Re: [RFC PATCH V2 01/18] x86/sev: Pvalidate memory gab for decompressing kernel

[Tianyu Lan <ltykernel@gmail.com>]

On 12/6/2022 5:16 PM, Gupta, Pankaj wrote:
> Hi Tianyu,
> 
> Some minor nits below.
>
Thanks a lot to review. I will change these in the next version.

>> Pvalidate needed pages for decompressing kernel. The E820_TYPE_RAM
>> entry includes only validated memory. The kernel expects that the
>> RAM entry's addr is fixed while the entry size is to be extended
>> to cover addresses to the start of next entry. This patch increases
>> the RAM entry size to cover all possilble memory addresses until
> 
> s/possilble/possible
> 
>> init_size.
>>
>> Signed-off-by: Tianyu Lan <tiala@microsoft.com>
>> ---
>>   arch/x86/boot/compressed/head_64.S |  8 +++
>>   arch/x86/boot/compressed/sev.c     | 84 ++++++++++++++++++++++++++++++
>>   2 files changed, 92 insertions(+)
>>
>> diff --git a/arch/x86/boot/compressed/head_64.S 
>> b/arch/x86/boot/compressed/head_64.S
>> index d33f060900d2..818edaf5d0cf 100644
>> --- a/arch/x86/boot/compressed/head_64.S
>> +++ b/arch/x86/boot/compressed/head_64.S
>> @@ -348,6 +348,14 @@ SYM_CODE_START(startup_64)
>>       cld
>>       cli
>> +#ifdef CONFIG_AMD_MEM_ENCRYPT
>> +    /* pvalidate memory on demand if SNP is enabled. */
>> +    pushq    %rsi
>> +    movq    %rsi, %rdi
>> +    call     pvalidate_for_startup_64
>> +    popq    %rsi
>> +#endif
>> +
>>       /* Setup data segments. */
>>       xorl    %eax, %eax
>>       movl    %eax, %ds
>> diff --git a/arch/x86/boot/compressed/sev.c 
>> b/arch/x86/boot/compressed/sev.c
>> index 960968f8bf75..3a5a1ab16095 100644
>> --- a/arch/x86/boot/compressed/sev.c
>> +++ b/arch/x86/boot/compressed/sev.c
>> @@ -12,8 +12,10 @@
>>    */
>>   #include "misc.h"
>> +#include <asm/msr-index.h>
>>   #include <asm/pgtable_types.h>
>>   #include <asm/sev.h>
>> +#include <asm/svm.h>
>>   #include <asm/trapnr.h>
>>   #include <asm/trap_pf.h>
>>   #include <asm/msr-index.h>
>> @@ -21,6 +23,7 @@
>>   #include <asm/ptrace.h>
>>   #include <asm/svm.h>
>>   #include <asm/cpuid.h>
>> +#include <asm/e820/types.h>
>>   #include "error.h"
>>   #include "../msr.h"
>> @@ -117,6 +120,22 @@ static enum es_result vc_read_mem(struct 
>> es_em_ctxt *ctxt,
>>   /* Include code for early handlers */
>>   #include "../../kernel/sev-shared.c"
>> +/* Check SEV-SNP via MSR */
>> +static bool sev_snp_runtime_check(void)
>> +{
>> +    unsigned long low, high;
>> +    u64 val;
>> +
>> +    asm volatile("rdmsr\n" : "=a" (low), "=d" (high) :
>> +            "c" (MSR_AMD64_SEV));
>> +
>> +    val = (high << 32) | low;
>> +    if (val & MSR_AMD64_SEV_SNP_ENABLED)
>> +        return true;
>> +
>> +    return false;
>> +}
>> +
>>   static inline bool sev_snp_enabled(void)
>>   {
>>       return sev_status & MSR_AMD64_SEV_SNP_ENABLED;
>> @@ -456,3 +475,68 @@ void sev_prep_identity_maps(unsigned long 
>> top_level_pgt)
>>       sev_verify_cbit(top_level_pgt);
>>   }
>> +
>> +static void extend_e820_on_demand(struct boot_e820_entry *e820_entry,
>> +                  u64 needed_ram_end)
>> +{
>> +    u64 end, paddr;
>> +    unsigned long eflags;
>> +    int rc;
>> +
>> +    if (!e820_entry)
>> +        return;
>> +
>> +    /* Validated memory must be aligned by PAGE_SIZE. */
>> +    end = ALIGN(e820_entry->addr + e820_entry->size, PAGE_SIZE);
>> +    if (needed_ram_end > end && e820_entry->type == E820_TYPE_RAM) {
> 
> maybe "if check" can be used to return from here, would read code better?
> 
>> +        for (paddr = end; paddr < needed_ram_end; paddr += PAGE_SIZE) 
> Maybe we could reuse 'pvalidate_pages' here?
> 
>> +            rc = pvalidate(paddr, RMP_PG_SIZE_4K, true);
>> +            if (rc) {
>> +                error("Failed to validate address.n");
> 
> Would it make sense to print the cause of failure here?
> 
>> +                return;
>> +            }
>> +        }
>> +        e820_entry->size = needed_ram_end - e820_entry->addr;
>> +    }
>> +}
>> +
>> +/*
>> + * Explicitly pvalidate needed pages for decompressing the kernel.
>> + * The E820_TYPE_RAM entry includes only validated memory. The kernel
>> + * expects that the RAM entry's addr is fixed while the entry size is 
>> to be
>> + * extended to cover addresses to the start of next entry.
>> + * The function increases the RAM entry size to cover all possible 
>> memory
>> + * addresses until init_size.
>> + * For example,  init_end = 0x4000000,
>> + * [RAM: 0x0 - 0x0],                       M[RAM: 0x0 - 0xa0000]
>> + * [RSVD: 0xa0000 - 0x10000]                [RSVD: 0xa0000 - 0x10000]
>> + * [ACPI: 0x10000 - 0x20000]      ==>       [ACPI: 0x10000 - 0x20000]
>> + * [RSVD: 0x800000 - 0x900000]              [RSVD: 0x800000 - 0x900000]
>> + * [RAM: 0x1000000 - 0x2000000]            M[RAM: 0x1000000 - 0x2001000]
>> + * [RAM: 0x2001000 - 0x2007000]            M[RAM: 0x2001000 - 0x4000000]
>> + * Other RAM memory after init_end is pvalidated by 
>> ms_hyperv_init_platform
>> + */
>> +__visible void pvalidate_for_startup_64(struct boot_params *boot_params)
>> +{
>> +    struct boot_e820_entry *e820_entry;
>> +    u64 init_end =
>> +        boot_params->hdr.pref_address + boot_params->hdr.init_size;
>> +    u8 i, nr_entries = boot_params->e820_entries;
>> +    u64 needed_end;
> 
> Could not very well interpret the name 'needed_end'.
> 
>> +
>> +    if (!sev_snp_runtime_check())
>> +        return;
>> +
>> +    for (i = 0; i < nr_entries; ++i) {
>> +        /* Pvalidate memory holes in e820 RAM entries. */
> 
> Pvalidate and pvalidate names are used interchangeably in comments.
> 
>> +        e820_entry = &boot_params->e820_table[i];
>> +        if (i < nr_entries - 1) {
>> +            needed_end = boot_params->e820_table[i + 1].addr;
>> +            if (needed_end < e820_entry->addr)
>> +                error("e820 table is not sorted.\n");
>> +        } else {
>> +            needed_end = init_end;
>> +        }
>> +        extend_e820_on_demand(e820_entry, needed_end);
>> +    }
>> +}
>