Re: [PATCH v6 06/12] powerpc/fsl_booke/32: implement KASLR infrastructure

[Scott Wood]

On Fri, Aug 09, 2019 at 06:07:54PM +0800, Jason Yan wrote:
> This patch add support to boot kernel from places other than KERNELBASE.
> Since CONFIG_RELOCATABLE has already supported, what we need to do is
> map or copy kernel to a proper place and relocate. Freescale Book-E
> parts expect lowmem to be mapped by fixed TLB entries(TLB1). The TLB1
> entries are not suitable to map the kernel directly in a randomized
> region, so we chose to copy the kernel to a proper place and restart to
> relocate.
> 
> The offset of the kernel was not randomized yet(a fixed 64M is set). We
> will randomize it in the next patch.
> 
> Signed-off-by: Jason Yan <yanaijie@xxxxxxxxxx>
> Cc: Diana Craciun <diana.craciun@xxxxxxx>
> Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> Cc: Christophe Leroy <christophe.leroy@xxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Tested-by: Diana Craciun <diana.craciun@xxxxxxx>
> Reviewed-by: Christophe Leroy <christophe.leroy@xxxxxx>
> ---
>  arch/powerpc/Kconfig                          | 11 ++++
>  arch/powerpc/kernel/Makefile                  |  1 +
>  arch/powerpc/kernel/early_32.c                |  2 +-
>  arch/powerpc/kernel/fsl_booke_entry_mapping.S | 17 +++--
>  arch/powerpc/kernel/head_fsl_booke.S          | 13 +++-
>  arch/powerpc/kernel/kaslr_booke.c             | 62 +++++++++++++++++++
>  arch/powerpc/mm/mmu_decl.h                    |  7 +++
>  arch/powerpc/mm/nohash/fsl_booke.c            |  7 ++-
>  8 files changed, 105 insertions(+), 15 deletions(-)
>  create mode 100644 arch/powerpc/kernel/kaslr_booke.c
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 77f6ebf97113..710c12ef7159 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -548,6 +548,17 @@ config RELOCATABLE
>  	  setting can still be useful to bootwrappers that need to know the
>  	  load address of the kernel (eg. u-boot/mkimage).
>  
> +config RANDOMIZE_BASE
> +	bool "Randomize the address of the kernel image"
> +	depends on (FSL_BOOKE && FLATMEM && PPC32)
> +	depends on RELOCATABLE
> +	help
> +	  Randomizes the virtual address at which the kernel image is
> +	  loaded, as a security feature that deters exploit attempts
> +	  relying on knowledge of the location of kernel internals.
> +
> +	  If unsure, say N.
> +

Why is N the safe default (other than concerns about code maturity,
though arm64 and mips don't seem to have updated this recommendation
after several years)?  On x86 this defaults to Y.

> diff --git a/arch/powerpc/kernel/fsl_booke_entry_mapping.S b/arch/powerpc/kernel/fsl_booke_entry_mapping.S
> index f4d3eaae54a9..641920d4f694 100644
> --- a/arch/powerpc/kernel/fsl_booke_entry_mapping.S
> +++ b/arch/powerpc/kernel/fsl_booke_entry_mapping.S
> @@ -155,23 +155,22 @@ skpinv:	addi	r6,r6,1				/* Increment */
>  
>  #if defined(ENTRY_MAPPING_BOOT_SETUP)
>  
> -/* 6. Setup KERNELBASE mapping in TLB1[0] */
> +/* 6. Setup kernstart_virt_addr mapping in TLB1[0] */
>  	lis	r6,0x1000		/* Set MAS0(TLBSEL) = TLB1(1), ESEL = 0 */
>  	mtspr	SPRN_MAS0,r6
>  	lis	r6,(MAS1_VALID|MAS1_IPROT)@h
>  	ori	r6,r6,(MAS1_TSIZE(BOOK3E_PAGESZ_64M))@l
>  	mtspr	SPRN_MAS1,r6
> -	lis	r6,MAS2_VAL(PAGE_OFFSET, BOOK3E_PAGESZ_64M, MAS2_M_IF_NEEDED)@h
> -	ori	r6,r6,MAS2_VAL(PAGE_OFFSET, BOOK3E_PAGESZ_64M, MAS2_M_IF_NEEDED)@l
> -	mtspr	SPRN_MAS2,r6
> +	lis     r6,MAS2_EPN_MASK(BOOK3E_PAGESZ_64M)@h
> +	ori     r6,r6,MAS2_EPN_MASK(BOOK3E_PAGESZ_64M)@l
> +	and     r6,r6,r20
> +	ori	r6,r6,MAS2_M_IF_NEEDED@l
> +	mtspr   SPRN_MAS2,r6

Please use tabs rather than spaces between the mnemonic and the
arguments.

It looks like that was the last user of MAS2_VAL so let's remove it.

> diff --git a/arch/powerpc/kernel/kaslr_booke.c b/arch/powerpc/kernel/kaslr_booke.c
> new file mode 100644
> index 000000000000..f8dc60534ac1
> --- /dev/null
> +++ b/arch/powerpc/kernel/kaslr_booke.c

Shouldn't this go under arch/powerpc/mm/nohash?

> +/*
> + * To see if we need to relocate the kernel to a random offset
> + * void *dt_ptr - address of the device tree
> + * phys_addr_t size - size of the first memory block
> + */
> +notrace void __init kaslr_early_init(void *dt_ptr, phys_addr_t size)
> +{
> +	unsigned long tlb_virt;
> +	phys_addr_t tlb_phys;
> +	unsigned long offset;
> +	unsigned long kernel_sz;
> +
> +	kernel_sz = (unsigned long)_end - KERNELBASE;

Why KERNELBASE and not kernstart_addr?

> +
> +	offset = kaslr_choose_location(dt_ptr, size, kernel_sz);
> +
> +	if (offset == 0)
> +		return;
> +
> +	kernstart_virt_addr += offset;
> +	kernstart_addr += offset;
> +
> +	is_second_reloc = 1;
> +
> +	if (offset >= SZ_64M) {
> +		tlb_virt = round_down(kernstart_virt_addr, SZ_64M);
> +		tlb_phys = round_down(kernstart_addr, SZ_64M);

If kernstart_addr wasn't 64M-aligned before adding offset, then "offset
>= SZ_64M" is not necessarily going to detect when you've crossed a
mapping boundary.

> +
> +		/* Create kernel map to relocate in */
> +		create_tlb_entry(tlb_phys, tlb_virt, 1);
> +	}
> +
> +	/* Copy the kernel to it's new location and run */
> +	memcpy((void *)kernstart_virt_addr, (void *)KERNELBASE, kernel_sz);
> +
> +	reloc_kernel_entry(dt_ptr, kernstart_virt_addr);
> +}

After copying, call flush_icache_range() on the destination.

> diff --git a/arch/powerpc/mm/nohash/fsl_booke.c b/arch/powerpc/mm/nohash/fsl_booke.c
> index 556e3cd52a35..2dc27cf88add 100644
> --- a/arch/powerpc/mm/nohash/fsl_booke.c
> +++ b/arch/powerpc/mm/nohash/fsl_booke.c
> @@ -263,7 +263,8 @@ void setup_initial_memory_limit(phys_addr_t first_memblock_base,
>  int __initdata is_second_reloc;
>  notrace void __init relocate_init(u64 dt_ptr, phys_addr_t start)
>  {
> -	unsigned long base = KERNELBASE;
> +	unsigned long base = kernstart_virt_addr;
> +	phys_addr_t size;
>  
>  	kernstart_addr = start;
>  	if (is_second_reloc) {
> @@ -291,7 +292,7 @@ notrace void __init relocate_init(u64 dt_ptr, phys_addr_t start)
>  	start &= ~0x3ffffff;
>  	base &= ~0x3ffffff;
>  	virt_phys_offset = base - start;
> -	early_get_first_memblock_info(__va(dt_ptr), NULL);
> +	early_get_first_memblock_info(__va(dt_ptr), &size);
>  	/*
>  	 * We now get the memstart_addr, then we should check if this
>  	 * address is the same as what the PAGE_OFFSET map to now. If
> @@ -316,6 +317,8 @@ notrace void __init relocate_init(u64 dt_ptr, phys_addr_t start)
>  		/* We should never reach here */
>  		panic("Relocation error");
>  	}
> +
> +	kaslr_early_init(__va(dt_ptr), size);

Are you assuming that available memory starts at physical address zero? 
This isn't true of some partitioning scenarios, or in a kdump crash
kernel.

-Scott