Re: [PATCH v5 28/32] x86/mm, kexec: Allow kexec to be used with SME

Tom Lendacky
2017-05-19T20:45:44Z

On 5/17/2017 2:17 PM, Borislav Petkov wrote:
> On Tue, Apr 18, 2017 at 04:21:21PM -0500, 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
>> 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 (or vice-versa).
>> 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/init.h | 1 +
>> arch/x86/include/asm/irqflags.h | 5 +++++
>> arch/x86/include/asm/kexec.h | 8 ++++++++
>> arch/x86/include/asm/pgtable_types.h | 1 +
>> arch/x86/kernel/machine_kexec_64.c | 35 +++++++++++++++++++++++++++++++++-
>> arch/x86/kernel/process.c | 26 +++++++++++++++++++++++--
>> arch/x86/mm/ident_map.c | 11 +++++++----
>> include/linux/kexec.h | 14 ++++++++++++++
>> kernel/kexec_core.c | 7 +++++++
>> 9 files changed, 101 insertions(+), 7 deletions(-)
>
> ...
>
>> @@ -86,7 +86,7 @@ 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);
>> @@ -114,6 +114,7 @@ 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;
>> @@ -597,3 +598,35 @@ void arch_kexec_unprotect_crashkres(void)
>> {
>> kexec_mark_crashkres(false);
>> }
>> +
>> +int arch_kexec_post_alloc_pages(void *vaddr, unsigned int pages, gfp_t gfp)
>> +{
>> + int ret;
>> +
>> + if (sme_active()) {
>
> if (!sme_active())
> return 0;
>
> /*
> * If SME...
>

Ok.

>
>> + /*
>> + * 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).
>> + */
>> + ret = set_memory_decrypted((unsigned long)vaddr, pages);
>> + if (ret)
>> + return ret;
>> +
>> + if (gfp & __GFP_ZERO)
>> + memset(vaddr, 0, pages * PAGE_SIZE);
>
> This function is called after alloc_pages() which already zeroes memory
> when __GFP_ZERO is supplied.
>
> If you need to clear the memory *after* set_memory_encrypted() happens,
> then you should probably mask out __GFP_ZERO before the alloc_pages()
> call so as not to do it twice.

I'll look into that. I could put the memset() at the end of this
function so that it is done here no matter what. And update the
default arch_kexec_post_alloc_pages() to also do the memset(). It
just hides the clearing of the pages a bit though by doing that.

>
>> + }
>> +
>> + return 0;
>> +}
>> +
>> +void arch_kexec_pre_free_pages(void *vaddr, unsigned int pages)
>> +{
>> + if (sme_active()) {
>> + /*
>> + * If SME is active we need to reset the pages back to being
>> + * an encrypted mapping before freeing them.
>> + */
>> + set_memory_encrypted((unsigned long)vaddr, pages);
>> + }
>> +}
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index 0bb8842..f4e5de6 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -24,6 +24,7 @@
>> #include <linux/cpuidle.h>
>> #include <trace/events/power.h>
>> #include <linux/hw_breakpoint.h>
>> +#include <linux/kexec.h>
>> #include <asm/cpu.h>
>> #include <asm/apic.h>
>> #include <asm/syscalls.h>
>> @@ -355,8 +356,25 @@ bool xen_set_default_idle(void)
>> return ret;
>> }
>> #endif
>> +
>> void stop_this_cpu(void *dummy)
>> {
>> + bool do_wbinvd_halt = false;
>> +
>> + if (kexec_in_progress && boot_cpu_has(X86_FEATURE_SME)) {
>> + /*
>> + * If we are performing a kexec and the processor supports
>> + * SME then we need to clear out cache information before
>> + * halting. 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. Perform a wbinvd followed by a halt to achieve
>> + * this.
>> + */
>> + do_wbinvd_halt = true;
>> + }
>> +
>> local_irq_disable();
>> /*
>> * Remove this CPU:
>> @@ -365,8 +383,12 @@ void stop_this_cpu(void *dummy)
>> disable_local_APIC();
>> mcheck_cpu_clear(this_cpu_ptr(&cpu_info));
>>
>> - for (;;)
>> - halt();
>> + for (;;) {
>> + if (do_wbinvd_halt)
>> + native_wbinvd_halt();
>
> No need for that native_wbinvd_halt() thing:
>
> for (;;) {
> if (do_wbinvd)
> wbinvd();
>
> halt();
> }
>

Actually there is. The above will result in data in the cache because
halt() turns into a function call if CONFIG_PARAVIRT is defined (refer
to the comment above where do_wbinvd_halt is set to true). I could make
this a native_wbinvd() and native_halt() as long as those are
guaranteed to never turn into function calls. But never say never, so
that's why I created native_wbinvd_halt().

Thanks,
Tom

>> /*
>> diff --git a/arch/x86/mm/ident_map.c b/arch/x86/mm/ident_map.c
>> index 04210a2..2c9fd3e 100644
>> --- a/arch/x86/mm/ident_map.c
>> +++ b/arch/x86/mm/ident_map.c
>> @@ -20,6 +20,7 @@ static void ident_pmd_init(struct x86_mapping_info *info, pmd_t *pmd_page,
>> static int ident_pud_init(struct x86_mapping_info *info, pud_t *pud_page,
>> unsigned long addr, unsigned long end)
>> {
>> + unsigned long kernpg_flag = info->kernpg_flag ? : _KERNPG_TABLE;
>
> You're already supplying a x86_mapping_info and thus you can init
> kernpg_flag to default _KERNPG_TABLE and override it in the SME+kexec
> case, as you already do. And this way you can simply do:
>
> set_pud(pud, __pud(__pa(pmd) | info->kernpg_flag));
>
> here and in the other pagetable functions I've snipped below, and save
> yourself some lines.

Ok, I'll check into that.

Thanks,
Tom

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