[PATCH v3] arm64/efi: don't pad between EFI_MEMORY_RUNTIME regions

Mark Rutland
2015-09-10T16:08:10Z

On Thu, Sep 10, 2015 at 04:41:39PM +0100, Ard Biesheuvel wrote:
> The new Properties Table feature introduced in UEFIv2.5 may split
> memory regions that cover PE/COFF memory images into separate code
> and data regions. Since these regions only differ in the type (runtime
> code vs runtime data) and the permission bits, but not in the memory
> type attributes (UC/WC/WT/WB), the spec does not require them to be
> aligned to 64 KB.
>
> Since the relative offset of PE/COFF .text and .data segments cannot
> be changed on the fly, this means that we can no longer pad out those
> regions to be mappable using 64 KB pages.
> Unfortunately, there is no annotation in the UEFI memory map that
> identifies data regions that were split off from a code region, so we
> must apply this logic to all adjacent runtime regions whose attributes
> only differ in the permission bits.
>
> So instead of rounding each memory region to 64 KB alignment at both
> ends, only round down regions that are not directly preceded by another
> runtime region with the same type attributes. Since the UEFI spec does
> not mandate that the memory map be sorted, this means we also need to
> sort it first.
>
> Note that this change will result in all EFI_MEMORY_RUNTIME regions whose
> start addresses are not aligned to the OS page size to be mapped with
> executable permissions (i.e., on kernels compiled with 64 KB pages).
> However, since these mappings are only active during the time that UEFI
> Runtime Services are being invoked, the window for abuse is rather small.
>
> Tested-by: Mark Salter <msalter@redhat.com>
> Reviewed-by: Mark Salter <msalter@redhat.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

I've given this a spin on Juno and Seattle, with both 4K and 64K pages.
Both are EFI 2.4, so that only tells us we haven't regressed things.

The code itself looks good to me. Feel free to add:

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com> [UEFI 2.4 only]

Thanks,
Mark.

> ---
> Changes since v2:
> - break down complex if() condition into helper functions to test for adjacency
> and compatible memory types
> - update commit log to emphasize that this patch may result in data regions to
> be mapped with exec permissions on 64k pages kernels.
> - no functional changes
>
> Changes since v1:
> - Ensure that we don't inadvertently set the XN bit on the preceding region at
> mapping time if we the OS is running with >4 KB pages.
>
> arch/arm64/kernel/efi.c | 3 +-
> drivers/firmware/efi/libstub/arm-stub.c | 88 ++++++++++++++++----
> 2 files changed, 75 insertions(+), 16 deletions(-)
>
> diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> index e8ca6eaedd02..13671a9cf016 100644
> --- a/arch/arm64/kernel/efi.c
> +++ b/arch/arm64/kernel/efi.c
> @@ -258,7 +258,8 @@ static bool __init efi_virtmap_init(void)
> */
> if (!is_normal_ram(md))
> prot = __pgprot(PROT_DEVICE_nGnRE);
> - else if (md->type == EFI_RUNTIME_SERVICES_CODE)
> + else if (md->type == EFI_RUNTIME_SERVICES_CODE ||
> + !PAGE_ALIGNED(md->phys_addr))
> prot = PAGE_KERNEL_EXEC;
> else
> prot = PAGE_KERNEL;
> diff --git a/drivers/firmware/efi/libstub/arm-stub.c b/drivers/firmware/efi/libstub/arm-stub.c
> index e29560e6b40b..950c87f5d279 100644
> --- a/drivers/firmware/efi/libstub/arm-stub.c
> +++ b/drivers/firmware/efi/libstub/arm-stub.c
> @@ -13,6 +13,7 @@
> */
>
> #include <linux/efi.h>
> +#include <linux/sort.h>
> #include <asm/efi.h>
>
> #include "efistub.h"
> @@ -305,6 +306,44 @@ fail:
> */
> #define EFI_RT_VIRTUAL_BASE 0x40000000
>
> +static int cmp_mem_desc(const void *l, const void *r)
> +{
> + const efi_memory_desc_t *left = l, *right = r;
> +
> + return (left->phys_addr > right->phys_addr) ? 1 : -1;
> +}
> +
> +/*
> + * Returns whether region @left ends exactly where region @right starts,
> + * or false if either argument is NULL.
> + */
> +static bool regions_are_adjacent(efi_memory_desc_t *left,
> + efi_memory_desc_t *right)
> +{
> + u64 left_end;
> +
> + if (left == NULL || right == NULL)
> + return false;
> +
> + left_end = left->phys_addr + left->num_pages * EFI_PAGE_SIZE;
> +
> + return left_end == right->phys_addr;
> +}
> +
> +/*
> + * Returns whether region @left and region @right have compatible memory type
> + * mapping attributes, and are both EFI_MEMORY_RUNTIME regions.
> + */
> +static bool regions_have_compatible_memory_type_attrs(efi_memory_desc_t *left,
> + efi_memory_desc_t *right)
> +{
> + static const u64 mem_type_mask = EFI_MEMORY_WB | EFI_MEMORY_WT |
> + EFI_MEMORY_WC | EFI_MEMORY_UC |
> + EFI_MEMORY_RUNTIME;
> +
> + return ((left->attribute ^ right->attribute) & mem_type_mask) == 0;
> +}
> +
> /*
> * efi_get_virtmap() - create a virtual mapping for the EFI memory map
> *
> @@ -317,33 +356,52 @@ void efi_get_virtmap(efi_memory_desc_t *memory_map, unsigned long map_size,
> int *count)
> {
> u64 efi_virt_base = EFI_RT_VIRTUAL_BASE;
> - efi_memory_desc_t *out = runtime_map;
> + efi_memory_desc_t *in, *prev = NULL, *out = runtime_map;
> int l;
>
> - for (l = 0; l < map_size; l += desc_size) {
> - efi_memory_desc_t *in = (void *)memory_map + l;
> + /*
> + * To work around potential issues with the Properties Table feature
> + * introduced in UEFI 2.5, which may split PE/COFF executable images
> + * in memory into several RuntimeServicesCode and RuntimeServicesData
> + * regions, we need to preserve the relative offsets between adjacent
> + * EFI_MEMORY_RUNTIME regions with the same memory type attributes.
> + * The easiest way to find adjacent regions is to sort the memory map
> + * before traversing it.
> + */
> + sort(memory_map, map_size / desc_size, desc_size, cmp_mem_desc, NULL);
> +
> + for (l = 0; l < map_size; l += desc_size, prev = in) {
> u64 paddr, size;
>
> + in = (void *)memory_map + l;
> if (!(in->attribute & EFI_MEMORY_RUNTIME))
> continue;
>
> + paddr = in->phys_addr;
> + size = in->num_pages * EFI_PAGE_SIZE;
> +
> /*
> * Make the mapping compatible with 64k pages: this allows
> * a 4k page size kernel to kexec a 64k page size kernel and
> * vice versa.
> */
> - paddr = round_down(in->phys_addr, SZ_64K);
> - size = round_up(in->num_pages * EFI_PAGE_SIZE +
> - in->phys_addr - paddr, SZ_64K);
> -
> - /*
> - * Avoid wasting memory on PTEs by choosing a virtual base that
> - * is compatible with section mappings if this region has the
> - * appropriate size and physical alignment. (Sections are 2 MB
> - * on 4k granule kernels)
> - */
> - if (IS_ALIGNED(in->phys_addr, SZ_2M) && size >= SZ_2M)
> - efi_virt_base = round_up(efi_virt_base, SZ_2M);
> + if (!regions_are_adjacent(prev, in) ||
> + !regions_have_compatible_memory_type_attrs(prev, in)) {
> +
> + paddr = round_down(in->phys_addr, SZ_64K);
> + size += in->phys_addr - paddr;
> +
> + /*
> + * Avoid wasting memory on PTEs by choosing a virtual
> + * base that is compatible with section mappings if this
> + * region has the appropriate size and physical
> + * alignment. (Sections are 2 MB on 4k granule kernels)
> + */
> + if (IS_ALIGNED(in->phys_addr, SZ_2M) && size >= SZ_2M)
> + efi_virt_base = round_up(efi_virt_base, SZ_2M);
> + else
> + efi_virt_base = round_up(efi_virt_base, SZ_64K);
> + }
>
> in->virt_addr = efi_virt_base + in->phys_addr - paddr;
> efi_virt_base += size;
> --
> 1.9.1
>