Re: [PATCH] arm64: kaslr: Fix kaslr end boundary of virt addr

[Ard Biesheuvel]

On 21 November 2017 at 03:44, Chen Feng <puck.chen@xxxxxxxxxxxxx> wrote:
> With kaslr and kasan enable both, I got the follow issue.
>
> [   16.130523s]kasan: reg->base = 100000000, phys_end =1c0000000,start = ffffffff40000000, end = ffffffc000000000
> [   16.142517s]___alloc_bootmem_nopanic:257
> [   16.148284s]__alloc_memory_core_early:63, addr = 197fc7fc0
> [   16.155670s]__alloc_memory_core_early:65, virt = ffffffffd7fc7fc0
> [   16.163635s]__alloc_memory_core_early:67, toshow = ffffff8ffaff8ff8
> [   16.171783s]__alloc_memory_core_early:69, show_phy = ffffffe2649f8ff8
> [   16.180145s]Unable to handle kernel paging request at virtual address ffffff8ffaff8ff8
> [   16.189971s]pgd = ffffffad9c507000
> [   16.195220s][ffffff8ffaff8ff8] *pgd=0000000197fc8003, *pud=0000000197fc8003
>
> *reg->base = 100000000, phys_end =1c0000000,start = ffffffff40000000, end = ffffffc000000000*
>
> memstart_addr 0
> ARM64_MEMSTART_ALIGN 0x40000000
> memstart_offset_seed 0xffc7
> PHYS_OFFSET = 0 - memstart_addr = 0 - 3E40000000 = FFFFFFC1C0000000
>
> reg->base = 0x100000000  -> 0xffffffff40000000
> phys_end  = 0x1c0000000  -> 0xffffffc000000000  This is confused, end less than start.
>

This looks a bit weird because we add the PAGE_OFFSET, but it simply
wraps at the top of the address space.

So this code in kasan_init()

void *start = (void *)__phys_to_virt(reg->base);
void *end = (void *)__phys_to_virt(reg->base + reg->size);

if (start >= end)
     break;

is essentially incorrect, because it translates an address that is
strictly outside of the current memblock region. If the KASLR code
happens to map DRAM all the way at the top of the linear region (which
is what occurs in your case), end - 1 is the last valid address.

So I think the minimal correct fix would be

diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c
index acba49fb5aac..3214aa9d90be 100644
--- a/arch/arm64/mm/kasan_init.c
+++ b/arch/arm64/mm/kasan_init.c
@@ -216,7 +216,7 @@ void __init kasan_init(void)

        for_each_memblock(memory, reg) {
                void *start = (void *)__phys_to_virt(reg->base);
-               void *end = (void *)__phys_to_virt(reg->base + reg->size);
+               void *end = start + reg->size;

                if (start >= end)
                        break;

given that mappings in the linear region are congruent with the
underlying physical regions (unless I am missing something wrt special
start/end values in memblock, but in that case, they should not be p2v
translated before the evaluation)

However, since having DRAM at the very top appears to break other things as well

vmemmap : 0xffffffbf00000000 - 0xffffffc000000000   (     4 GB maximum)
          0xffffffbfff000000 - 0xffffffbf00000000   (17592186040336 MB actual)
memory  : 0xffffffffc0000000 - 0x               0   (  1024 MB)

I will leave it to Will and/or Catalin to decide whether they prefer
to follow your approach instead, and prevent KASLR from mapping DRAM
all the way at the top of the address space. Otherwise, we'll need to
track down all problematic uses of __phys_to_virt() et al, because
there will surely be more.

Thanks,
Ard.



> And In memblock it use "start_addr + size" as the end addr. So in function kasan_init,
> if the start >= end, it will not map the hole block address. But the memory in this
> block is valid. And it can be allocated as well.
>
> So donot use the last memory region. Changing "range = range / ARM64_MEMSTART_ALIGN + 1" to
> range = range / ARM64_MEMSTART_ALIGN;
>
> Signed-off-by: Chen Feng <puck.chen@xxxxxxxxxxxxx>
> Signed-off-by: Chen Xiang <chenxiang9@xxxxxxxxxx>
> ---
>  arch/arm64/mm/init.c | 7 ++-----
>  1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 716d122..60112c0 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -267,11 +267,8 @@ void __init arm64_memblock_init(void)
>                  * margin, the size of the region that the available physical
>                  * memory spans, randomize the linear region as well.
>                  */
> -               if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN) {
> -                       range = range / ARM64_MEMSTART_ALIGN + 1;
> -                       memstart_addr -= ARM64_MEMSTART_ALIGN *
> -                                        ((range * memstart_offset_seed) >> 16);
> -               }
> +               if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN)
> +                       memstart_addr -= (range * memstart_offset_seed) >> 16;
>         }
>
>         /*
> --
> 1.9.1
>