[PATCH 11/16] x86: add a boot option to enable and disable the direct map

[Hongyan Xia <hx242@xen.org>]

From: Hongyan Xia <hongyxia@amazon.com>

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

This is added as a boot command line option, not a Kconfig. We do not
produce different builds for EC2 so this is not introduced as a
compile-time configuration.

Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
---
 docs/misc/xen-command-line.pandoc | 12 ++++++++++++
 xen/arch/x86/mm.c                 |  3 +++
 xen/arch/x86/setup.c              |  2 ++
 xen/include/asm-arm/mm.h          |  5 +++++
 xen/include/asm-x86/mm.h          | 17 ++++++++++++++++-
 5 files changed, 38 insertions(+), 1 deletion(-)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index ee12b0f53f..7027e3a15c 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -652,6 +652,18 @@ Specify the size of the console debug trace buffer. By specifying `cpu:`
 additionally a trace buffer of the specified size is allocated per cpu.
 The debug trace feature is only enabled in debugging builds of Xen.
 
+### 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/mm.c b/xen/arch/x86/mm.c
index b3530d2763..64da997764 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -162,6 +162,9 @@ l1_pgentry_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
 l1_pgentry_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
     l1_fixmap_x[L1_PAGETABLE_ENTRIES];
 
+bool __read_mostly opt_directmap = true;
+boolean_param("directmap", opt_directmap);
+
 paddr_t __read_mostly mem_hotplug;
 
 /* Frame table size in pages. */
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index faca8c9758..60fc4038be 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1282,6 +1282,8 @@ void __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", arch_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/include/asm-arm/mm.h b/xen/include/asm-arm/mm.h
index 7df91280bc..e6fd934113 100644
--- a/xen/include/asm-arm/mm.h
+++ b/xen/include/asm-arm/mm.h
@@ -366,6 +366,11 @@ int arch_acquire_resource(struct domain *d, unsigned int type, unsigned int id,
     return -EOPNOTSUPP;
 }
 
+static inline bool arch_has_directmap(void)
+{
+    return true;
+}
+
 #endif /*  __ARCH_ARM_MM__ */
 /*
  * Local variables:
diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
index ef7a20ac7d..7ff99ee8e3 100644
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -454,6 +454,8 @@ int check_descriptor(const struct domain *d, seg_desc_t *desc);
 
 extern paddr_t mem_hotplug;
 
+extern bool opt_directmap;
+
 /******************************************************************************
  * With shadow pagetables, the different kinds of address start
  * to get get confusing.
@@ -637,13 +639,26 @@ extern const char zero_page[];
 /* Build a 32bit PSE page table using 4MB pages. */
 void write_32bit_pse_identmap(uint32_t *l2);
 
+static inline bool arch_has_directmap(void)
+{
+    return opt_directmap;
+}
+
 /*
  * x86 maps part of physical memory via the directmap region.
  * Return whether the input MFN falls in that range.
+ *
+ * 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_mfn_in_directmap(unsigned long mfn)
 {
-    unsigned long eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END);
+    unsigned long eva;
+
+    if ( !arch_has_directmap() )
+        return false;
+
+    eva = min(DIRECTMAP_VIRT_END, HYPERVISOR_VIRT_END);
 
     return mfn <= (virt_to_mfn(eva - 1) + 1);
 }
-- 
2.24.1.AMZN