Re: [PATCHv11 4/9] x86/boot/compressed: Handle unaccepted memory

[Liam Merwick <liam.merwick@oracle.com>]

On 13/05/2023 23:04, Kirill A. Shutemov wrote:
> The firmware will pre-accept the memory used to run the stub. But, the
> stub is responsible for accepting the memory into which it decompresses
> the main kernel. Accept memory just before decompression starts.
> 
> The stub is also responsible for choosing a physical address in which to
> place the decompressed kernel image. The KASLR mechanism will randomize
> this physical address. Since the unaccepted memory region is relatively


Reading this sentence, should "unaccepted" be "accepted" here?
(i.e. most memory at the start is unaccepted and the accepted region is 
the smaller one).

> small, KASLR would be quite ineffective if it only used the pre-accepted
> area (EFI_CONVENTIONAL_MEMORY). Ensure that KASLR randomizes among the
> entire physical address space by also including EFI_UNACCEPTED_MEMORY.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Otherwise

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>


> ---
>   arch/x86/boot/compressed/efi.h   |  1 +
>   arch/x86/boot/compressed/kaslr.c | 35 +++++++++++++++++++++-----------
>   arch/x86/boot/compressed/misc.c  |  6 ++++++
>   arch/x86/boot/compressed/misc.h  |  6 ++++++
>   4 files changed, 36 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/efi.h b/arch/x86/boot/compressed/efi.h
> index 7db2f41b54cd..cf475243b6d5 100644
> --- a/arch/x86/boot/compressed/efi.h
> +++ b/arch/x86/boot/compressed/efi.h
> @@ -32,6 +32,7 @@ typedef	struct {
>   } efi_table_hdr_t;
>   
>   #define EFI_CONVENTIONAL_MEMORY		 7
> +#define EFI_UNACCEPTED_MEMORY		15
>   
>   #define EFI_MEMORY_MORE_RELIABLE \
>   				((u64)0x0000000000010000ULL)	/* higher reliability */
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 454757fbdfe5..749f0fe7e446 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -672,6 +672,28 @@ static bool process_mem_region(struct mem_vector *region,
>   }
>   
>   #ifdef CONFIG_EFI
> +
> +/*
> + * Only EFI_CONVENTIONAL_MEMORY and EFI_UNACCEPTED_MEMORY (if supported) are
> + * guaranteed to be free.
> + *
> + * It is more conservative in picking free memory than the EFI spec allows:
> + *
> + * According to the spec, EFI_BOOT_SERVICES_{CODE|DATA} are also free memory
> + * and thus available to place the kernel image into, but in practice there's
> + * firmware where using that memory leads to crashes.
> + */
> +static inline bool memory_type_is_free(efi_memory_desc_t *md)
> +{
> +	if (md->type == EFI_CONVENTIONAL_MEMORY)
> +		return true;
> +
> +	if (md->type == EFI_UNACCEPTED_MEMORY)
> +		return IS_ENABLED(CONFIG_UNACCEPTED_MEMORY);
> +
> +	return false;
> +}
> +
>   /*
>    * Returns true if we processed the EFI memmap, which we prefer over the E820
>    * table if it is available.
> @@ -716,18 +738,7 @@ process_efi_entries(unsigned long minimum, unsigned long image_size)
>   	for (i = 0; i < nr_desc; i++) {
>   		md = efi_early_memdesc_ptr(pmap, e->efi_memdesc_size, i);
>   
> -		/*
> -		 * Here we are more conservative in picking free memory than
> -		 * the EFI spec allows:
> -		 *
> -		 * According to the spec, EFI_BOOT_SERVICES_{CODE|DATA} are also
> -		 * free memory and thus available to place the kernel image into,
> -		 * but in practice there's firmware where using that memory leads
> -		 * to crashes.
> -		 *
> -		 * Only EFI_CONVENTIONAL_MEMORY is guaranteed to be free.
> -		 */
> -		if (md->type != EFI_CONVENTIONAL_MEMORY)
> +		if (!memory_type_is_free(md))
>   			continue;
>   
>   		if (efi_soft_reserve_enabled() &&
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index 014ff222bf4b..eb8df0d4ad51 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -455,6 +455,12 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
>   #endif
>   
>   	debug_putstr("\nDecompressing Linux... ");
> +
> +	if (IS_ENABLED(CONFIG_UNACCEPTED_MEMORY)) {
> +		debug_putstr("Accepting memory... ");
> +		accept_memory(__pa(output), __pa(output) + needed_size);
> +	}
> +
>   	__decompress(input_data, input_len, NULL, NULL, output, output_len,
>   			NULL, error);
>   	entry_offset = parse_elf(output);
> diff --git a/arch/x86/boot/compressed/misc.h b/arch/x86/boot/compressed/misc.h
> index 2f155a0e3041..9663d1839f54 100644
> --- a/arch/x86/boot/compressed/misc.h
> +++ b/arch/x86/boot/compressed/misc.h
> @@ -247,4 +247,10 @@ static inline unsigned long efi_find_vendor_table(struct boot_params *bp,
>   }
>   #endif /* CONFIG_EFI */
>   
> +#ifdef CONFIG_UNACCEPTED_MEMORY
> +void accept_memory(phys_addr_t start, phys_addr_t end);
> +#else
> +static inline void accept_memory(phys_addr_t start, phys_addr_t end) {}
> +#endif
> +
>   #endif /* BOOT_COMPRESSED_MISC_H */