Re: [PATCH v2 2/3] x86/mm: add .data..decrypted section to hold shared variables

From: Brijesh Singh
Date: Wed Aug 29 2018 - 10:41:03 EST



On 08/29/2018 08:59 AM, Borislav Petkov wrote:
On Tue, Aug 28, 2018 at 05:12:56PM -0500, Brijesh Singh wrote:
kvmclock defines few static variables which are shared with hypervisor

... with the hypervisor ...

during the kvmclock initialization.

When SEV is active, memory is encrypted with a guest-specific key, and
if guest OS wants to share the memory region with 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 the kernel_physical_mapping_init fails when

"But it fails when..."

called from the kvmclock initialization (mainly because memblock allocator
was not ready).

"... is not ready that early during boot)."

The '__decrypted' can be used to define a shared variable; the variables

"Add a __decrypted section attribute which can be used when defining
such shared variable. The so-defined variables will be placed..."

will be put in the .data.decryption section. This section is mapped with

" ... in the .data..decrypted section."

C=0 early in the boot, we also ensure that the initialized values are

"... early during boot,"

updated to match with C=0 (i.e perform an in-place decryption). The
.data..decrypted section is PMD aligned and sized so that we avoid the

"... PMD-aligned ..."

need to split the large pages when mapping this section.

The sme_encrypt_kernel() was used to perform the in-place encryption
of the Linux kernel and initrd when SME is active. The routine has been
enhanced to decrypt the .data..decryption section for both SME and SEV
cases.

".data..decrypted"


While reusing the sme_populate_pgd() we found that the function does not
update the flags if the pte/pmd entry already exists. The patch updates
the function to take care of it.


Change the tone to impartial:

"While at it, fix sme_populate_pgd() to update page flags if the PMD/PTE
entry already exists."

And avoid using "This patch" - what this patch does, should be visible
to the enlightened onlooker.



Thanks Boris, I will incorporate your edits in commit message.


Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Cc: kvm@xxxxxxxxxxxxxxx
Cc: "Radim KrÄmÃÅ" <rkrcmar@xxxxxxxxxx>
---
arch/x86/include/asm/mem_encrypt.h | 6 +++
arch/x86/kernel/head64.c | 9 ++++
arch/x86/kernel/vmlinux.lds.S | 17 +++++++
arch/x86/mm/mem_encrypt_identity.c | 100 +++++++++++++++++++++++++++++--------
4 files changed, 112 insertions(+), 20 deletions(-)

diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
index c064383..802b2eb 100644
--- a/arch/x86/include/asm/mem_encrypt.h
+++ b/arch/x86/include/asm/mem_encrypt.h
@@ -52,6 +52,8 @@ void __init mem_encrypt_init(void);
bool sme_active(void);
bool sev_active(void);
+#define __decrypted __attribute__((__section__(".data..decrypted")))
+
#else /* !CONFIG_AMD_MEM_ENCRYPT */
#define sme_me_mask 0ULL
@@ -77,6 +79,8 @@ 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 __decrypted
+
#endif /* CONFIG_AMD_MEM_ENCRYPT */
/*
@@ -88,6 +92,8 @@ 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_data_decrypted[], __end_data_decrypted[];
+
#endif /* __ASSEMBLY__ */
#endif /* __X86_MEM_ENCRYPT_H__ */
diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
index 8047379..3e03129 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;
@@ -234,6 +235,14 @@ unsigned long __head __startup_64(unsigned long physaddr,
/* Encrypt the kernel and related (if SME is active) */
sme_encrypt_kernel(bp);
+ /* Clear the memory encryption mask from the decrypted section */

End sentences with a fullstop.


Noted.


+ vaddr = (unsigned long)__start_data_decrypted;
+ vaddr_end = (unsigned long)__end_data_decrypted;
+ for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
+ i = pmd_index(vaddr);
+ pmd[i] -= sme_get_me_mask();
+ }

This needs to be no-op on !SME machines. Hint: if (sme_active())...

Sure, I will add the check before calling this loop.


/*
* 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 8bde0a4..0ef9320 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -89,6 +89,21 @@ PHDRS {
note PT_NOTE FLAGS(0); /* ___ */
}
+/*
+ * This section contains data which will be mapped as decrypted. Memory
+ * encryption operates on a page basis. But we make this section a pmd

"... make this section PMD-aligned ..."

Also, avoid the "we" and formulate in passive voic


Noted.



+ * aligned to avoid spliting 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 DATA_DECRYPTED_SECTION \

DATA_DECRYPTED is perfectly fine.


Noted.


+ . = ALIGN(PMD_SIZE); \
+ __start_data_decrypted = .; \
+ *(.data..decrypted); \
+ . = ALIGN(PMD_SIZE); \
+ __end_data_decrypted = .; \
+
SECTIONS
{
#ifdef CONFIG_X86_32
@@ -171,6 +186,8 @@ SECTIONS
/* rarely changed data like cpu maps */
READ_MOSTLY_DATA(INTERNODE_CACHE_BYTES)
+ DATA_DECRYPTED_SECTION
+
/* End of data section */
_edata = .;
} :data
diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index bf6097e..88c1cce 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -51,6 +51,8 @@
(_PAGE_PAT | _PAGE_PWT))
#define PMD_FLAGS_ENC (PMD_FLAGS_LARGE | _PAGE_ENC)
+#define PMD_FLAGS_ENC_WP ((PMD_FLAGS_ENC & ~_PAGE_CACHE_MASK) | \
+ (_PAGE_PAT | _PAGE_PWT))
#define PTE_FLAGS (__PAGE_KERNEL_EXEC & ~_PAGE_GLOBAL)
@@ -59,6 +61,8 @@
(_PAGE_PAT | _PAGE_PWT))
#define PTE_FLAGS_ENC (PTE_FLAGS | _PAGE_ENC)
+#define PTE_FLAGS_ENC_WP ((PTE_FLAGS_ENC & ~_PAGE_CACHE_MASK) | \
+ (_PAGE_PAT | _PAGE_PWT))
struct sme_populate_pgd_data {
void *pgtable_area;
@@ -154,9 +158,6 @@ static void __init sme_populate_pgd_large(struct sme_populate_pgd_data *ppd)
return;
pmd = pmd_offset(pud, ppd->vaddr);
- if (pmd_large(*pmd))
- return;
-
set_pmd(pmd, __pmd(ppd->paddr | ppd->pmd_flags));
}
@@ -182,8 +183,7 @@ static void __init sme_populate_pgd(struct sme_populate_pgd_data *ppd)
return;
pte = pte_offset_map(pmd, ppd->vaddr);
- if (pte_none(*pte))
- set_pte(pte, __pte(ppd->paddr | ppd->pte_flags));
+ set_pte(pte, __pte(ppd->paddr | ppd->pte_flags));
}

This looks like it belongs in a prepatch fix.


Sure, I can do a prepatch for this change.



static void __init __sme_map_range_pmd(struct sme_populate_pgd_data *ppd)
@@ -235,6 +235,11 @@ static void __init sme_map_range_encrypted(struct sme_populate_pgd_data *ppd)
__sme_map_range(ppd, PMD_FLAGS_ENC, PTE_FLAGS_ENC);
}
+static void __init sme_map_range_encrypted_wp(struct sme_populate_pgd_data *ppd)
+{
+ __sme_map_range(ppd, PMD_FLAGS_ENC_WP, PTE_FLAGS_ENC_WP);
+}
+
static void __init sme_map_range_decrypted(struct sme_populate_pgd_data *ppd)
{
__sme_map_range(ppd, PMD_FLAGS_DEC, PTE_FLAGS_DEC);

These changes with the _WP flags and helper addition belong in a pre-patch.

@@ -382,7 +387,10 @@ static void __init build_workarea_map(struct boot_params *bp,
ppd->paddr = workarea_start;
ppd->vaddr = workarea_start;
ppd->vaddr_end = workarea_end;
- sme_map_range_decrypted(ppd);
+ if (sev_active())
+ sme_map_range_encrypted(ppd);
+ else
+ sme_map_range_decrypted(ppd);
/* Flush the TLB - no globals so cr3 is enough */
native_write_cr3(__native_read_cr3());
@@ -439,16 +447,27 @@ static void __init build_workarea_map(struct boot_params *bp,
sme_map_range_decrypted_wp(ppd);
}
- /* Add decrypted workarea mappings to both kernel mappings */
+ /*
+ * When SEV is active, kernel is already encrypted hence mapping
+ * the initial workarea_start as encrypted. When SME is active,
+ * the kernel is not encrypted hence add a decrypted workarea

s/ a//

Noted.



+ * mappings to both kernel mappings.
+ */
ppd->paddr = workarea_start;
ppd->vaddr = workarea_start;
ppd->vaddr_end = workarea_end;
- sme_map_range_decrypted(ppd);
+ if (sev_active())
+ sme_map_range_encrypted(ppd);
+ else
+ sme_map_range_decrypted(ppd);
ppd->paddr = workarea_start;
ppd->vaddr = workarea_start + decrypted_base;
ppd->vaddr_end = workarea_end + decrypted_base;
- sme_map_range_decrypted(ppd);
+ if (sev_active())
+ sme_map_range_encrypted(ppd);
+ else
+ sme_map_range_decrypted(ppd);
wa->kernel_start = kernel_start;
wa->kernel_end = kernel_end;
@@ -491,28 +510,69 @@ static void __init remove_workarea_map(struct sme_workarea_data *wa,
native_write_cr3(__native_read_cr3());
}
+static void __init decrypt_data_decrypted_section(struct sme_workarea_data *wa,

That function name could use some clarifying change...


How about decrypt_shared_data() ?



+ struct sme_populate_pgd_data *ppd)
+{
+ unsigned long decrypted_start, decrypted_end, decrypted_len;
+
+ /* Physical addresses of decrypted data section */
+ decrypted_start = __pa_symbol(__start_data_decrypted);
+ decrypted_end = ALIGN(__pa_symbol(__end_data_decrypted), PMD_PAGE_SIZE);
+ decrypted_len = decrypted_end - decrypted_start;
+
+ if (!decrypted_len)
+ return;
+
+ /* Add decrypted mapping for the section (identity) */
+ ppd->paddr = decrypted_start;
+ ppd->vaddr = decrypted_start;
+ ppd->vaddr_end = decrypted_end;
+ sme_map_range_decrypted(ppd);
+
+ /* Add encrypted-wp mapping for the section (non-identity) */
+ ppd->paddr = decrypted_start;
+ ppd->vaddr = decrypted_start + wa->decrypted_base;
+ ppd->vaddr_end = decrypted_end + wa->decrypted_base;
+ sme_map_range_encrypted_wp(ppd);
+
+ /* Perform in-place decryption */
+ sme_encrypt_execute(decrypted_start,
+ decrypted_start + wa->decrypted_base,
+ decrypted_len, wa->workarea_start,
+ (unsigned long)ppd->pgd);
+
+ ppd->vaddr = decrypted_start + wa->decrypted_base;
+ ppd->vaddr_end = decrypted_end + wa->decrypted_base;
+ sme_clear_pgd(ppd);
+}
+
void __init sme_encrypt_kernel(struct boot_params *bp)
{
struct sme_populate_pgd_data ppd;
struct sme_workarea_data wa;
- if (!sme_active())
+ if (!mem_encrypt_active())
return;
build_workarea_map(bp, &wa, &ppd);
- /* When SEV is active, encrypt kernel and initrd */
- sme_encrypt_execute(wa.kernel_start,
- wa.kernel_start + wa.decrypted_base,
- wa.kernel_len, wa.workarea_start,
- (unsigned long)ppd.pgd);
-
- if (wa.initrd_len)
- sme_encrypt_execute(wa.initrd_start,
- wa.initrd_start + wa.decrypted_base,
- wa.initrd_len, wa.workarea_start,
+ /* When SME is active, encrypt kernel and initrd */
+ if (sme_active()) {
+ sme_encrypt_execute(wa.kernel_start,
+ wa.kernel_start + wa.decrypted_base,
+ wa.kernel_len, wa.workarea_start,
(unsigned long)ppd.pgd);
+ if (wa.initrd_len)
+ sme_encrypt_execute(wa.initrd_start,
+ wa.initrd_start + wa.decrypted_base,
+ wa.initrd_len, wa.workarea_start,
+ (unsigned long)ppd.pgd);
+ }
+
+ /* Decrypt the contents of .data..decrypted section */
+ decrypt_data_decrypted_section(&wa, &ppd);
+
remove_workarea_map(&wa, &ppd);
}
--
2.7.4