Re: [PATCH v4 07/10] powerpc/fsl_booke/32: randomize the kernel image offset

[Jason Yan <yanaijie@huawei.com>]

On 2019/8/6 15:56, Christophe Leroy wrote:
> 
> 
> Le 05/08/2019 à 08:43, Jason Yan a écrit :
>> After we have the basic support of relocate the kernel in some
>> appropriate place, we can start to randomize the offset now.
>>
>> Entropy is derived from the banner and timer, which will change every
>> build and boot. This not so much safe so additionally the bootloader may
>> pass entropy via the /chosen/kaslr-seed node in device tree.
>>
>> We will use the first 512M of the low memory to randomize the kernel
>> image. The memory will be split in 64M zones. We will use the lower 8
>> bit of the entropy to decide the index of the 64M zone. Then we chose a
>> 16K aligned offset inside the 64M zone to put the kernel in.
>>
>>      KERNELBASE
>>
>>          |-->   64M   <--|
>>          |               |
>>          +---------------+    +----------------+---------------+
>>          |               |....|    |kernel|    |               |
>>          +---------------+    +----------------+---------------+
>>          |                         |
>>          |----->   offset    <-----|
>>
>>                                kimage_vaddr
>>
>> We also check if we will overlap with some areas like the dtb area, the
>> initrd area or the crashkernel area. If we cannot find a proper area,
>> kaslr will be disabled and boot from the original kernel.
>>
>> Signed-off-by: Jason Yan <yanaijie@huawei.com>
>> Cc: Diana Craciun <diana.craciun@nxp.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> Reviewed-by: Diana Craciun <diana.craciun@nxp.com>
>> Tested-by: Diana Craciun <diana.craciun@nxp.com>
> 
> Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>
> 

Thanks for your help,

> One small comment below
> 
>> ---
>>   arch/powerpc/kernel/kaslr_booke.c | 322 +++++++++++++++++++++++++++++-
>>   1 file changed, 320 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/kaslr_booke.c 
>> b/arch/powerpc/kernel/kaslr_booke.c
>> index 30f84c0321b2..97250cad71de 100644
>> --- a/arch/powerpc/kernel/kaslr_booke.c
>> +++ b/arch/powerpc/kernel/kaslr_booke.c
>> @@ -23,6 +23,8 @@
>>   #include <linux/delay.h>
>>   #include <linux/highmem.h>
>>   #include <linux/memblock.h>
>> +#include <linux/libfdt.h>
>> +#include <linux/crash_core.h>
>>   #include <asm/pgalloc.h>
>>   #include <asm/prom.h>
>>   #include <asm/io.h>
>> @@ -34,15 +36,329 @@
>>   #include <asm/machdep.h>
>>   #include <asm/setup.h>
>>   #include <asm/paca.h>
>> +#include <asm/kdump.h>
>>   #include <mm/mmu_decl.h>
>> +#include <generated/compile.h>
>> +#include <generated/utsrelease.h>
>> +
>> +#ifdef DEBUG
>> +#define DBG(fmt...) printk(KERN_ERR fmt)
>> +#else
>> +#define DBG(fmt...)
>> +#endif
>> +
>> +struct regions {
>> +    unsigned long pa_start;
>> +    unsigned long pa_end;
>> +    unsigned long kernel_size;
>> +    unsigned long dtb_start;
>> +    unsigned long dtb_end;
>> +    unsigned long initrd_start;
>> +    unsigned long initrd_end;
>> +    unsigned long crash_start;
>> +    unsigned long crash_end;
>> +    int reserved_mem;
>> +    int reserved_mem_addr_cells;
>> +    int reserved_mem_size_cells;
>> +};
>>   extern int is_second_reloc;
>> +/* Simplified build-specific string for starting entropy. */
>> +static const char build_str[] = UTS_RELEASE " (" LINUX_COMPILE_BY "@"
>> +        LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION;
>> +
>> +static __init void kaslr_get_cmdline(void *fdt)
>> +{
>> +    int node = fdt_path_offset(fdt, "/chosen");
>> +
>> +    early_init_dt_scan_chosen(node, "chosen", 1, boot_command_line);
>> +}
>> +
>> +static unsigned long __init rotate_xor(unsigned long hash, const void 
>> *area,
>> +                       size_t size)
>> +{
>> +    size_t i;
>> +    unsigned long *ptr = (unsigned long *)area;
> 
> As area is a void *, this cast shouldn't be necessary. Or maybe it is 
> necessary because it discards the const ?
> 

It's true the cast is not necessary. The ptr can be made const and then 
remove the cast.

> Christophe
>