xen/hypercall: Don't use BUG() for parameter checking in hypercall_create_continuation()

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 <andrew.cooper3@citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>
master commit: 0bf9f8d3e399a0e1d2b717f71b4776172446184b
master date: 2019-10-31 16:07:11 +0100

diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
index 2ab315ea6fd8fba0ce4316c4bfe21501b85d1fba..0b6d477d7f3e1ddda25b8146325534ce996b0996 100644 (file)
--- a/xen/arch/arm/domain.c
+++ b/xen/arch/arm/domain.c
@@ -347,14 +347,15 @@ 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;                                                                  \
 })
@@ -374,9 +375,6 @@ unsigned long hypercall_create_continuation(
     unsigned int i;
     va_list args;
 
-    /* All hypercalls take at least one argument */
-    BUG_ON( !p || *p == '\0' );
-
     current->hcall_preempted = true;
 
     va_start(args, format);
@@ -384,7 +382,7 @@ unsigned long hypercall_create_continuation(
     if ( mcs->flags & MCSF_in_multicall )
     {
         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;
@@ -400,7 +398,7 @@ unsigned long hypercall_create_continuation(
 
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
 
                 switch ( i )
                 {
@@ -423,7 +421,7 @@ unsigned long hypercall_create_continuation(
 
             for ( i = 0; *p != '\0'; i++ )
             {
-                arg = next_arg(p, args);
+                arg = NEXT_ARG(p, args);
 
                 switch ( i )
                 {
@@ -444,8 +442,16 @@ 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/hypercall.c b/xen/arch/x86/hypercall.c
index e30181817a15c4977428756f5814f752f3425867..ae2f1ba187a869c16baa2b766fbd00434c4c02b5 100644 (file)
--- a/xen/arch/x86/hypercall.c
+++ b/xen/arch/x86/hypercall.c
@@ -74,14 +74,15 @@ const hypercall_args_t hypercall_args_table[NR_hypercalls] =
 #undef COMP
 #undef ARGS
 
-#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;                                                                  \
 })
@@ -108,7 +109,7 @@ unsigned long hypercall_create_continuation(
     if ( mcs->flags & MCSF_in_multicall )
     {
         for ( i = 0; *p != '\0'; i++ )
-            mcs->call.args[i] = next_arg(p, args);
+            mcs->call.args[i] = NEXT_ARG(p, args);
     }
     else
     {
@@ -120,7 +121,7 @@ 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;
@@ -136,7 +137,7 @@ 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->rbx = arg; break;
@@ -153,8 +154,16 @@ 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(curr->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 88bfdc836dd0527866b01a269fd38e33a59c49c9..d446ed131b9a4d2c3854c0eb5b2569415ff45b82 100644 (file)
--- 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 1bfec32e9c03fa06393c19fa9b589e3de14c0caf..a49545121bef62fe7267318ca5a16a2c3120be4b 100644 (file)
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -1273,7 +1273,7 @@ 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;