Re: [PATCH] arm64: kaslr: Reserve size of ARM64_MEMSTART_ALIGN in linear region

[Ard Biesheuvel]

On Mon, 24 Dec 2018 at 08:40, Yueyi Li <liyueyi@xxxxxxxx> wrote:
>
> When KASLR enaled(CONFIG_RANDOMIZE_BASE=y), the top 4K virtual
> address have chance to be mapped to physical address, but which
> is expected to leave room for ERR_PTR.
>
> Also, it might cause some other warparound issue when somewhere
> use the last memory page but no overflow check. Such as the last
> page compressed by LZO:
>
> [ 2738.034508] Unable to handle kernel NULL pointer dereference at virtual address 00000009
> [ 2738.034515] Mem abort info:
> [ 2738.034518]   Exception class = DABT (current EL), IL = 32 bits
> [ 2738.034520]   SET = 0, FnV = 0
> [ 2738.034523]   EA = 0, S1PTW = 0
> [ 2738.034524]   FSC = 5
> [ 2738.034526] Data abort info:
> [ 2738.034528]   ISV = 0, ISS = 0x00000005
> [ 2738.034530]   CM = 0, WnR = 0
> [ 2738.034533] user pgtable: 4k pages, 39-bit VAs, pgd = ffffffff94cee000
> [ 2738.034535] [0000000000000009] *pgd=0000000000000000, *pud=0000000000000000
> ...
> [ 2738.034592] pc : lzo1x_1_do_compress+0x198/0x610
> [ 2738.034595] lr : lzo1x_1_compress+0x98/0x3d8
> [ 2738.034597] sp : ffffff801caa3470 pstate : 00c00145
> [ 2738.034598] x29: ffffff801caa3500 x28: 0000000000001000
> [ 2738.034601] x27: 0000000000001000 x26: fffffffffffff000
> [ 2738.034604] x25: ffffffff4ebc0000 x24: 0000000000000000
> [ 2738.034607] x23: 000000000000004c x22: fffffffffffff7b8
> [ 2738.034610] x21: ffffffff2e2ee0b3 x20: ffffffff2e2ee0bb
> [ 2738.034612] x19: 0000000000000fcc x18: fffffffffffff84a
> [ 2738.034615] x17: 00000000801b03d6 x16: 0000000000000782
> [ 2738.034618] x15: ffffffff2e2ee0bf x14: fffffffffffffff0
> [ 2738.034620] x13: 000000000000000f x12: 0000000000000020
> [ 2738.034623] x11: 000000001824429d x10: ffffffffffffffec
> [ 2738.034626] x9 : 0000000000000009 x8 : 0000000000000000
> [ 2738.034628] x7 : 0000000000000868 x6 : 0000000000000434
> [ 2738.034631] x5 : ffffffff4ebc0000 x4 : 0000000000000000
> [ 2738.034633] x3 : ffffff801caa3510 x2 : ffffffff2e2ee000
> [ 2738.034636] x1 : 0000000000000000 x0 : fffffffffffff000
> ...
> [ 2738.034717] Process kworker/u16:1 (pid: 8705, stack limit = 0xffffff801caa0000)
> [ 2738.034720] Call trace:
> [ 2738.034722]  lzo1x_1_do_compress+0x198/0x610
> [ 2738.034725]  lzo_compress+0x48/0x88
> [ 2738.034729]  crypto_compress+0x14/0x20
> [ 2738.034733]  zcomp_compress+0x2c/0x38
> [ 2738.034736]  zram_bvec_rw+0x3d0/0x860
> [ 2738.034738]  zram_rw_page+0x88/0xe0
> [ 2738.034742]  bdev_write_page+0x70/0xc0
> [ 2738.034745]  __swap_writepage+0x58/0x3f8
> [ 2738.034747]  swap_writepage+0x40/0x50
> [ 2738.034750]  shrink_page_list+0x4fc/0xe58
> [ 2738.034753]  reclaim_pages_from_list+0xa0/0x150
> [ 2738.034756]  reclaim_pte_range+0x18c/0x1f8
> [ 2738.034759]  __walk_page_range+0xf8/0x1e0
> [ 2738.034762]  walk_page_range+0xf8/0x130
> [ 2738.034765]  reclaim_task_anon+0xcc/0x168
> [ 2738.034767]  swap_fn+0x438/0x668
> [ 2738.034771]  process_one_work+0x1fc/0x460
> [ 2738.034773]  worker_thread+0x2d0/0x478
> [ 2738.034775]  kthread+0x110/0x120
> [ 2738.034778]  ret_from_fork+0x10/0x18
> [ 2738.034781] Code: 3800167f 54ffffa8 d100066f 14000031 (b9400131)
> [ 2738.034784] ---[ end trace 9b5cca106f0e54d1 ]---
> [ 2738.035473] Kernel panic - not syncing: Fatal exception
>
> in = 0xfffffffffffff000
> in_len = 4096
> ip = x9 = 0x0000000000000009 overflowed.
>
> Always leave room the last size of ARM64_MEMSTART_ALIGN region
> in linear region.
>
> Signed-off-by: liyueyi <liyueyi@xxxxxxxx>
> ---
>  arch/arm64/mm/init.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
> index 0340e45..20fe11e 100644
> --- a/arch/arm64/mm/init.c
> +++ b/arch/arm64/mm/init.c
> @@ -439,7 +439,8 @@ void __init arm64_memblock_init(void)
>         if (IS_ENABLED(CONFIG_RANDOMIZE_BASE)) {
>                 extern u16 memstart_offset_seed;
>                 u64 range = linear_region_size -
> -                           (memblock_end_of_DRAM() - memblock_start_of_DRAM());
> +                          (memblock_end_of_DRAM() - memblock_start_of_DRAM()) -
> +                          ARM64_MEMSTART_ALIGN;
>
>                 /*
>                  * If the size of the linear region exceeds, by a sufficient

Does the following change fix your issue as well?

index 9b432d9fcada..9dcf0ff75a11 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -447,7 +447,7 @@ void __init arm64_memblock_init(void)
                 * memory spans, randomize the linear region as well.
                 */
                if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN) {
-                       range = range / ARM64_MEMSTART_ALIGN + 1;
+                       range /= ARM64_MEMSTART_ALIGN;
                        memstart_addr -= ARM64_MEMSTART_ALIGN *
                                         ((range * memstart_offset_seed) >> 16);
                }