linux-kernel - Re: [PATCH v7] x86/boot/KASLR: Restrict kernel to be randomized in mirror regions

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Date:   Fri, 28 Jul 2017 08:46:14 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, keescook@...omium.org,
        matt@...eblueprint.co.uk, tglx@...utronix.de, hpa@...or.com,
        izumi.taku@...fujitsu.com, fanc.fnst@...fujitsu.com,
        thgarnie@...gle.com, n-horiguchi@...jp.nec.com, dyoung@...hat.com
Subject: Re: [PATCH v7] x86/boot/KASLR: Restrict kernel to be randomized in
 mirror regions


* Baoquan He <bhe@...hat.com> wrote:

> Currently KASLR will parse all e820 entries of RAM type and add all
> candidate position into slots array. Then we will choose one slot
> randomly as the new position which kernel will be decompressed into
> and run at.
> 
> On system with EFI enabled, e820 memory regions are coming from EFI
> memory regions by combining adjacent regions. While these EFI memory
> regions have more attributes to mark their different use. Mirror
> attribute is such kind. The physical memory region whose descriptors
> in EFI memory map has EFI_MEMORY_MORE_RELIABLE attribute (bit: 16) are
> mirrored. The address range mirroring feature of kernel arranges such
> mirror region into normal zone and other region into movable zone. And
> with mirroring feature enabled, the code and date of kernel can only be
> located in more reliable mirror region. However, the current KASLR code
> doesn't check EFI memory entries, and could choose new position in
> non-mirrored region. This will break the functionality of the address
> range mirroring feature.
> 
> So if EFI is detected, iterate EFI memory map and pick the mirror region
> to process for adding candidate of randomization slot. If EFI is disabled
> or no mirror region found, still process e820 memory map.
> 
> Signed-off-by: Baoquan He <bhe@...hat.com>
> ---
> v6->v7:
>   Ingo pointed out several incorrect line break issues and unclear 
>   description of patch log. Correct them and rewrite patch log.
> 
>   And also rewrite the EFI warning message that if EFI memmap is above
>   4G in 32bit system since 32bit system can not handle data above 4G at
>   kernel decompression stage. This is suggested by Ingo too.
> 
> v5->v6:
>   Code style issue fix according to Kees's comment.
> 
>   This is based on tip/x86/boot, patch 1,2,3/4 in v5 post has
>   been put into tip/x86/boot now.
> 
>  arch/x86/boot/compressed/kaslr.c | 68 ++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 66 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/kaslr.c b/arch/x86/boot/compressed/kaslr.c
> index 99c7194f7ea6..e1dd59ea8a8f 100644
> --- a/arch/x86/boot/compressed/kaslr.c
> +++ b/arch/x86/boot/compressed/kaslr.c
> @@ -37,7 +37,9 @@
>  #include <linux/uts.h>
>  #include <linux/utsname.h>
>  #include <linux/ctype.h>
> +#include <linux/efi.h>
>  #include <generated/utsrelease.h>
> +#include <asm/efi.h>
>  
>  /* Macros used by the included decompressor code below. */
>  #define STATIC
> @@ -558,6 +560,65 @@ static void process_mem_region(struct mem_vector *entry,
>  	}
>  }
>  
> +#ifdef CONFIG_EFI
> +/*
> + * Returns true if mirror region found (and must have been processed
> + * for slots adding)
> + */
> +static bool
> +process_efi_entries(unsigned long minimum, unsigned long image_size)
> +{
> +	struct efi_info *e = &boot_params->efi_info;
> +	bool efi_mirror_found = false;
> +	struct mem_vector region;
> +	efi_memory_desc_t *md;
> +	unsigned long pmap;
> +	char *signature;
> +	u32 nr_desc;
> +	int i;
> +
> +	signature = (char *)&e->efi_loader_signature;
> +	if (strncmp(signature, EFI32_LOADER_SIGNATURE, 4) &&
> +	    strncmp(signature, EFI64_LOADER_SIGNATURE, 4))
> +		return false;
> +
> +#ifdef CONFIG_X86_32
> +	/* Can't handle data above 4GB at this time */
> +	if (e->efi_memmap_hi) {
> +		warn("EFI memmap is above 4GB, can't be handled now on x86_32. EFI should be disabled.\n");
> +		return false;
> +	}
> +	pmap =  e->efi_memmap;
> +#else
> +	pmap = (e->efi_memmap | ((__u64)e->efi_memmap_hi << 32));
> +#endif
> +
> +	nr_desc = e->efi_memmap_size / e->efi_memdesc_size;
> +	for (i = 0; i < nr_desc; i++) {
> +		md = (efi_memory_desc_t *)(pmap + (i * e->efi_memdesc_size));
> +		if (md->attribute & EFI_MEMORY_MORE_RELIABLE) {
> +			region.start = md->phys_addr;
> +			region.size = md->num_pages << EFI_PAGE_SHIFT;
> +			process_mem_region(&region, minimum, image_size);
> +			efi_mirror_found = true;
> +
> +			if (slot_area_index == MAX_SLOT_AREA) {
> +				debug_putstr("Aborted EFI scan (slot_areas full)!\n");
> +				break;
> +			}
> +		}
> +	}

So I suggested this before: if you treat 'md' as an array of elements (which it 
is), then the type cast can be moved to a more natural point, where we do address 
calculations anyway:

	md = (efi_memory_desc_t *)(e->efi_memmap | ((__u64)e->efi_memmap_hi << 32)));

The 'pmap' variable can be eliminated and all places in the loop that use 'md->' 
can use 'md[i].'.

Thanks,

	Ingo