[PATCH] arm64: set UXN on swapper page tables

[Will Deacon]

On Tue, Jul 19, 2022 at 04:49:09PM -0700, Peter Collingbourne wrote:
> On a system that implements FEAT_EPAN, read/write access to the idmap
> is denied because UXN is not set on the swapper PTEs. As a result,
> idmap_kpti_install_ng_mappings panics the kernel when accessing
> __idmap_kpti_flag. Fix it by setting UXN on these PTEs.
> 
> Fixes: 18107f8a2df6 ("arm64: Support execute-only permissions with Enhanced PAN")
> Cc: <stable at vger.kernel.org> # 5.15
> Link: https://linux-review.googlesource.com/id/Ic452fa4b4f74753e54f71e61027e7222a0fae1b1
> Signed-off-by: Peter Collingbourne <pcc at google.com>
> ---
> This fix is no longer needed since commit c3cee924bd85 ("arm64: head:
> cover entire kernel image in initial ID map"), which moved __idmap_kpti_flag
> to .data, but that commit is currently only present in next.
> 
>  arch/arm64/include/asm/kernel-pgtable.h | 4 ++--
>  arch/arm64/kernel/head.S                | 2 +-
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kernel-pgtable.h b/arch/arm64/include/asm/kernel-pgtable.h
> index 96dc0f7da258..a971d462f531 100644
> --- a/arch/arm64/include/asm/kernel-pgtable.h
> +++ b/arch/arm64/include/asm/kernel-pgtable.h
> @@ -103,8 +103,8 @@
>  /*
>   * Initial memory map attributes.
>   */
> -#define SWAPPER_PTE_FLAGS	(PTE_TYPE_PAGE | PTE_AF | PTE_SHARED)
> -#define SWAPPER_PMD_FLAGS	(PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S)
> +#define SWAPPER_PTE_FLAGS	(PTE_TYPE_PAGE | PTE_AF | PTE_SHARED | PTE_UXN)
> +#define SWAPPER_PMD_FLAGS	(PMD_TYPE_SECT | PMD_SECT_AF | PMD_SECT_S | PMD_SECT_UXN)
>  
>  #if ARM64_KERNEL_USES_PMD_MAPS
>  #define SWAPPER_MM_MMUFLAGS	(PMD_ATTRINDX(MT_NORMAL) | SWAPPER_PMD_FLAGS)
> diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S
> index 6a98f1a38c29..8a93a0a7489b 100644
> --- a/arch/arm64/kernel/head.S
> +++ b/arch/arm64/kernel/head.S
> @@ -285,7 +285,7 @@ SYM_FUNC_START_LOCAL(__create_page_tables)
>  	subs	x1, x1, #64
>  	b.ne	1b
>  
> -	mov	x7, SWAPPER_MM_MMUFLAGS
> +	mov_q	x7, SWAPPER_MM_MMUFLAGS

Heh, nice find:

Acked-by: Will Deacon <will at kernel.org>

Catalin, I guess this needs to land for 5.19 given that it's no longer
relevant after the idmap rework pending for 5.20? If it all explodes in
-next, I can merge in for-next/fixes.

Will