[v2] x86: Add a boot option to enable and disable the direct map

Commit Message

Elias El Yandouzi Jan. 16, 2024, 6:50 p.m. UTC

From: Hongyan Xia <[e-mail]>

Also add a helper function to retrieve it. Change arch_mfns_in_direct_map
to check this option before returning.

This is added as a Kconfig option as well as a boot command line option.
While being generic, the Kconfig option is only usable for x86 at the moment.

Signed-off-by: Hongyan Xia <[e-mail]>
Signed-off-by: Julien Grall <[e-mail]>

----

    Changes in V2:
        * Introduce a Kconfig option
        * Reword the commit message
        * Make opt_directmap and helper generic

    Changes since Hongyan's version:
        * Reword the commit message
        * opt_directmap is only modified during boot so mark it as
          __ro_after_init

Patch

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index 8e65f8bd18..63c946f482 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -799,6 +799,18 @@  that enabling this option cannot guarantee anything beyond what underlying
 hardware guarantees (with, where available and known to Xen, respective
 tweaks applied).
 
+### directmap (x86)
+> `= <boolean>`
+
+> Default: `true`
+
+Enable or disable the direct map region in Xen.
+
+By default, Xen creates the direct map region which maps physical memory
+in that region. Setting this to no will remove the direct map, blocking
+exploits that leak secrets via speculative memory access in the direct
+map.
+
 ### dma_bits
 > `= <integer>`
 
diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 1acdffc51c..350f41b832 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -29,6 +29,7 @@  config X86
 	select HAS_UBSAN
 	select HAS_VPCI if HVM
 	select NEEDS_LIBELF
+	select HAS_SECRET_HIDING
 
 config ARCH_DEFCONFIG
 	string
diff --git a/xen/arch/x86/include/asm/mm.h b/xen/arch/x86/include/asm/mm.h
index 7d26d9cd2f..4aae270a78 100644
--- a/xen/arch/x86/include/asm/mm.h
+++ b/xen/arch/x86/include/asm/mm.h
@@ -620,10 +620,18 @@  void write_32bit_pse_identmap(uint32_t *l2);
 /*
  * x86 maps part of physical memory via the directmap region.
  * Return whether the range of MFN falls in the directmap region.
+ *
+ * When boot command line sets directmap=no, we will not have a direct map at
+ * all so this will always return false.
  */
 static inline bool arch_mfns_in_directmap(unsigned long mfn, unsigned long nr)
 {
-    unsigned long eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END);
+    unsigned long eva;
+
+    if ( !has_directmap() )
+        return false;
+
+    eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END);
 
     return (mfn + nr) <= (virt_to_mfn(eva - 1) + 1);
 }
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 4d0c90b7a0..b813ea75b5 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1512,6 +1512,8 @@  void asmlinkage __init noreturn __start_xen(unsigned long mbi_p)
     if ( highmem_start )
         xenheap_max_mfn(PFN_DOWN(highmem_start - 1));
 
+    printk("Booting with directmap %s\n", has_directmap() ? "on" : "off");
+
     /*
      * Walk every RAM region and map it in its entirety (on x86/64, at least)
      * and notify it to the boot allocator.
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index 310ad4229c..9a24c89ac5 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -83,6 +83,23 @@  config HAS_UBSAN
 config MEM_ACCESS_ALWAYS_ON
 	bool
 
+config HAS_SECRET_HIDING
+	bool
+
+config SECRET_HIDING
+    bool "Secret hiding"
+    depends on HAS_SECRET_HIDING
+    ---help---
+    The directmap contains mapping for most of the RAM which makes domain
+    memory easily accessible. While making the performance better, it also makes
+    the hypervisor more vulnerable to speculation attacks.
+
+    Enabling this feature will allow the user to decide whether the memory
+    is always mapped at boot or mapped only on demand (see the command line
+    option "directmap").
+
+    If unsure, say N.
+
 config MEM_ACCESS
 	def_bool MEM_ACCESS_ALWAYS_ON
 	prompt "Memory Access and VM events" if !MEM_ACCESS_ALWAYS_ON
diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index 740b6f0ff7..a3746cfbcf 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -173,6 +173,11 @@  paddr_t __ro_after_init mem_hotplug;
 static char __initdata opt_badpage[100] = "";
 string_param("badpage", opt_badpage);
 
+bool __ro_after_init opt_directmap = true;
+#ifdef CONFIG_HAS_SECRET_HIDING
+boolean_param("directmap", opt_directmap);
+#endif
+
 /*
  * no-bootscrub -> Free pages are not zeroed during boot.
  */
diff --git a/xen/include/xen/mm.h b/xen/include/xen/mm.h
index 3d9b2d05a5..f860e98ee4 100644
--- a/xen/include/xen/mm.h
+++ b/xen/include/xen/mm.h
@@ -165,6 +165,13 @@  extern unsigned long max_page;
 extern unsigned long total_pages;
 extern paddr_t mem_hotplug;
 
+extern bool opt_directmap;
+
+static inline bool has_directmap(void)
+{
+    return opt_directmap;
+}
+
 /*
  * Extra fault info types which are used to further describe
  * the source of an access violation.