tools/misc: xen-hvmcrash: Inject #DF instead of overwriting RIP

xen-hvmcrash would previously save records, overwrite the instruction
pointer with a bogus value, and then restore them to crash a domain
just enough to cause the guest OS to memdump.

This approach is found to be unreliable when tested on a guest running
Windows 10 x64, with some executions doing nothing at all.

Another approach would be to trigger NMIs. This approach is found to be
unreliable when tested on Linux (Ubuntu 22.04), as Linux will ignore
NMIs if it is not configured to handle such.

Injecting a double fault abort to all vCPUs is found to be more
reliable at crashing and invoking memdumps from Windows and Linux
domains.

This patch modifies the xen-hvmcrash tool to inject #DF to all vCPUs
belonging to the specified domain, instead of overwriting RIP.

Signed-off-by: Matthew Barnes <matthew.barnes@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
master commit: e42e4d8c3e
master date: 2024-07-22 10:50:03 +0100

tools/misc/Makefile

@@ -81,7 +81,7 @@ xen-hvmctx: xen-hvmctx.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
 
 xen-hvmcrash: xen-hvmcrash.o
-	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)
+	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(LDLIBS_libxendevicemodel) $(APPEND_LDFLAGS)
 
 xen-memshare: xen-memshare.o
 	$(CC) $(LDFLAGS) -o $@ $< $(LDLIBS_libxenctrl) $(APPEND_LDFLAGS)

tools/misc/xen-hvmcrash.c

@@ -1,7 +1,7 @@
 /* 
  * xen-hvmcrash.c
  *
- * Attempt to crash an HVM guest by overwriting RIP/EIP with a bogus value
+ * Attempt to crash an HVM guest by injecting #DF to every vcpu
  * 
  * Copyright (c) 2010 Citrix Systems, Inc.
  *
@@ -24,36 +24,25 @@
  * DEALINGS IN THE SOFTWARE.
  */
 
-#include <inttypes.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <stddef.h>
-#include <stdint.h>
-#include <unistd.h>
 #include <string.h>
 #include <errno.h>
-#include <limits.h>
-
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <arpa/inet.h>
 
 #include <xenctrl.h>
-#include <xen/xen.h>
-#include <xen/domctl.h>
-#include <xen/hvm/save.h>
+#include <xendevicemodel.h>
+
+#include <xen/asm/x86-defns.h>
 
 int
 main(int argc, char **argv)
 {
     int domid;
     xc_interface *xch;
+    xendevicemodel_handle *dmod;
     xc_domaininfo_t dominfo;
-    int ret;
-    uint32_t len;
-    uint8_t *buf;
-    uint32_t off;
-    struct hvm_save_descriptor *descriptor;
+    int vcpu_id, ret;
+    bool injected = false;
 
     if (argc != 2 || !argv[1] || (domid = atoi(argv[1])) < 0) {
         fprintf(stderr, "usage: %s <domid>\n", argv[0]);
@@ -83,54 +72,19 @@ main(int argc, char **argv)
         exit(-1);
     }
 
-    /*
-     * Calling with zero buffer length should return the buffer length
-     * required.
-     */
-    ret = xc_domain_hvm_getcontext(xch, domid, 0, 0);
-    if (ret < 0) {
-        perror("xc_domain_hvm_getcontext");
-        exit(1);
-    }
-    
-    len = ret;
-    buf = malloc(len);
-    if (buf == NULL) {
-        perror("malloc");
-        exit(1);
-    }
+    dmod = xc_interface_dmod_handle(xch);
 
-    ret = xc_domain_hvm_getcontext(xch, domid, buf, len);
-    if (ret < 0) {
-        perror("xc_domain_hvm_getcontext");
-        exit(1);
-    }
-
-    off = 0;
-
-    while (off < len) {
-        descriptor = (struct hvm_save_descriptor *)(buf + off);
-
-        off += sizeof (struct hvm_save_descriptor);
-
-        if (descriptor->typecode == HVM_SAVE_CODE(CPU)) {
-            HVM_SAVE_TYPE(CPU) *cpu;
-
-            /* Overwrite EIP/RIP with some recognisable but bogus value */
-            cpu = (HVM_SAVE_TYPE(CPU) *)(buf + off);
-            printf("CPU[%d]: RIP = %" PRIx64 "\n", descriptor->instance, cpu->rip);
-            cpu->rip = 0xf001;
-        } else if (descriptor->typecode == HVM_SAVE_CODE(END)) {
-            break;
+    for (vcpu_id = 0; vcpu_id <= dominfo.max_vcpu_id; vcpu_id++) {
+        printf("Injecting #DF to vcpu ID #%d...\n", vcpu_id);
+        ret = xendevicemodel_inject_event(dmod, domid, vcpu_id,
+                                X86_EXC_DF,
+                                XEN_DMOP_EVENT_hw_exc, 0, 0, 0);
+        if (ret < 0) {
+            fprintf(stderr, "Could not inject #DF to vcpu ID #%d: %s\n",
+                    vcpu_id, strerror(errno));
+            continue;
         }
-
-        off += descriptor->length;
-    }
-
-    ret = xc_domain_hvm_setcontext(xch, domid, buf, len);
-    if (ret < 0) {
-        perror("xc_domain_hvm_setcontext");
-        exit(1);
+        injected = true;
     }
 
     ret = xc_domain_unpause(xch, domid);
@@ -139,5 +93,10 @@ main(int argc, char **argv)
         exit(1);
     }
 
+    if (!injected) {
+        fprintf(stderr, "Could not inject #DF to any vcpu!\n");
+        exit(1);
+    }
+
     return 0;
 }