Re: [PATCH v4 23/26] efi/libstub: Use memory attribute protocol

[Ard Biesheuvel]

On Thu, 15 Dec 2022 at 13:42, Evgeniy Baskov <baskov@xxxxxxxxx> 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@xxxxxxx>
> Tested-by: Peter Jones <pjones@xxxxxxxxxx>
> Signed-off-by: Evgeniy Baskov <baskov@xxxxxxxxx>

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.




> ---
>  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
>