Re: [RFC PATCH v4 26/28] x86: Allow kexec to be used with SME

Borislav Petkov
2017-02-28T10:35:05Z

On Thu, Feb 16, 2017 at 09:47:55AM -0600, Tom Lendacky wrote:
> Provide support so that kexec can be used to boot a kernel when SME is
> enabled.
>
> Support is needed to allocate pages for kexec without encryption. This
> is needed in order to be able to reboot in the kernel in the same manner
> as originally booted.
>
> Additionally, when shutting down all of the CPUs we need to be sure to
> disable caches, flush the caches and then halt. This is needed when booting
> from a state where SME was not active into a state where SME is active.
> Without these steps, it is possible for cache lines to exist for the same
> physical location but tagged both with and without the encryption bit. This
> can cause random memory corruption when caches are flushed depending on
> which cacheline is written last.
>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
> arch/x86/include/asm/cacheflush.h | 2 ++
> arch/x86/include/asm/init.h | 1 +
> arch/x86/include/asm/mem_encrypt.h | 10 ++++++++
> arch/x86/include/asm/pgtable_types.h | 1 +
> arch/x86/kernel/machine_kexec_64.c | 3 ++
> arch/x86/kernel/process.c | 43 +++++++++++++++++++++++++++++++++-
> arch/x86/kernel/smp.c | 4 ++-
> arch/x86/mm/ident_map.c | 6 +++--
> arch/x86/mm/pageattr.c | 2 ++
> include/linux/mem_encrypt.h | 10 ++++++++
> kernel/kexec_core.c | 24 +++++++++++++++++++
> 11 files changed, 100 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/include/asm/cacheflush.h b/arch/x86/include/asm/cacheflush.h
> index 33ae60a..2180cd5 100644
> --- a/arch/x86/include/asm/cacheflush.h
> +++ b/arch/x86/include/asm/cacheflush.h
> @@ -48,8 +48,10 @@
> int set_memory_rw(unsigned long addr, int numpages);
> int set_memory_np(unsigned long addr, int numpages);
> int set_memory_4k(unsigned long addr, int numpages);
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> int set_memory_encrypted(unsigned long addr, int numpages);
> int set_memory_decrypted(unsigned long addr, int numpages);
> +#endif
>
> int set_memory_array_uc(unsigned long *addr, int addrinarray);
> int set_memory_array_wc(unsigned long *addr, int addrinarray);

Hmm, why is this ifdeffery creeping in now?

Just supply !CONFIG_AMD_MEM_ENCRYPT versions which don't do anything but
return the address.

> diff --git a/arch/x86/include/asm/init.h b/arch/x86/include/asm/init.h
> index 737da62..b2ec511 100644
> --- a/arch/x86/include/asm/init.h
> +++ b/arch/x86/include/asm/init.h
> @@ -6,6 +6,7 @@ struct x86_mapping_info {
> void *context; /* context for alloc_pgt_page */
> unsigned long pmd_flag; /* page flag for PMD entry */
> unsigned long offset; /* ident mapping offset */
> + unsigned long kernpg_flag; /* kernel pagetable flag override */
> };
>
> int kernel_ident_mapping_init(struct x86_mapping_info *info, pgd_t *pgd_page,
> diff --git a/arch/x86/include/asm/mem_encrypt.h b/arch/x86/include/asm/mem_encrypt.h
> index 5a17f1b..1fd5426 100644
> --- a/arch/x86/include/asm/mem_encrypt.h
> +++ b/arch/x86/include/asm/mem_encrypt.h
> @@ -64,6 +64,16 @@ static inline u64 sme_dma_mask(void)
> return 0ULL;
> }
>
> +static inline int set_memory_encrypted(unsigned long vaddr, int numpages)
> +{
> + return 0;
> +}
> +
> +static inline int set_memory_decrypted(unsigned long vaddr, int numpages)
> +{
> + return 0;
> +}
> +
> #endif
>
> static inline void __init sme_early_encrypt(resource_size_t paddr,
> diff --git a/arch/x86/include/asm/pgtable_types.h b/arch/x86/include/asm/pgtable_types.h
> index f00e70f..456c5cc 100644
> --- a/arch/x86/include/asm/pgtable_types.h
> +++ b/arch/x86/include/asm/pgtable_types.h
> @@ -213,6 +213,7 @@ enum page_cache_mode {
> #define PAGE_KERNEL __pgprot(__PAGE_KERNEL | _PAGE_ENC)
> #define PAGE_KERNEL_RO __pgprot(__PAGE_KERNEL_RO | _PAGE_ENC)
> #define PAGE_KERNEL_EXEC __pgprot(__PAGE_KERNEL_EXEC | _PAGE_ENC)
> +#define PAGE_KERNEL_EXEC_NOENC __pgprot(__PAGE_KERNEL_EXEC)
> #define PAGE_KERNEL_RX __pgprot(__PAGE_KERNEL_RX | _PAGE_ENC)
> #define PAGE_KERNEL_NOCACHE __pgprot(__PAGE_KERNEL_NOCACHE | _PAGE_ENC)
> #define PAGE_KERNEL_LARGE __pgprot(__PAGE_KERNEL_LARGE | _PAGE_ENC)
> diff --git a/arch/x86/kernel/machine_kexec_64.c b/arch/x86/kernel/machine_kexec_64.c
> index 307b1f4..b01648c 100644
> --- a/arch/x86/kernel/machine_kexec_64.c
> +++ b/arch/x86/kernel/machine_kexec_64.c
> @@ -76,7 +76,7 @@ static int init_transition_pgtable(struct kimage *image, pgd_t *pgd)
> set_pmd(pmd, __pmd(__pa(pte) | _KERNPG_TABLE));
> }
> pte = pte_offset_kernel(pmd, vaddr);
> - set_pte(pte, pfn_pte(paddr >> PAGE_SHIFT, PAGE_KERNEL_EXEC));
> + set_pte(pte, pfn_pte(paddr >> PAGE_SHIFT, PAGE_KERNEL_EXEC_NOENC));
> return 0;
> err:
> free_transition_pgtable(image);
> @@ -104,6 +104,7 @@ static int init_pgtable(struct kimage *image, unsigned long start_pgtable)
> .alloc_pgt_page = alloc_pgt_page,
> .context = image,
> .pmd_flag = __PAGE_KERNEL_LARGE_EXEC,
> + .kernpg_flag = _KERNPG_TABLE_NOENC,
> };
> unsigned long mstart, mend;
> pgd_t *level4p;
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index 3ed869c..9b01261 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -279,8 +279,43 @@ bool xen_set_default_idle(void)
> return ret;
> }
> #endif
> -void stop_this_cpu(void *dummy)
> +
> +static bool is_smt_thread(int cpu)
> {
> +#ifdef CONFIG_SCHED_SMT
> + if (cpumask_test_cpu(smp_processor_id(), cpu_smt_mask(cpu)))
> + return true;
> +#endif

No, no sched stuff in here. Just

if (cpumask_test_cpu(smp_processor_id(), topology_sibling_cpumask(cpu)))

> + return false;
> +}
> +
> +void stop_this_cpu(void *data)
> +{
> + atomic_t *stopping_cpu = data;
> + bool do_cache_disable = false;
> + bool do_wbinvd = false;
> +
> + if (stopping_cpu) {
> + int stopping_id = atomic_read(stopping_cpu);
> + struct cpuinfo_x86 *c = &cpu_data(stopping_id);
> +
> + /*
> + * If the processor supports SME then we need to clear
> + * out cache information before halting it because we could
> + * be performing a kexec. With kexec, going from SME
> + * inactive to SME active requires clearing cache entries
> + * so that addresses without the encryption bit set don't
> + * corrupt the same physical address that has the encryption
> + * bit set when caches are flushed. If this is not an SMT
> + * thread of the stopping CPU then we disable caching at this
> + * point to keep the cache clean.
> + */
> + if (cpu_has(c, X86_FEATURE_SME)) {
> + do_cache_disable = !is_smt_thread(stopping_id);
> + do_wbinvd = true;
> + }
> + }

Let's simplify this (diff ontop of yours). Notice the sme_active() call
in there - I believe we want to do this only when SME is active - not on
any CPU which merely supports SME.

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 9b012612698d..e771d7a42e49 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -296,9 +296,6 @@ void stop_this_cpu(void *data)
bool do_wbinvd = false;

if (stopping_cpu) {
- int stopping_id = atomic_read(stopping_cpu);
- struct cpuinfo_x86 *c = &cpu_data(stopping_id);
-
/*
* If the processor supports SME then we need to clear
* out cache information before halting it because we could
@@ -310,8 +307,8 @@ void stop_this_cpu(void *data)
* thread of the stopping CPU then we disable caching at this
* point to keep the cache clean.
*/
- if (cpu_has(c, X86_FEATURE_SME)) {
- do_cache_disable = !is_smt_thread(stopping_id);
+ if (sme_active()) {
+ do_cache_disable = !is_smt_thread(atomic_read(stopping_cpu));
do_wbinvd = true;
}
}

> local_irq_disable();
> /*
> * Remove this CPU:
> @@ -289,6 +324,12 @@ void stop_this_cpu(void *dummy)
> disable_local_APIC();
> mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
>
> + if (do_cache_disable)
> + write_cr0(read_cr0() | X86_CR0_CD);

Question: what clears CD back again? The CPU online path?

> +
> + if (do_wbinvd)
> + wbinvd();
> +

Ok, so this whole shebang is pretty much crippling the machine.
And, AFAICT, you're doing this now from smp_stop_nmi_callback() and
smp_reboot_interrupt() as they both pass a !NULL arg to stop_this_cpu().

And AFAICT those are not all cases where we kexec.

What you need instead, IMO, is __crash_kexec() ->
machine_crash_shutdown() -> native_machine_crash_shutdown() and put all
the SME special handling there.

I *think*.

> for (;;)
> halt();
> }

...

> diff --git a/arch/x86/mm/pageattr.c b/arch/x86/mm/pageattr.c
> index 9710f5c..46cc89d 100644
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -1742,6 +1742,7 @@ int set_memory_4k(unsigned long addr, int numpages)
> __pgprot(0), 1, 0, NULL);
> }
>
> +#ifdef CONFIG_AMD_MEM_ENCRYPT
> static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> {
> struct cpa_data cpa;
> @@ -1807,6 +1808,7 @@ int set_memory_decrypted(unsigned long addr, int numpages)
> return __set_memory_enc_dec(addr, numpages, false);
> }
> EXPORT_SYMBOL(set_memory_decrypted);
> +#endif /* CONFIG_AMD_MEM_ENCRYPT */

Btw, I don't see those things used in modules to justify the
EXPORT_SYMBOL(). And it should be EXPORT_SYMBOL_GPL() since it is a new
symbol.

So you could put those wrappers in a header and do the ifdeffery there and
__set_memory_enc_dec() you can do like this:

static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
{
if (!IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT))
return 0;

...

}

so that you can save yourself the ifdeffery. The compiler would still
parse the function body so everything else used in there would have to
be defined too, even in the !CONFIG_AMD_MEM_ENCRYPT case.

>
> int set_pages_uc(struct page *page, int numpages)
> {
> diff --git a/include/linux/mem_encrypt.h b/include/linux/mem_encrypt.h
> index 6829ff1..913cf80 100644
> --- a/include/linux/mem_encrypt.h
> +++ b/include/linux/mem_encrypt.h
> @@ -34,6 +34,16 @@ static inline u64 sme_dma_mask(void)
> return 0ULL;
> }
>
> +static inline int set_memory_encrypted(unsigned long vaddr, int numpages)
> +{
> + return 0;
> +}
> +
> +static inline int set_memory_decrypted(unsigned long vaddr, int numpages)
> +{
> + return 0;
> +}
> +
> #endif
>
> #endif /* CONFIG_AMD_MEM_ENCRYPT */
> diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
> index 5617cc4..ab62f41 100644
> --- a/kernel/kexec_core.c
> +++ b/kernel/kexec_core.c
> @@ -38,6 +38,7 @@
> #include <linux/syscore_ops.h>
> #include <linux/compiler.h>
> #include <linux/hugetlb.h>
> +#include <linux/mem_encrypt.h>
>
> #include <asm/page.h>
> #include <asm/sections.h>
> @@ -315,6 +316,18 @@ static struct page *kimage_alloc_pages(gfp_t gfp_mask, unsigned int order)
> count = 1 << order;
> for (i = 0; i < count; i++)
> SetPageReserved(pages + i);
> +
> + /*
> + * If SME is active we need to be sure that kexec pages are
> + * not encrypted because when we boot to the new kernel the
> + * pages won't be accessed encrypted (initially).
> + */
> + if (sme_active()) {
> + void *vaddr = page_address(pages);
> +
> + set_memory_decrypted((unsigned long)vaddr, count);
> + memset(vaddr, 0, count * PAGE_SIZE);

Why the memset?

> + }
> }
>
> return pages;
> @@ -326,6 +339,17 @@ static void kimage_free_pages(struct page *page)
>
> order = page_private(page);
> count = 1 << order;
> +
> + /*
> + * If SME is active we need to reset the pages back to being an
> + * encrypted mapping before freeing them.
> + */
> + if (sme_active()) {
> + void *vaddr = page_address(page);
> +
> + set_memory_encrypted((unsigned long)vaddr, count);

if (sme_active())
set_memory_encrypted((unsigned long)page_address(page), count);

looks ok to me too.

--
Regards/Gruss,
Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.

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