Re: [PATCH v2] arm64: kaslr: Fix up the kernel image alignment

[Ard Biesheuvel]

On 22 March 2017 at 11:38, Srinivas Ramana <sramana@xxxxxxxxxxxxxx> wrote:
> From: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
>
> If kernel image extends across alignment boundary, existing
> code increases the KASLR offset by size of kernel image. The
> offset is masked after resizing. There are cases, where after
> masking, we may still have kernel image extending across
> boundary. This eventually results in only 2MB block getting
> mapped while creating the page tables. This results in data aborts
> while accessing unmapped regions during second relocation (with
> kaslr offset) in __primary_switch. To fix this problem, round up the
> kernel image size, by swapper block size, before adding it for
> correction.
>
> For example consider below case, where kernel image still crosses
> 1GB alignment boundary, after masking the offset, which is fixed
> by rounding up kernel image size.
>
> SWAPPER_TABLE_SHIFT = 30
> Swapper using section maps with section size 2MB.
> CONFIG_PGTABLE_LEVELS = 3
> VA_BITS = 39
>
> _text  : 0xffffff8008080000
> _end   : 0xffffff800aa1b000
> offset : 0x1f35600000
> mask = ((1UL << (VA_BITS - 2)) - 1) & ~(SZ_2M - 1)
>
> (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
> (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
>
> offset after existing correction (before mask) = 0x1f37f9b000
> (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
> (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
>
> offset (after mask) = 0x1f37e00000
> (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7c
> (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
>
> new offset w/ rounding up = 0x1f38000000
> (_text + offset) >> SWAPPER_TABLE_SHIFT = 0x3fffffe7d
> (_end + offset) >> SWAPPER_TABLE_SHIFT  = 0x3fffffe7d
>
> Fixes: f80fb3a3d508 ("arm64: add support for kernel ASLR")
> Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Signed-off-by: Srinivas Ramana <sramana@xxxxxxxxxxxxxx>

Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>

... and thanks for the excellent commit log message!

> ---
>  arch/arm64/kernel/kaslr.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
> index 769f24ef628c..d7e90d97f5c4 100644
> --- a/arch/arm64/kernel/kaslr.c
> +++ b/arch/arm64/kernel/kaslr.c
> @@ -131,11 +131,15 @@ u64 __init kaslr_early_init(u64 dt_phys, u64 modulo_offset)
>         /*
>          * The kernel Image should not extend across a 1GB/32MB/512MB alignment
>          * boundary (for 4KB/16KB/64KB granule kernels, respectively). If this
> -        * happens, increase the KASLR offset by the size of the kernel image.
> +        * happens, increase the KASLR offset by the size of the kernel image
> +        * rounded up by SWAPPER_BLOCK_SIZE.
>          */
>         if ((((u64)_text + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT) !=
> -           (((u64)_end + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT))
> -               offset = (offset + (u64)(_end - _text)) & mask;
> +           (((u64)_end + offset + modulo_offset) >> SWAPPER_TABLE_SHIFT)) {
> +               u64 kimg_sz = _end - _text;
> +               offset = (offset + round_up(kimg_sz, SWAPPER_BLOCK_SIZE))
> +                               & mask;
> +       }
>
>         if (IS_ENABLED(CONFIG_KASAN))
>                 /*
> --
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
> is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
>