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

Hi Hongyan,

On 30/04/2020 21:44, Hongyan Xia wrote:
From: Hongyan Xia <hongyxia@xxxxxxxxxx>

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@xxxxxxxxxx>
  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 
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
  ### 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") 
  l1_pgentry_t __section(".bss.page_aligned") __aligned(PAGE_SIZE)
+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;

arm32 doesn't have a directmap, so this needs to be false for arm32 and true for arm64.

I would also like the implementation of the helper close to arch_mfn_in_directmap() in asm-arm/arm*/mm.h.


Julien Grall



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