[PATCH v4 15/15] x86/tdx: Add cmdline option to force use of ioremap_shared

[Kuppuswamy Sathyanarayanan]

Add a command line option to force all the enabled drivers to use
shared memory mappings. This will be useful when enabling new drivers
in the protected guest without making all the required changes to use
shared mappings in it.

Note that this might also allow other non explicitly enabled drivers
to interact with the host, which could cause other security risks.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---
 .../admin-guide/kernel-parameters.rst         |  1 +
 .../admin-guide/kernel-parameters.txt         | 12 ++++++++++++
 arch/x86/include/asm/io.h                     |  2 ++
 arch/x86/mm/ioremap.c                         | 19 ++++++++++++++++++-
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
index 01ba293a2d70..bdf3896a100c 100644
--- a/Documentation/admin-guide/kernel-parameters.rst
+++ b/Documentation/admin-guide/kernel-parameters.rst
@@ -147,6 +147,7 @@ parameter is applicable::
 	PCI	PCI bus support is enabled.
 	PCIE	PCI Express support is enabled.
 	PCMCIA	The PCMCIA subsystem is enabled.
+	PG	Protected guest is enabled.
 	PNP	Plug & Play support is enabled.
 	PPC	PowerPC architecture is enabled.
 	PPT	Parallel port support is enabled.
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index bdb22006f713..ba390be62f89 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2062,6 +2062,18 @@
 			1 - Bypass the IOMMU for DMA.
 			unset - Use value of CONFIG_IOMMU_DEFAULT_PASSTHROUGH.
 
+	ioremap_force_shared= [X86_64, PG]
+			Force the kernel to use shared memory mappings which do
+			not use ioremap_shared/pcimap_shared to opt-in to shared
+			mappings with the host. This feature is mainly used by
+			a protected guest when enabling new drivers without
+			proper shared memory related changes. Please note that
+			this option might also allow other non explicitly enabled
+			drivers to interact with the host in protected guest,
+			which could cause other security risks. This option will
+			also cause BIOS data structures to be shared with the host,
+			which might open security holes.
+
 	io7=		[HW] IO7 for Marvel-based Alpha systems
 			See comment before marvel_specify_io7 in
 			arch/alpha/kernel/core_marvel.c.
diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index 51c2c45456bf..744f72835a30 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -413,6 +413,8 @@ extern bool arch_memremap_can_ram_remap(resource_size_t offset,
 extern bool phys_mem_access_encrypted(unsigned long phys_addr,
 				      unsigned long size);
 
+extern bool ioremap_force_shared;
+
 /**
  * iosubmit_cmds512 - copy data to single MMIO location, in 512-bit units
  * @dst: destination, in MMIO space (must be 512-bit aligned)
diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 74260aaa494b..7576e886fad8 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -28,6 +28,7 @@
 #include <asm/memtype.h>
 #include <asm/setup.h>
 #include <asm/tdx.h>
+#include <asm/cmdline.h>
 
 #include "physaddr.h"
 
@@ -162,6 +163,17 @@ static void __ioremap_check_mem(resource_size_t addr, unsigned long size,
 	__ioremap_check_other(addr, desc);
 }
 
+/*
+ * Normally only drivers that are hardened for use in confidential guests
+ * force shared mappings. But if device filtering is disabled other
+ * devices can be loaded, and these need shared mappings too. This
+ * variable is set to true if these filters are disabled.
+ *
+ * Note this has some side effects, e.g. various BIOS tables
+ * get shared too which is risky.
+ */
+bool ioremap_force_shared;
+
 /*
  * Remap an arbitrary physical address space into the kernel virtual
  * address space. It transparently creates kernel huge I/O mapping when
@@ -249,7 +261,7 @@ __ioremap_caller(resource_size_t phys_addr, unsigned long size,
 	prot = PAGE_KERNEL_IO;
 	if ((io_desc.flags & IORES_MAP_ENCRYPTED) || encrypted)
 		prot = pgprot_encrypted(prot);
-	else if (shared)
+	else if (shared || ioremap_force_shared)
 		prot = pgprot_protected_guest(prot);
 
 	switch (pcm) {
@@ -847,6 +859,11 @@ void __init early_ioremap_init(void)
 	WARN_ON((fix_to_virt(0) + PAGE_SIZE) & ((1 << PMD_SHIFT) - 1));
 #endif
 
+	/* Parse cmdline params for ioremap_force_shared */
+	if (cmdline_find_option_bool(boot_command_line,
+				     "ioremap_force_shared"))
+		ioremap_force_shared = 1;
+
 	early_ioremap_setup();
 
 	pmd = early_ioremap_pmd(fix_to_virt(FIX_BTMAP_BEGIN));
-- 
2.25.1