livepatch: Add support for inline asm livepatching expectations

This is the initial implementation of the expectations enhancement
to improve inline asm livepatching.

Expectations are designed as optional feature, since the main use of
them is planned for inline asm livepatching. The flag enabled allows
to control the expectation state.
Each expectation has data and len fields that describe the data
that is expected to be found at a given patching (old_addr) location.
The len must not exceed the data array size. The data array size
follows the size of the opaque array, since the opaque array holds
the original data and therefore must match what is specified in the
expectation (if enabled).

The payload structure is modified as each expectation structure is
part of the livepatch_func structure and hence extends the payload.

Each expectation is checked prior to the apply action (i.e. as late
as possible to check against the most current state of the code).

For the replace action a new payload's expectations are checked AFTER
all applied payloads are successfully reverted, but BEFORE new payload
is applied. That breaks the replace action's atomicity and in case of
an expectation check failure would leave a system with all payloads
reverted. That is obviously insecure. Use it with caution and act
upon replace errors!

Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Andra-Irina Paraschiv <andraprs@amazon.com>
Reviewed-by: Martin Pohlack <mpohlack@amazon.de>
Reviewed-by: Norbert Manthey <nmanthey@amazon.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

.gitignore

@@ -308,6 +308,7 @@ xen/include/xen/acm_policy.h
 xen/include/xen/compile.h
 xen/include/xen/lib/x86/cpuid-autogen.h
 xen/test/livepatch/config.h
+xen/test/livepatch/expect_config.h
 xen/test/livepatch/*.livepatch
 xen/tools/kconfig/.tmp_gtkcheck
 xen/tools/kconfig/.tmp_qtcheck

docs/misc/livepatch.pandoc

@@ -300,10 +300,11 @@ which describe the functions to be patched:
         /* Added to livepatch payload version 2: */
         uint8_t applied;
         uint8_t _pad[7];
+        livepatch_expectation_t expect;
     };
 
-The size of the structure is 64 bytes on 64-bit hypervisors. It will be
+The size of the structure is 104 bytes on 64-bit hypervisors. It will be
-52 on 32-bit hypervisors.
+92 on 32-bit hypervisors.
 The version 2 of the payload adds additional 8 bytes to the structure size.
 
  * `name` is the symbol name of the old function. Only used if `old_addr` is
@@ -336,6 +337,28 @@ The version 2 of the payload adds the following fields to the structure:
   * `applied` tracks function's applied/reverted state. It has a boolean type
     either LIVEPATCH_FUNC_NOT_APPLIED or LIVEPATCH_FUNC_APPLIED.
   * `_pad[7]` adds padding to align to 8 bytes.
+  * `expect` is an optional structure containing expected to-be-replaced data
+    (mostly for inline asm patching). The `expect` structure format is:
+
+    struct livepatch_expectation {
+        uint8_t enabled : 1;
+        uint8_t len : 5;
+        uint8_t rsv: 2;
+        uint8_t data[LIVEPATCH_OPAQUE_SIZE]; /* Same size as opaque[] buffer of
+                                            struct livepatch_func. This is the
+                                            max number of bytes to be patched */
+    };
+    typedef struct livepatch_expectation livepatch_expectation_t;
+
+    * `enabled` allows to enable the expectation check for given function.
+      Default state is disabled.
+    * `len` specifies the number of valid bytes in `data` array. 5 bits is
+      enough to specify values up to 32 (of bytes), which is above the array
+      size.
+    * `rsv` reserved bitfields. **MUST** be zero.
+    * `data` contains expected bytes of content to be replaced. Same size as
+      `opaque` buffer of `struct livepatch_func` (max number of bytes to be
+      patched).
 
 The size of the `livepatch_func` array is determined from the ELF section
 size.
@@ -391,6 +414,7 @@ A simple example of what a payload file can be:
         /* Added to livepatch payload version 2: */
         uint8_t applied;
         uint8_t _pad[7];
+        livepatch_expectation_t expect;
     };
 
     /* Our replacement function for xen_extra_version. */
@@ -408,6 +432,13 @@ A simple example of what a payload file can be:
         .old_addr = (void *)0xffff82d08013963c, /* Extracted from xen-syms. */
         .new_size = 13, /* To be be computed by scripts. */
         .old_size = 13, /* -----------""---------------  */
+        /* Added to livepatch payload version 2: */
+        .expect = { /* All fields to be filled manually */
+            .enabled = 1,
+            .len = 5,
+            .rsv = 0,
+            .data = { 0x48, 0x8d, 0x05, 0x33, 0x1C }
+        },
     } __attribute__((__section__(".livepatch.funcs")));
 
 Code must be compiled with `-fPIC`.

xen/common/livepatch.c

@@ -560,6 +560,61 @@ static int check_patching_sections(const struct livepatch_elf *elf)
     return 0;
 }
 
+static inline int livepatch_verify_expectation_fn(const struct livepatch_func *func)
+{
+    const livepatch_expectation_t *exp = &func->expect;
+
+    /* Ignore disabled expectations. */
+    if ( !exp->enabled )
+        return 0;
+
+    /* There is nothing to expect */
+    if ( !func->old_addr )
+        return -EFAULT;
+
+    if ( exp->len > sizeof(exp->data))
+        return -EOVERFLOW;
+
+    if ( exp->rsv )
+        return -EINVAL;
+
+    /* Incorrect expectation */
+    if ( func->old_size < exp->len )
+        return -ERANGE;
+
+    if ( memcmp(func->old_addr, exp->data, exp->len) )
+    {
+        printk(XENLOG_ERR LIVEPATCH "%s: expectation failed: expected:%*phN, actual:%*phN\n",
+               func->name, exp->len, exp->data, exp->len, func->old_addr);
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
+static inline int livepatch_check_expectations(const struct payload *payload)
+{
+    int i, rc;
+
+    printk(XENLOG_INFO LIVEPATCH "%s: Verifying enabled expectations for all functions\n",
+           payload->name);
+
+    for ( i = 0; i < payload->nfuncs; i++ )
+    {
+        const struct livepatch_func *func = &(payload->funcs[i]);
+
+        rc = livepatch_verify_expectation_fn(func);
+        if ( rc )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s: expectations of %s failed (rc=%d), aborting!\n",
+                   payload->name, func->name ?: "unknown", rc);
+            return rc;
+        }
+    }
+
+    return 0;
+}
+
 /*
  * Lookup specified section and when exists assign its address to a specified hook.
  * Perform section pointer and size validation: single hook sections must contain a
@@ -1352,6 +1407,20 @@ static void livepatch_do_action(void)
 
         if ( rc == 0 )
         {
+            /*
+             * Make sure all expectation requirements are met.
+             * Beware all the payloads are reverted at this point.
+             * If expectations are not met the system is left in a
+             * completely UNPATCHED state!
+             */
+            rc = livepatch_check_expectations(data);
+            if ( rc )
+            {
+                printk(XENLOG_ERR LIVEPATCH "%s: SYSTEM MIGHT BE INSECURE: "
+                       "Replace action has been aborted after reverting ALL payloads!\n", data->name);
+                break;
+            }
+
             if ( is_hook_enabled(data->hooks.apply.action) )
             {
                 printk(XENLOG_INFO LIVEPATCH "%s: Calling apply action hook function\n", data->name);
@@ -1808,6 +1877,11 @@ static int livepatch_action(struct xen_sysctl_livepatch_action *action)
                     break;
             }
 
+            /* Make sure all expectation requirements are met. */
+            rc = livepatch_check_expectations(data);
+            if ( rc )
+                break;
+
             if ( is_hook_enabled(data->hooks.apply.pre) )
             {
                 printk(XENLOG_INFO LIVEPATCH "%s: Calling pre-apply hook function\n", data->name);

xen/include/public/sysctl.h

@@ -840,6 +840,19 @@ struct xen_sysctl_cpu_featureset {
  * We guard this with __XEN__ as toolstacks SHOULD not use it.
  */
 #ifdef __XEN__
+#define LIVEPATCH_OPAQUE_SIZE 31
+
+struct livepatch_expectation {
+    uint8_t enabled : 1;
+    uint8_t len : 5;        /* Length of data up to LIVEPATCH_OPAQUE_SIZE
+                               (5 bits is enough for now) */
+    uint8_t rsv : 2;        /* Reserved. Zero value */
+    uint8_t data[LIVEPATCH_OPAQUE_SIZE]; /* Same size as opaque[] buffer of
+                                            struct livepatch_func. This is the
+                                            max number of bytes to be patched */
+};
+typedef struct livepatch_expectation livepatch_expectation_t;
+
 typedef enum livepatch_func_state {
     LIVEPATCH_FUNC_NOT_APPLIED,
     LIVEPATCH_FUNC_APPLIED
@@ -852,9 +865,10 @@ struct livepatch_func {
     uint32_t new_size;
     uint32_t old_size;
     uint8_t version;        /* MUST be LIVEPATCH_PAYLOAD_VERSION. */
-    uint8_t opaque[31];
+    uint8_t opaque[LIVEPATCH_OPAQUE_SIZE];
     uint8_t applied;
     uint8_t _pad[7];
+    livepatch_expectation_t expect;
 };
 typedef struct livepatch_func livepatch_func_t;
 #endif

xen/test/livepatch/Makefile

@@ -27,6 +27,8 @@ LIVEPATCH_ACTION_HOOKS_NOFUNC := xen_action_hooks_nofunc.livepatch
 LIVEPATCH_ACTION_HOOKS_MARKER:= xen_action_hooks_marker.livepatch
 LIVEPATCH_ACTION_HOOKS_NOAPPLY:= xen_action_hooks_noapply.livepatch
 LIVEPATCH_ACTION_HOOKS_NOREVERT:= xen_action_hooks_norevert.livepatch
+LIVEPATCH_EXPECTATIONS:= xen_expectations.livepatch
+LIVEPATCH_EXPECTATIONS_FAIL:= xen_expectations_fail.livepatch
 
 LIVEPATCHES += $(LIVEPATCH)
 LIVEPATCHES += $(LIVEPATCH_BYE)
@@ -40,6 +42,8 @@ LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOFUNC)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_MARKER)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOAPPLY)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOREVERT)
+LIVEPATCHES += $(LIVEPATCH_EXPECTATIONS)
+LIVEPATCHES += $(LIVEPATCH_EXPECTATIONS_FAIL)
 
 LIVEPATCH_DEBUG_DIR ?= $(DEBUG_DIR)/xen-livepatch
 
@@ -54,7 +58,7 @@ uninstall:
 
 .PHONY: clean
 clean::
-	rm -f *.o .*.o.d *.livepatch config.h
+	rm -f *.o .*.o.d *.livepatch config.h expect_config.h
 
 #
 # To compute these values we need the binary files: xen-syms
@@ -182,8 +186,27 @@ xen_actions_hooks_norevert.o: config.h
 $(LIVEPATCH_ACTION_HOOKS_NOREVERT): xen_action_hooks_marker.o xen_hello_world_func.o note.o xen_note.o
 	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_NOREVERT) $^
 
+EXPECT_BYTES_COUNT := 8
+CODE_GET_EXPECT=$(shell $(OBJDUMP) -d --insn-width=1 $(1) | sed -n -e '/<'$(2)'>:$$/,/^$$/ p' | tail -n +2 | head -n $(EXPECT_BYTES_COUNT) | awk '{$$0=$$2; printf "%s", substr($$0,length-1)}' | sed 's/.\{2\}/0x&,/g' | sed 's/^/{/;s/,$$/}/g')
+.PHONY: expect_config.h
+expect_config.h: EXPECT_BYTES=$(call CODE_GET_EXPECT,$(BASEDIR)/xen-syms,xen_extra_version)
+expect_config.h: xen_expectations.o
+	(set -e; \
+	 echo "#define EXPECT_BYTES $(EXPECT_BYTES)"; \
+         echo "#define EXPECT_BYTES_COUNT $(EXPECT_BYTES_COUNT)") > $@
+
+xen_expectations.o: expect_config.h
+
+.PHONY: $(LIVEPATCH_EXPECTATIONS)
+$(LIVEPATCH_EXPECTATIONS): xen_expectations.o xen_hello_world_func.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_EXPECTATIONS) $^
+
+.PHONY: $(LIVEPATCH_EXPECTATIONS_FAIL)
+$(LIVEPATCH_EXPECTATIONS_FAIL): xen_expectations_fail.o xen_hello_world_func.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_EXPECTATIONS_FAIL) $^
+
 .PHONY: livepatch
 livepatch: $(LIVEPATCH) $(LIVEPATCH_BYE) $(LIVEPATCH_REPLACE) $(LIVEPATCH_NOP) $(LIVEPATCH_NO_XEN_BUILDID) \
            $(LIVEPATCH_PREPOST_HOOKS) $(LIVEPATCH_PREPOST_HOOKS_FAIL) $(LIVEPATCH_ACTION_HOOKS) \
            $(LIVEPATCH_ACTION_HOOKS_NOFUNC) $(LIVEPATCH_ACTION_HOOKS_MARKER) $(LIVEPATCH_ACTION_HOOKS_NOAPPLY) \
-           $(LIVEPATCH_ACTION_HOOKS_NOREVERT)
+           $(LIVEPATCH_ACTION_HOOKS_NOREVERT) $(LIVEPATCH_EXPECTATIONS) $(LIVEPATCH_EXPECTATIONS_FAIL)

xen/test/livepatch/xen_expectations.c

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include "expect_config.h"
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static const char livepatch_exceptions_str[] = "xen_extra_version";
+extern const char *xen_hello_world(void);
+
+struct livepatch_func __section(".livepatch.funcs") livepatch_exceptions = {
+    .version = LIVEPATCH_PAYLOAD_VERSION,
+    .name = livepatch_exceptions_str,
+    .new_addr = xen_hello_world,
+    .old_addr = xen_extra_version,
+    .new_size = EXPECT_BYTES_COUNT,
+    .old_size = EXPECT_BYTES_COUNT,
+    .expect = {
+        .enabled = 1,
+        .len = EXPECT_BYTES_COUNT,
+        .data = EXPECT_BYTES
+    },
+
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */

xen/test/livepatch/xen_expectations_fail.c

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static const char livepatch_exceptions_str[] = "xen_extra_version";
+extern const char *xen_hello_world(void);
+
+#define EXPECT_BYTES_COUNT 6
+
+struct livepatch_func __section(".livepatch.funcs") livepatch_exceptions = {
+    .version = LIVEPATCH_PAYLOAD_VERSION,
+    .name = livepatch_exceptions_str,
+    .new_addr = xen_hello_world,
+    .old_addr = xen_extra_version,
+    .new_size = EXPECT_BYTES_COUNT,
+    .old_size = EXPECT_BYTES_COUNT,
+    .expect = {
+        .enabled = 1,
+        .len = EXPECT_BYTES_COUNT,
+        .data = { 0xDE, 0xAD, 0xC0, 0xDE, 0xBA, 0xBE }
+    },
+
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */