Re: [PATCHv4 3/8] efi/x86: Implement support for unaccepted memory

From: Borislav Petkov
Date: Fri Apr 15 2022 - 18:24:47 EST

On Wed, Apr 06, 2022 at 02:43:38AM +0300, Kirill A. Shutemov wrote:
> diff --git a/Documentation/x86/zero-page.rst b/Documentation/x86/zero-page.rst
> index f088f5881666..8e3447a4b373 100644
> --- a/Documentation/x86/zero-page.rst
> +++ b/Documentation/x86/zero-page.rst
> @@ -42,4 +42,5 @@ Offset/Size Proto Name Meaning
> 2D0/A00 ALL e820_table E820 memory map table
> (array of struct e820_entry)
> D00/1EC ALL eddbuf EDD data (array of struct edd_info)
> +ECC/008 ALL unaccepted_memory Bitmap of unaccepted memory (1bit == 2M)

There's a perfectly fine spot at 0x78:

__u8 _pad3[8]; /* 0x078 */

why not take that one?

> =========== ===== ======================= =================================================
> diff --git a/arch/x86/boot/compressed/Makefile b/arch/x86/boot/compressed/Makefile
> index 8fd0e6ae2e1f..09993797efa2 100644
> --- a/arch/x86/boot/compressed/Makefile
> +++ b/arch/x86/boot/compressed/Makefile
> @@ -102,6 +102,7 @@ endif
>
> vmlinux-objs-$(CONFIG_ACPI) += $(obj)/acpi.o
> vmlinux-objs-$(CONFIG_INTEL_TDX_GUEST) += $(obj)/tdx.o $(obj)/tdcall.o
> +vmlinux-objs-$(CONFIG_UNACCEPTED_MEMORY) += $(obj)/bitmap.o $(obj)/unaccepted_memory.o
>
> vmlinux-objs-$(CONFIG_EFI_MIXED) += $(obj)/efi_thunk_$(BITS).o
> efi-obj-$(CONFIG_EFI_STUB) = $(objtree)/drivers/firmware/efi/libstub/lib.a
> diff --git a/arch/x86/boot/compressed/bitmap.c b/arch/x86/boot/compressed/bitmap.c
> new file mode 100644
> index 000000000000..bf58b259380a
> --- /dev/null
> +++ b/arch/x86/boot/compressed/bitmap.c
> @@ -0,0 +1,24 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/* Taken from lib/string.c */
> +
> +#include <linux/bitmap.h>

verify_include_paths: Warning: Kernel-proper include at arch/x86/boot/compressed/bitmap.c:4 [+#include <linux/bitmap.h>]

Same game as before: put the stuff you need into a separate or a shared
header and avoid the linux/ namespace include.

> +void __bitmap_set(unsigned long *map, unsigned int start, int len)
> +{
> + unsigned long *p = map + BIT_WORD(start);
> + const unsigned int size = start + len;
> + int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
> + unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
> +
> + while (len - bits_to_set >= 0) {
> + *p |= mask_to_set;
> + len -= bits_to_set;
> + bits_to_set = BITS_PER_LONG;
> + mask_to_set = ~0UL;
> + p++;
> + }
> + if (len) {
> + mask_to_set &= BITMAP_LAST_WORD_MASK(size);
> + *p |= mask_to_set;
> + }
> +}
> diff --git a/arch/x86/boot/compressed/unaccepted_memory.c b/arch/x86/boot/compressed/unaccepted_memory.c
> new file mode 100644
> index 000000000000..d363acf59c08
> --- /dev/null
> +++ b/arch/x86/boot/compressed/unaccepted_memory.c

arch/x86/boot/compressed/mem.c

simply. That "unaccepted_memory" everywhere is a mouthful and too specific.

> @@ -0,0 +1,53 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +
> +#include "error.h"
> +#include "misc.h"
> +
> +static inline void __accept_memory(phys_addr_t start, phys_addr_t end)
> +{
> + /* Platform-specific memory-acceptance call goes here */
> + error("Cannot accept memory");
> +}
> +
> +void mark_unaccepted(struct boot_params *params, u64 start, u64 end)

That name is kinda misleading? It is not only marking as unaccepted - it
is also accepting weird 2M misaligned chunks...

> +{
> + /*
> + * The accepted memory bitmap only works at PMD_SIZE granularity.
> + * If a request comes in to mark memory as unaccepted which is not
> + * PMD_SIZE-aligned, simply accept the memory now since it can not be
> + * *marked* as unaccepted.
> + */

That comment goes over the function name.

> + /*
> + * Accept small regions that might not be able to be represented
> + * in the bitmap:
> + */
> + if (end - start < 2 * PMD_SIZE) {
> + __accept_memory(start, end);
> + return;
> + }
> +
> + /*
> + * No matter how the start and end are aligned, at least one unaccepted
> + * PMD_SIZE area will remain.
> + */
> +
> + /* Immediately accept a <PMD_SIZE piece at the start: */

Immediately? As opposed to delayed?

> + if (start & ~PMD_MASK) {
> + __accept_memory(start, round_up(start, PMD_SIZE));
> + start = round_up(start, PMD_SIZE);
> + }
> +
> + /* Immediately accept a <PMD_SIZE piece at the end: */
> + if (end & ~PMD_MASK) {
> + __accept_memory(round_down(end, PMD_SIZE), end);
> + end = round_down(end, PMD_SIZE);
> + }
> +
> + /*
> + * 'start' and 'end' are now both PMD-aligned.
> + * Record the range as being unaccepted:
> + */
> + bitmap_set((unsigned long *)params->unaccepted_memory,
> + start / PMD_SIZE, (end - start) / PMD_SIZE);
> +}
> diff --git a/arch/x86/include/asm/unaccepted_memory.h b/arch/x86/include/asm/unaccepted_memory.h

Why do you need a separate header?

We already have

arch/x86/include/asm/mem_encrypt.h

and this is kinda very much related...

> diff --git a/drivers/firmware/efi/Kconfig b/drivers/firmware/efi/Kconfig
> index 2c3dac5ecb36..b17ceec757d0 100644
> --- a/drivers/firmware/efi/Kconfig
> +++ b/drivers/firmware/efi/Kconfig
> @@ -243,6 +243,21 @@ config EFI_DISABLE_PCI_DMA
> options "efi=disable_early_pci_dma" or "efi=no_disable_early_pci_dma"
> may be used to override this option.
>
> +config UNACCEPTED_MEMORY
> + bool
> + depends on EFI_STUB
> + depends on !KEXEC_CORE
> + help
> + Some Virtual Machine platforms, such as Intel TDX, require
> + some memory to be "accepted" by the guest before it can be used.
> + This mechanism helps prevent malicious hosts from making changes
> + to guest memory.
> +
> + UEFI specification v2.9 introduced EFI_UNACCEPTED_MEMORY memory type.
> +
> + This option adds support for unaccepted memory and makes such memory
> + usable by kernel.

... by *the* kernel.

> +
> endmenu
>
> config EFI_EMBEDDED_FIRMWARE
> diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
> index 5502e176d51b..2c055afb1b11 100644
> --- a/drivers/firmware/efi/efi.c
> +++ b/drivers/firmware/efi/efi.c
> @@ -747,6 +747,7 @@ static __initdata char memory_type_name[][13] = {
> "MMIO Port",
> "PAL Code",
> "Persistent",
> + "Unaccepted",
> };
>
> char * __init efi_md_typeattr_format(char *buf, size_t size,
> diff --git a/drivers/firmware/efi/libstub/x86-stub.c b/drivers/firmware/efi/libstub/x86-stub.c
> index d18cac8ab436..e7601fd612aa 100644
> --- a/drivers/firmware/efi/libstub/x86-stub.c
> +++ b/drivers/firmware/efi/libstub/x86-stub.c
> @@ -9,12 +9,14 @@
> #include <linux/efi.h>
> #include <linux/pci.h>
> #include <linux/stddef.h>
> +#include <linux/bitmap.h>
>
> #include <asm/efi.h>
> #include <asm/e820/types.h>
> #include <asm/setup.h>
> #include <asm/desc.h>
> #include <asm/boot.h>
> +#include <asm/unaccepted_memory.h>
>
> #include "efistub.h"
>
> @@ -504,6 +506,13 @@ setup_e820(struct boot_params *params, struct setup_data *e820ext, u32 e820ext_s
> e820_type = E820_TYPE_PMEM;
> break;
>
> + case EFI_UNACCEPTED_MEMORY:
> + if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> + continue;
> + e820_type = E820_TYPE_RAM;
> + mark_unaccepted(params, d->phys_addr,
> + d->phys_addr + PAGE_SIZE * d->num_pages);
> + break;
> default:
> continue;
> }
> @@ -575,6 +584,9 @@ static efi_status_t allocate_e820(struct boot_params *params,
> {
> efi_status_t status;
> __u32 nr_desc;
> + bool unaccepted_memory_present = false;

This wholly written out "unaccepted_memory" everywhere is too much and
too long. How about

bool unaccept_mem;

or so?

> + u64 max_addr = 0;
> + int i;
>
> status = efi_get_memory_map(map);
> if (status != EFI_SUCCESS)
> @@ -589,9 +601,57 @@ static efi_status_t allocate_e820(struct boot_params *params,
> if (status != EFI_SUCCESS)
> goto out;
> }

This whole chunk you're adding here begs to be a separate function with
the big fat comment placed over the function name.

Might just as well call it after allocate_e820() has been called.

> +
> + if (!IS_ENABLED(CONFIG_UNACCEPTED_MEMORY))
> + goto out;
> +
> + /* Check if there's any unaccepted memory and find the max address */
> + for (i = 0; i < nr_desc; i++) {
> + efi_memory_desc_t *d;
> +
> + d = efi_early_memdesc_ptr(*map->map, *map->desc_size, i);
> + if (d->type == EFI_UNACCEPTED_MEMORY)
> + unaccepted_memory_present = true;
> + if (d->phys_addr + d->num_pages * PAGE_SIZE > max_addr)
> + max_addr = d->phys_addr + d->num_pages * PAGE_SIZE;
> + }
> +
> + /*
> + * If unaccepted memory is present allocate a bitmap to track what
> + * memory has to be accepted before access.
> + *
> + * One bit in the bitmap represents 2MiB in the address space:
> + * A 4k bitmap can track 64GiB of physical address space.
> + *
> + * In the worst case scenario -- a huge hole in the middle of the
> + * address space -- It needs 256MiB to handle 4PiB of the address
> + * space.

And you're saying that that efi_allocate_pages() below can really give a
256M contiguous chunk?

> + *
> + * TODO: handle situation if params->unaccepted_memory has already set.
> + * It's required to deal with kexec.
> + *
> + * The bitmap will be populated in setup_e820() according to the memory
> + * map after efi_exit_boot_services().
> + */
> + if (unaccepted_memory_present) {
> + unsigned long *unaccepted_memory = NULL;

So if you call this simply

unsigned long *mem = ...

> + u64 size = DIV_ROUND_UP(max_addr, PMD_SIZE * BITS_PER_BYTE);
> +
> + status = efi_allocate_pages(size,
> + (unsigned long *)&unaccepted_memory,
> + ULONG_MAX);

... you'd have this on a single line:

status = efi_allocate_pages(size, (unsigned long *)&mem, ULONG_MAX);

> + if (status != EFI_SUCCESS)
> + goto out;
> + memset(unaccepted_memory, 0, size);
> + params->unaccepted_memory = (unsigned long)unaccepted_memory;

... and then have this assignment more readable:

params->unaccepted_memory = (unsigned long)mem;

as it shows the important var being ->unaccepted_memory and mem only a
local helper.

--
Regards/Gruss,
Boris.

https://people.kernel.org/tglx/notes-about-netiquette