[PATCH] arm64: kaslr: don't pretend KASLR is enabled if offset < MIN_KIMG_ALIGN

[Mark Rutland]

On Thu, Feb 23, 2023 at 09:41:01PM +0100, Ard Biesheuvel wrote:
> Our virtual KASLR displacement is a randomly chosen multiple of
> 2 MiB plus an offset that is equal to the physical placement modulo 2
> MiB. This arrangement ensures that we can always use 2 MiB block
> mappings (or contiguous PTE mappings for 16k or 64k pages) to map the
> kernel.
> 
> This means that a KASLR offset of less than 2 MiB is simply the product
> of this physical displacement, and no randomization has actually taken
> place. Currently, we use 'kaslr_offset() > 0' to decide whether or not
> randomization has occurred, and so we misidentify this case.

> Fix this, by correctly identifying the case where the virtual
> displacement is a result of the physical displacement only.
> 
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>

Acked-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
> I have sent out the same patch with a much shorter commit log as part of
> my LPA2 series before, but I only realized today that this is likely to
> fix most occurrences of the issue where a single 10g vmalloc() call
> break subsequent module loading, so I am reposting it in isolation.
> 
>  arch/arm64/include/asm/memory.h | 11 +++++++++++
>  arch/arm64/kernel/cpufeature.c  |  2 +-
>  arch/arm64/kernel/kaslr.c       |  2 +-
>  3 files changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
> index 9dd08cd339c3f028..78e5163836a0ab95 100644
> --- a/arch/arm64/include/asm/memory.h
> +++ b/arch/arm64/include/asm/memory.h
> @@ -180,6 +180,7 @@
>  #include <linux/compiler.h>
>  #include <linux/mmdebug.h>
>  #include <linux/types.h>
> +#include <asm/boot.h>
>  #include <asm/bug.h>
>  
>  #if VA_BITS > 48
> @@ -203,6 +204,16 @@ static inline unsigned long kaslr_offset(void)
>  	return kimage_vaddr - KIMAGE_VADDR;
>  }
>  
> +static inline bool kaslr_enabled(void)
> +{
> +	/*
> +	 * The KASLR offset modulo MIN_KIMG_ALIGN is taken from the physical
> +	 * placement of the image rather than from the seed, so a displacement
> +	 * of less than MIN_KIMG_ALIGN means that no seed was provided.
> +	 */
> +	return kaslr_offset() >= MIN_KIMG_ALIGN;
> +}
> +
>  /*
>   * Allow all memory at the discovery stage. We will clip it later.
>   */
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 45a42cf2191c36c3..5643a9ca502af207 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1633,7 +1633,7 @@ bool kaslr_requires_kpti(void)
>  			return false;
>  	}
>  
> -	return kaslr_offset() > 0;
> +	return kaslr_enabled();
>  }
>  
>  static bool __meltdown_safe = true;
> diff --git a/arch/arm64/kernel/kaslr.c b/arch/arm64/kernel/kaslr.c
> index 325455d16dbcb31a..e7477f21a4c9d062 100644
> --- a/arch/arm64/kernel/kaslr.c
> +++ b/arch/arm64/kernel/kaslr.c
> @@ -41,7 +41,7 @@ static int __init kaslr_init(void)
>  		return 0;
>  	}
>  
> -	if (!kaslr_offset()) {
> +	if (!kaslr_enabled()) {
>  		pr_warn("KASLR disabled due to lack of seed\n");
>  		return 0;
>  	}
> -- 
> 2.39.1
>