[PATCHv3 2/2] x86/mm: Make memremap(MEMREMAP_WB) map memory as encrypted by default

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

Currently memremap(MEMREMAP_WB) can produce decrypted/shared mapping:

memremap(MEMREMAP_WB)
  arch_memremap_wb()
    ioremap_cache()
      __ioremap_caller(.encrytped = false)

In such cases, the IORES_MAP_ENCRYPTED flag on the memory will determine
if the resulting mapping is encrypted or decrypted.

Creating a decrypted mapping without explicit request from the caller is
risky:

  - It can inadvertently expose the guest's data and compromise the
    guest.

  - Accessing private memory via shared/decrypted mapping on TDX will
    either trigger implicit conversion to shared or #VE (depending on
    VMM implementation).

    Implicit conversion is destructive: subsequent access to the same
    memory via private mapping will trigger a hard-to-debug #VE crash.

The kernel already provides a way to request decrypted mapping
explicitly via the MEMREMAP_DEC flag.

Modify memremap(MEMREMAP_WB) to produce encrypted/private mapping by
default unless MEMREMAP_DEC is specified.

Fix the crash due to #VE on kexec in TDX guests if CONFIG_EISA is enabled.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: stable@vger.kernel.org # 6.11+
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Ashish Kalra <ashish.kalra@amd.com>
Cc: "Maciej W. Rozycki" <macro@orcam.me.uk>
---
 arch/x86/include/asm/io.h | 3 +++
 arch/x86/mm/ioremap.c     | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index ed580c7f9d0a..1a0dc2b2bf5b 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -175,6 +175,9 @@ extern void __iomem *ioremap_prot(resource_size_t offset, unsigned long size, un
 extern void __iomem *ioremap_encrypted(resource_size_t phys_addr, unsigned long size);
 #define ioremap_encrypted ioremap_encrypted
 
+void *arch_memremap_wb(phys_addr_t phys_addr, size_t size, unsigned long flags);
+#define arch_memremap_wb arch_memremap_wb
+
 /**
  * ioremap     -   map bus memory into CPU space
  * @offset:    bus address of the memory
diff --git a/arch/x86/mm/ioremap.c b/arch/x86/mm/ioremap.c
index 8d29163568a7..3c36f3f5e688 100644
--- a/arch/x86/mm/ioremap.c
+++ b/arch/x86/mm/ioremap.c
@@ -503,6 +503,14 @@ void iounmap(volatile void __iomem *addr)
 }
 EXPORT_SYMBOL(iounmap);
 
+void *arch_memremap_wb(phys_addr_t phys_addr, size_t size, unsigned long flags)
+{
+	if (flags & MEMREMAP_DEC)
+		return (void __force *)ioremap_cache(phys_addr, size);
+
+	return (void __force *)ioremap_encrypted(phys_addr, size);
+}
+
 /*
  * Convert a physical pointer to a virtual kernel pointer for /dev/mem
  * access
-- 
2.45.2