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@xxxxxxxxxx>
Acked-by: Mark Rutland <mark.rutland@xxxxxxx>
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
>
