xen/version: Introduce non-truncating deterministically-signed XENVER_* subops

In XenServer, we have encountered problems caused by both XENVER_extraversion
and XENVER_commandline having fixed bounds.

More than just the invariant size, the APIs/ABIs also broken by typedef-ing an
array, and using an unqualified 'char' which has implementation-specific
signed-ness.

Provide brand new ops, which are capable of expressing variable length
strings, and mark the older ops as broken.

This fixes all issues around XENVER_extraversion being longer than 15 chars.
Further work beyond just this API is needed to remove other assumptions about
XENVER_commandline being 1023 chars long.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jason Andryuk <jandryuk@gmail.com>
---
Non-technical objections to this patch were raised, and subsequently rejected
by a community wide vote.  The results of the vote have not been shared with
the community at the time of committing.

xen/common/kernel.c

@@ -24,6 +24,7 @@
 CHECK_build_id;
 CHECK_compile_info;
 CHECK_feature_info;
+CHECK_varbuf;
 #endif
 
 enum system_state system_state = SYS_STATE_early_boot;
@@ -503,6 +504,56 @@ static int __init cf_check param_init(void)
 __initcall(param_init);
 #endif
 
+static long xenver_varbuf_op(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
+{
+    struct xen_varbuf user_str;
+    const char *str = NULL;
+    size_t sz;
+
+    switch ( cmd )
+    {
+    case XENVER_extraversion2:
+        str = xen_extra_version();
+        break;
+
+    case XENVER_changeset2:
+        str = xen_changeset();
+        break;
+
+    case XENVER_commandline2:
+        str = saved_cmdline;
+        break;
+
+    case XENVER_capabilities2:
+        str = xen_cap_info;
+        break;
+
+    default:
+        ASSERT_UNREACHABLE();
+        return -ENODATA;
+    }
+
+    sz = strlen(str);
+
+    if ( sz > KB(64) ) /* Arbitrary limit.  Avoid long-running operations. */
+        return -E2BIG;
+
+    if ( guest_handle_is_null(arg) ) /* Length request */
+        return sz;
+
+    if ( copy_from_guest(&user_str, arg, 1) )
+        return -EFAULT;
+
+    if ( sz > user_str.len )
+        return -ENOBUFS;
+
+    if ( copy_to_guest_offset(arg, offsetof(struct xen_varbuf, buf),
+                              str, sz) )
+        return -EFAULT;
+
+    return sz;
+}
+
 long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 {
     bool deny = xsm_xen_version(XSM_OTHER, cmd);
@@ -721,6 +772,14 @@ long do_xen_version(int cmd, XEN_GUEST_HANDLE_PARAM(void) arg)
 
         return sz;
     }
+
+    case XENVER_extraversion2:
+    case XENVER_capabilities2:
+    case XENVER_changeset2:
+    case XENVER_commandline2:
+        if ( deny )
+            return -EPERM;
+        return xenver_varbuf_op(cmd, arg);
     }
 
     return -ENOSYS;

xen/include/public/version.h

@@ -19,12 +19,20 @@
 /* arg == NULL; returns major:minor (16:16). */
 #define XENVER_version      0
 
-/* arg == xen_extraversion_t. */
+/*
+ * arg == xen_extraversion_t.
+ *
+ * This API/ABI is broken.  Use XENVER_extraversion2 where possible.
+ */
 #define XENVER_extraversion 1
 typedef char xen_extraversion_t[16];
 #define XEN_EXTRAVERSION_LEN (sizeof(xen_extraversion_t))
 
-/* arg == xen_compile_info_t. */
+/*
+ * arg == xen_compile_info_t.
+ *
+ * This API/ABI is broken and truncates data.
+ */
 #define XENVER_compile_info 2
 struct xen_compile_info {
     char compiler[64];
@@ -34,10 +42,20 @@ struct xen_compile_info {
 };
 typedef struct xen_compile_info xen_compile_info_t;
 
+/*
+ * arg == xen_capabilities_info_t.
+ *
+ * This API/ABI is broken.  Use XENVER_capabilities2 where possible.
+ */
 #define XENVER_capabilities 3
 typedef char xen_capabilities_info_t[1024];
 #define XEN_CAPABILITIES_INFO_LEN (sizeof(xen_capabilities_info_t))
 
+/*
+ * arg == xen_changeset_info_t.
+ *
+ * This API/ABI is broken.  Use XENVER_changeset2 where possible.
+ */
 #define XENVER_changeset 4
 typedef char xen_changeset_info_t[64];
 #define XEN_CHANGESET_INFO_LEN (sizeof(xen_changeset_info_t))
@@ -95,6 +113,11 @@ typedef struct xen_feature_info xen_feature_info_t;
  */
 #define XENVER_guest_handle 8
 
+/*
+ * arg == xen_commandline_t.
+ *
+ * This API/ABI is broken.  Use XENVER_commandline2 where possible.
+ */
 #define XENVER_commandline 9
 typedef char xen_commandline_t[1024];
 
@@ -110,6 +133,42 @@ struct xen_build_id {
 };
 typedef struct xen_build_id xen_build_id_t;
 
+/*
+ * Container for an arbitrary variable length buffer.
+ */
+struct xen_varbuf {
+    uint32_t len;                          /* IN:  size of buf[] in bytes. */
+    unsigned char buf[XEN_FLEX_ARRAY_DIM]; /* OUT: requested data.         */
+};
+typedef struct xen_varbuf xen_varbuf_t;
+
+/*
+ * arg == xen_varbuf_t
+ *
+ * Equivalent to the original ops, but with a non-truncating API/ABI.
+ *
+ * These hypercalls can fail for a number of reasons.  All callers must handle
+ * -XEN_xxx return values appropriately.
+ *
+ * Passing arg == NULL is a request for size, which will be signalled with a
+ * non-negative return value.  Note: a return size of 0 may be legitimate for
+ * the requested subop.
+ *
+ * Otherwise, the input xen_varbuf_t provides the size of the following
+ * buffer.  Xen will fill the buffer, and return the number of bytes written
+ * (e.g. if the input buffer was longer than necessary).
+ *
+ * Some subops may return binary data.  Some subops may be expected to return
+ * textural data.  These are returned without a NUL terminator, and while the
+ * contents is expected to be ASCII/UTF-8, Xen makes no guarentees to this
+ * effect.  e.g. Xen has no control over the formatting used for the command
+ * line.
+ */
+#define XENVER_extraversion2 11
+#define XENVER_capabilities2 12
+#define XENVER_changeset2    13
+#define XENVER_commandline2  14
+
 #endif /* __XEN_PUBLIC_VERSION_H__ */
 
 /*

xen/include/xlat.lst

@@ -195,6 +195,7 @@
 ?	build_id			version.h
 ?	compile_info			version.h
 ?	feature_info			version.h
+?	varbuf                          version.h
 
 ?	xenoprof_init			xenoprof.h
 ?	xenoprof_passive		xenoprof.h

xen/include/xsm/dummy.h

@@ -828,9 +828,12 @@ static XSM_INLINE int cf_check xsm_xen_version(XSM_DEFAULT_ARG uint32_t op)
         block_speculation();
         return 0;
     case XENVER_extraversion:
+    case XENVER_extraversion2:
     case XENVER_compile_info:
     case XENVER_capabilities:
+    case XENVER_capabilities2:
     case XENVER_changeset:
+    case XENVER_changeset2:
     case XENVER_pagesize:
     case XENVER_guest_handle:
         /* These MUST always be accessible to any guest by default. */

xen/xsm/flask/hooks.c

@@ -1795,15 +1795,18 @@ static int cf_check flask_xen_version(uint32_t op)
         /* These sub-ops ignore the permission checks and return data. */
         return 0;
     case XENVER_extraversion:
+    case XENVER_extraversion2:
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_EXTRAVERSION, NULL);
     case XENVER_compile_info:
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_COMPILE_INFO, NULL);
     case XENVER_capabilities:
+    case XENVER_capabilities2:
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_CAPABILITIES, NULL);
     case XENVER_changeset:
+    case XENVER_changeset2:
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_CHANGESET, NULL);
     case XENVER_pagesize:
@@ -1813,6 +1816,7 @@ static int cf_check flask_xen_version(uint32_t op)
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_GUEST_HANDLE, NULL);
     case XENVER_commandline:
+    case XENVER_commandline2:
         return avc_has_perm(dsid, SECINITSID_XEN, SECCLASS_VERSION,
                             VERSION__XEN_COMMANDLINE, NULL);
     case XENVER_build_id: