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Date:   Sat, 11 Mar 2023 18:14:32 +0300
From:   Evgeniy Baskov <baskov@...ras.ru>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Borislav Petkov <bp@...en8.de>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        "Limonciello, Mario" <mario.limonciello@....com>,
        joeyli <jlee@...e.com>, lvc-project@...uxtesting.org,
        x86@...nel.org, linux-efi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 23/26] efi/libstub: Use memory attribute protocol

On 2023-03-10 19:13, Ard Biesheuvel wrote:
> On Thu, 15 Dec 2022 at 13:42, Evgeniy Baskov <baskov@...ras.ru> wrote:
>> 
>> Add EFI_MEMORY_ATTRIBUTE_PROTOCOL as preferred alternative to DXE
>> services for changing memory attributes in the EFISTUB.
>> 
>> Use DXE services only as a fallback in case aforementioned protocol
>> is not supported by UEFI implementation.
>> 
>> Move DXE services initialization code closer to the place they are 
>> used
>> to match EFI_MEMORY_ATTRIBUTE_PROTOCOL initialization code.
>> 
>> Tested-by: Mario Limonciello <mario.limonciello@....com>
>> Tested-by: Peter Jones <pjones@...hat.com>
>> Signed-off-by: Evgeniy Baskov <baskov@...ras.ru>
> 
> I'm not convinced about the use of the DXE services for this, and I
> think we should replace this patch with changes that base all the new
> protection code on the EFI memory attributes protocol only.
> 
> We introduced that DXE code to remove protections from memory that was
> mapped read-only and/or non-executable, and described as such in the
> GCD memory map.
> 
> Using it to manipulate restricted permissions like this is quite a
> different thing, and sadly (at least in EDK2), the GCD system memory
> map is not kept in sync with the updated permissions, i.e, the W^X
> protections for loaded images and the NX protection for arbitrary page
> allocations are both based on the PI CPU arch protocol, which
> manipulates the page tables directly, but does not record the modified
> attributes in the GCD or EFI memory maps, as this would result in
> massive fragmentation and break lots of other things.
> 
> That means that, except for the specific use case for which we
> introduced the DXE services calls, the only reliable way to figure out
> what permission attributes a certain range of memory is using is the
> EFI memory attributes protocol, and I don't think we should use
> anything else for tightening down these protections.
> 
> 

Makes sense. I'll change the patch to only widen the permissions with
DXE, so it aligns with the original intention. And only apply stricter
permissions with memory attribute protocol.

Thanks!

> 
> 
>> ---
>>  drivers/firmware/efi/libstub/mem.c      | 168 
>> ++++++++++++++++++------
>>  drivers/firmware/efi/libstub/x86-stub.c |  17 ---
>>  2 files changed, 128 insertions(+), 57 deletions(-)
>> 
>> diff --git a/drivers/firmware/efi/libstub/mem.c 
>> b/drivers/firmware/efi/libstub/mem.c
>> index 3e47e5931f04..07d54c88c62e 100644
>> --- a/drivers/firmware/efi/libstub/mem.c
>> +++ b/drivers/firmware/efi/libstub/mem.c
>> @@ -5,6 +5,9 @@
>> 
>>  #include "efistub.h"
>> 
>> +const efi_dxe_services_table_t *efi_dxe_table;
>> +efi_memory_attribute_protocol_t *efi_mem_attrib_proto;
>> +
>>  /**
>>   * efi_get_memory_map() - get memory map
>>   * @map:               pointer to memory map pointer to which to 
>> assign the
>> @@ -129,66 +132,47 @@ void efi_free(unsigned long size, unsigned long 
>> addr)
>>         efi_bs_call(free_pages, addr, nr_pages);
>>  }
>> 
>> -/**
>> - * efi_adjust_memory_range_protection() - change memory range 
>> protection attributes
>> - * @start:     memory range start address
>> - * @size:      memory range size
>> - *
>> - * Actual memory range for which memory attributes are modified is
>> - * the smallest ranged with start address and size aligned to 
>> EFI_PAGE_SIZE
>> - * that includes [start, start + size].
>> - *
>> - * @return: status code
>> - */
>> -efi_status_t efi_adjust_memory_range_protection(unsigned long start,
>> -                                               unsigned long size,
>> -                                               unsigned long 
>> attributes)
>> +static void retrieve_dxe_table(void)
>> +{
>> +       efi_dxe_table = 
>> get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
>> +       if (efi_dxe_table &&
>> +           efi_dxe_table->hdr.signature != 
>> EFI_DXE_SERVICES_TABLE_SIGNATURE) {
>> +               efi_warn("Ignoring DXE services table: invalid 
>> signature\n");
>> +               efi_dxe_table = NULL;
>> +       }
>> +}
>> +
>> +static efi_status_t adjust_mem_attrib_dxe(efi_physical_addr_t 
>> rounded_start,
>> +                                         efi_physical_addr_t 
>> rounded_end,
>> +                                         unsigned long attributes)
>>  {
>>         efi_status_t status;
>>         efi_gcd_memory_space_desc_t desc;
>> -       efi_physical_addr_t end, next;
>> -       efi_physical_addr_t rounded_start, rounded_end;
>> +       efi_physical_addr_t end, next, start;
>>         efi_physical_addr_t unprotect_start, unprotect_size;
>> 
>> -       if (efi_dxe_table == NULL)
>> -               return EFI_UNSUPPORTED;
>> +       if (!efi_dxe_table) {
>> +               retrieve_dxe_table();
>> 
>> -       /*
>> -        * This function should not be used to modify attributes
>> -        * other than writable/executable.
>> -        */
>> -
>> -       if ((attributes & ~(EFI_MEMORY_RO | EFI_MEMORY_XP)) != 0)
>> -               return EFI_INVALID_PARAMETER;
>> -
>> -       /*
>> -        * Disallow simultaniously executable and writable memory
>> -        * to inforce W^X policy if direct extraction code is enabled.
>> -        */
>> -
>> -       if ((attributes & (EFI_MEMORY_RO | EFI_MEMORY_XP)) == 0) {
>> -               efi_warn("W^X violation at [%08lx,%08lx]\n",
>> -                        (unsigned long)rounded_start,
>> -                        (unsigned long)rounded_end);
>> +               if (!efi_dxe_table)
>> +                       return EFI_UNSUPPORTED;
>>         }
>> 
>> -       rounded_start = rounddown(start, EFI_PAGE_SIZE);
>> -       rounded_end = roundup(start + size, EFI_PAGE_SIZE);
>> -
>>         /*
>>          * Don't modify memory region attributes, they are
>>          * already suitable, to lower the possibility to
>>          * encounter firmware bugs.
>>          */
>> 
>> -       for (end = start + size; start < end; start = next) {
>> +
>> +       for (start = rounded_start, end = rounded_end; start < end; 
>> start = next) {
>> 
>>                 status = efi_dxe_call(get_memory_space_descriptor,
>>                                       start, &desc);
>> 
>>                 if (status != EFI_SUCCESS) {
>>                         efi_warn("Unable to get memory descriptor at 
>> %lx\n",
>> -                                start);
>> +                                (unsigned long)start);
>>                         return status;
>>                 }
>> 
>> @@ -230,3 +214,107 @@ efi_status_t 
>> efi_adjust_memory_range_protection(unsigned long start,
>> 
>>         return EFI_SUCCESS;
>>  }
>> +
>> +static void retrieve_memory_attributes_proto(void)
>> +{
>> +       efi_status_t status;
>> +       efi_guid_t guid = EFI_MEMORY_ATTRIBUTE_PROTOCOL_GUID;
>> +
>> +       status = efi_bs_call(locate_protocol, &guid, NULL,
>> +                            (void **)&efi_mem_attrib_proto);
>> +       if (status != EFI_SUCCESS)
>> +               efi_mem_attrib_proto = NULL;
>> +}
>> +
>> +/**
>> + * efi_adjust_memory_range_protection() - change memory range 
>> protection attributes
>> + * @start:     memory range start address
>> + * @size:      memory range size
>> + *
>> + * Actual memory range for which memory attributes are modified is
>> + * the smallest ranged with start address and size aligned to 
>> EFI_PAGE_SIZE
>> + * that includes [start, start + size].
>> + *
>> + * This function first attempts to use EFI_MEMORY_ATTRIBUTE_PROTOCOL,
>> + * that is a part of UEFI Specification since version 2.10.
>> + * If the protocol is unavailable it falls back to DXE services 
>> functions.
>> + *
>> + * @return: status code
>> + */
>> +efi_status_t efi_adjust_memory_range_protection(unsigned long start,
>> +                                               unsigned long size,
>> +                                               unsigned long 
>> attributes)
>> +{
>> +       efi_status_t status;
>> +       efi_physical_addr_t rounded_start, rounded_end;
>> +       unsigned long attr_clear;
>> +
>> +       /*
>> +        * This function should not be used to modify attributes
>> +        * other than writable/executable.
>> +        */
>> +
>> +       if ((attributes & ~(EFI_MEMORY_RO | EFI_MEMORY_XP)) != 0)
>> +               return EFI_INVALID_PARAMETER;
>> +
>> +       /*
>> +        * Warn if requested to make memory simultaneously
>> +        * executable and writable to enforce W^X policy.
>> +        */
>> +
>> +       if ((attributes & (EFI_MEMORY_RO | EFI_MEMORY_XP)) == 0) {
>> +               efi_warn("W^X violation at  [%08lx,%08lx]",
>> +                        (unsigned long)rounded_start,
>> +                        (unsigned long)rounded_end);
>> +       }
>> +
>> +       rounded_start = rounddown(start, EFI_PAGE_SIZE);
>> +       rounded_end = roundup(start + size, EFI_PAGE_SIZE);
>> +
>> +       if (!efi_mem_attrib_proto) {
>> +               retrieve_memory_attributes_proto();
>> +
>> +               /* Fall back to DXE services if unsupported */
>> +               if (!efi_mem_attrib_proto) {
>> +                       return adjust_mem_attrib_dxe(rounded_start,
>> +                                                    rounded_end,
>> +                                                    attributes);
>> +               }
>> +       }
>> +
>> +       /*
>> +        * Unlike DXE services functions, 
>> EFI_MEMORY_ATTRIBUTE_PROTOCOL
>> +        * does not clear unset protection bit, so it needs to be 
>> cleared
>> +        * explcitly
>> +        */
>> +
>> +       attr_clear = ~attributes &
>> +                    (EFI_MEMORY_RO | EFI_MEMORY_XP | EFI_MEMORY_RP);
>> +
>> +       status = efi_call_proto(efi_mem_attrib_proto,
>> +                               clear_memory_attributes,
>> +                               rounded_start,
>> +                               rounded_end - rounded_start,
>> +                               attr_clear);
>> +       if (status != EFI_SUCCESS) {
>> +               efi_warn("Failed to clear memory attributes at 
>> [%08lx,%08lx]: %lx",
>> +                        (unsigned long)rounded_start,
>> +                        (unsigned long)rounded_end,
>> +                        status);
>> +               return status;
>> +       }
>> +
>> +       status = efi_call_proto(efi_mem_attrib_proto,
>> +                               set_memory_attributes,
>> +                               rounded_start,
>> +                               rounded_end - rounded_start,
>> +                               attributes);
>> +       if (status != EFI_SUCCESS) {
>> +               efi_warn("Failed to set memory attributes at 
>> [%08lx,%08lx]: %lx",
>> +                        (unsigned long)rounded_start,
>> +                        (unsigned long)rounded_end,
>> +                        status);
>> +       }
>> +
>> +       return status;
>> +}
>> diff --git a/drivers/firmware/efi/libstub/x86-stub.c 
>> b/drivers/firmware/efi/libstub/x86-stub.c
>> index 60697fcd8950..06a62b121521 100644
>> --- a/drivers/firmware/efi/libstub/x86-stub.c
>> +++ b/drivers/firmware/efi/libstub/x86-stub.c
>> @@ -23,7 +23,6 @@
>>  #define MAXMEM_X86_64_4LEVEL (1ull << 46)
>> 
>>  const efi_system_table_t *efi_system_table;
>> -const efi_dxe_services_table_t *efi_dxe_table;
>>  u32 image_offset __section(".data");
>>  static efi_loaded_image_t *image __section(".data");
>> 
>> @@ -357,15 +356,6 @@ void __noreturn efi_exit(efi_handle_t handle, 
>> efi_status_t status)
>>  static void setup_sections_memory_protection(unsigned long 
>> image_base)
>>  {
>>  #ifdef CONFIG_EFI_DXE_MEM_ATTRIBUTES
>> -       efi_dxe_table = 
>> get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
>> -
>> -       if (!efi_dxe_table ||
>> -           efi_dxe_table->hdr.signature != 
>> EFI_DXE_SERVICES_TABLE_SIGNATURE) {
>> -               efi_warn("Unable to locate EFI DXE services table\n");
>> -               efi_dxe_table = NULL;
>> -               return;
>> -       }
>> -
>>         /* .setup [image_base, _head] */
>>         efi_adjust_memory_range_protection(image_base,
>>                                            (unsigned long)_head - 
>> image_base,
>> @@ -732,13 +722,6 @@ asmlinkage unsigned long efi_main(efi_handle_t 
>> handle,
>>         if (efi_system_table->hdr.signature != 
>> EFI_SYSTEM_TABLE_SIGNATURE)
>>                 efi_exit(handle, EFI_INVALID_PARAMETER);
>> 
>> -       efi_dxe_table = 
>> get_efi_config_table(EFI_DXE_SERVICES_TABLE_GUID);
>> -       if (efi_dxe_table &&
>> -           efi_dxe_table->hdr.signature != 
>> EFI_DXE_SERVICES_TABLE_SIGNATURE) {
>> -               efi_warn("Ignoring DXE services table: invalid 
>> signature\n");
>> -               efi_dxe_table = NULL;
>> -       }
>> -
>>         setup_sections_memory_protection(bzimage_addr - image_offset);
>> 
>>  #ifdef CONFIG_CMDLINE_BOOL
>> --
>> 2.37.4
>> 

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