Re: [PATCH v8 1/2] x86/mm: add .bss..decrypted section to hold shared variables

Borislav Petkov
2018-09-14T07:11:09Z

On Thu, Sep 13, 2018 at 04:51:10PM -0500, Brijesh Singh wrote:
> kvmclock defines few static variables which are shared with the
> hypervisor during the kvmclock initialization.

...

> 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();
> + }
> + }

Why isn't this chunk at the end of sme_encrypt_kernel() instead of here?

> + /*
> * 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 = .;

Putting it in the BSS would need a bit of care in the future as it poses
a certain ordering on the calls in x86_64_start_kernel() (not that there
isn't any now :)):

You have:

x86_64_start_kernel:

...

clear_bss();

...

x86_64_start_reservations() -> ... -> setup_arch() -> kvmclock_init()

so it would be prudent to put at least a comment somewhere, say, over
the definition of BSS_DECRYPTED or so, that attention should be paid to
the clear_bss() call early.

Thx.

--
Regards/Gruss,
Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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