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Subject[PATCH v8 1/2] x86/mm: add .bss..decrypted section to hold shared variables
Date
kvmclock defines few static variables which are shared with the
hypervisor during the kvmclock initialization.

When SEV is active, memory is encrypted with a guest-specific key, and
if the guest OS wants to share the memory region with the hypervisor
then it must clear the C-bit before sharing it.

Currently, we use kernel_physical_mapping_init() to split large pages
before clearing the C-bit on shared pages. But it fails when called from
the kvmclock initialization (mainly because the memblock allocator is
not ready that early during boot).

Add a __bss_decrypted section attribute which can be used when defining
such shared variable. The so-defined variables will be placed in the
.bss..decrypted section. This section will be mapped with C=0 early
during boot.

The .bss..decrypted section has a big chunk of memory that may be unused
when memory encryption is not active, free it when memory encryption is
not active.

Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: kvm@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: linux-kernel@vger.kernel.org
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
---
arch/x86/include/asm/mem_encrypt.h | 7 +++++++
arch/x86/kernel/head64.c | 16 ++++++++++++++++
arch/x86/kernel/vmlinux.lds.S | 19 +++++++++++++++++++
arch/x86/mm/init.c | 4 ++++
arch/x86/mm/mem_encrypt.c | 10 ++++++++++
5 files changed, 56 insertions(+)

diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
index c064383..616f8e6 100644
--- a/arch/x86/include/asm/mem_encrypt.h
+++ b/arch/x86/include/asm/mem_encrypt.h
@@ -48,10 +48,13 @@ int __init early_set_memory_encrypted(unsigned long vaddr, unsigned long size);

/* Architecture __weak replacement functions */
void __init mem_encrypt_init(void);
+void __init mem_encrypt_free_decrypted_mem(void);

bool sme_active(void);
bool sev_active(void);

+#define __bss_decrypted __attribute__((__section__(".bss..decrypted")))
+
#else /* !CONFIG_AMD_MEM_ENCRYPT */

#define sme_me_mask 0ULL
@@ -77,6 +80,8 @@ early_set_memory_decrypted(unsigned long vaddr, unsigned long size) { return 0;
static inline int __init
early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0; }

+#define __bss_decrypted
+
#endif /* CONFIG_AMD_MEM_ENCRYPT */

/*
@@ -88,6 +93,8 @@ early_set_memory_encrypted(unsigned long vaddr, unsigned long size) { return 0;
#define __sme_pa(x) (__pa(x) | sme_me_mask)
#define __sme_pa_nodebug(x) (__pa_nodebug(x) | sme_me_mask)

+extern char __start_bss_decrypted[], __end_bss_decrypted[], __start_bss_decrypted_unused[];
+
#endif /* __ASSEMBLY__ */

#endif /* __X86_MEM_ENCRYPT_H__ */
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 8047379..c16af27 100644
--- a/arch/x86/kernel/head64.c
+++ b/arch/x86/kernel/head64.c
@@ -112,6 +112,7 @@ static bool __head check_la57_support(unsigned long physaddr)
unsigned long __head __startup_64(unsigned long physaddr,
struct boot_params *bp)
{
+ unsigned long vaddr, vaddr_end;
unsigned long load_delta, *p;
unsigned long pgtable_flags;
pgdval_t *pgd;
@@ -235,6 +236,21 @@ unsigned long __head __startup_64(unsigned long physaddr,
sme_encrypt_kernel(bp);

/*
+ * Clear the memory encryption mask from the .bss..decrypted section.
+ * The bss section will be memset to zero later in the initialization so
+ * there is no need to zero it after changing the memory encryption
+ * attribute.
+ */
+ if (mem_encrypt_active()) {
+ vaddr = (unsigned long)__start_bss_decrypted;
+ vaddr_end = (unsigned long)__end_bss_decrypted;
+ for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
+ i = pmd_index(vaddr);
+ pmd[i] -= sme_get_me_mask();
+ }
+ }
+
+ /*
* Return the SME encryption mask (if SME is active) to be used as a
* modifier for the initial pgdir entry programmed into CR3.
*/
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 9c77d2d..0d618ee 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -65,6 +65,23 @@ jiffies_64 = jiffies;
#define ALIGN_ENTRY_TEXT_BEGIN . = ALIGN(PMD_SIZE);
#define ALIGN_ENTRY_TEXT_END . = ALIGN(PMD_SIZE);

+/*
+ * This section contains data which will be mapped as decrypted. Memory
+ * encryption operates on a page basis. Make this section PMD-aligned
+ * to avoid splitting the pages while mapping the section early.
+ *
+ * Note: We use a separate section so that only this section gets
+ * decrypted to avoid exposing more than we wish.
+ */
+#define BSS_DECRYPTED \
+ . = ALIGN(PMD_SIZE); \
+ __start_bss_decrypted = .; \
+ *(.bss..decrypted); \
+ . = ALIGN(PAGE_SIZE); \
+ __start_bss_decrypted_unused = .; \
+ . = ALIGN(PMD_SIZE); \
+ __end_bss_decrypted = .; \
+
#else

#define X86_ALIGN_RODATA_BEGIN
@@ -74,6 +91,7 @@ jiffies_64 = jiffies;

#define ALIGN_ENTRY_TEXT_BEGIN
#define ALIGN_ENTRY_TEXT_END
+#define BSS_DECRYPTED

#endif

@@ -345,6 +363,7 @@ SECTIONS
__bss_start = .;
*(.bss..page_aligned)
*(.bss)
+ BSS_DECRYPTED
. = ALIGN(PAGE_SIZE);
__bss_stop = .;
}
diff --git a/arch/x86/mm/init.c b/arch/x86/mm/init.c
index 7a8fc26..faca978 100644
--- a/arch/x86/mm/init.c
+++ b/arch/x86/mm/init.c
@@ -815,10 +815,14 @@ void free_kernel_image_pages(void *begin, void *end)
set_memory_np_noalias(begin_ul, len_pages);
}

+void __weak mem_encrypt_free_decrypted_mem(void) { }
+
void __ref free_initmem(void)
{
e820__reallocate_tables();

+ mem_encrypt_free_decrypted_mem();
+
free_kernel_image_pages(&__init_begin, &__init_end);
}

diff --git a/arch/x86/mm/mem_encrypt.c b/arch/x86/mm/mem_encrypt.c
index b2de398..718acdf 100644
--- a/arch/x86/mm/mem_encrypt.c
+++ b/arch/x86/mm/mem_encrypt.c
@@ -348,6 +348,16 @@ bool sev_active(void)
EXPORT_SYMBOL(sev_active);

/* Architecture __weak replacement functions */
+void __init mem_encrypt_free_decrypted_mem(void)
+{
+ if (mem_encrypt_active())
+ return;
+
+ free_init_pages("unused decrypted",
+ (unsigned long)__start_bss_decrypted_unused,
+ (unsigned long)__end_bss_decrypted);
+}
+
void __init mem_encrypt_init(void)
{
if (!sme_me_mask)
--
2.7.4
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