
     }                                                                       \
     __arg;                                                                  \
 })
@@ -2449,7 +2450,7 @@ unsigned long hypercall_create_continuation(
         __set_bit(_MCSF_call_preempted, &mcs->flags);
 
         for ( i = 0; *p != '\0'; i++ )
-            mcs->call.args[i] = next_arg(p, args);
+            mcs->call.args[i] = NEXT_ARG(p, args);
     }
     else
     {
@@ -2470,7 +2471,7 @@ unsigned long hypercall_create_continuation(
         {
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
                 switch ( i )
                 {
                 case 0: regs->rdi = arg; break;
@@ -2486,7 +2487,7 @@ unsigned long hypercall_create_continuation(
         {
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
                 switch ( i )
                 {
                 case 0: regs->ebx = arg; break;
@@ -2503,8 +2504,16 @@ unsigned long hypercall_create_continuation(
     va_end(args);
 
     return op;
+
+ bad_fmt:
+    gprintk(XENLOG_ERR, "Bad hypercall continuation format '%c'\n", *p);
+    ASSERT_UNREACHABLE();
+    domain_crash(current->domain);
+    return 0;
 }
 
+#undef NEXT_ARG
+
 int hypercall_xlat_continuation(unsigned int *id, unsigned int nr,
                                 unsigned int mask, ...)
 {
diff --git a/xen/common/compat/domain.c b/xen/common/compat/domain.c
index 88bfdc836d..d446ed131b 100644
--- a/xen/common/compat/domain.c
+++ b/xen/common/compat/domain.c
@@ -81,7 +81,7 @@ int compat_vcpu_op(int cmd, unsigned int vcpuid, XEN_GUEST_HANDLE_PARAM(void) ar
         }
 
         if ( rc == -ERESTART )
-            rc = hypercall_create_continuation(__HYPERVISOR_vcpu_op, "iuh",
+            rc = hypercall_create_continuation(__HYPERVISOR_vcpu_op, "iih",
                                                cmd, vcpuid, arg);
 
         break;
diff --git a/xen/common/domain.c b/xen/common/domain.c
index 740163ee77..28d7903a96 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1277,7 +1277,7 @@ long do_vcpu_op(int cmd, unsigned int vcpuid, XEN_GUEST_HANDLE_PARAM(void) arg)
 
         rc = arch_initialise_vcpu(v, arg);
         if ( rc == -ERESTART )
-            rc = hypercall_create_continuation(__HYPERVISOR_vcpu_op, "iuh",
+            rc = hypercall_create_continuation(__HYPERVISOR_vcpu_op, "iih",
                                                cmd, vcpuid, arg);
 
         break;