Re: [PATCH 4.4 072/101] arm64: assembler: make adr_l work in modules under KASLR

Ard Biesheuvel
2017-07-04T09:24:39Z

On 3 July 2017 at 14:35, Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
> 4.4-stable review patch. If anyone has any objections, please let me know.
>
> ------------------
>
> From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>
>
> [ Upstream commit 41c066f2c4d436c535616fe182331766c57838f0 ]
>
> When CONFIG_RANDOMIZE_MODULE_REGION_FULL=y, the offset between loaded
> modules and the core kernel may exceed 4 GB, putting symbols exported
> by the core kernel out of the reach of the ordinary adrp/add instruction
> pairs used to generate relative symbol references. So make the adr_l
> macro emit a movz/movk sequence instead when executing in module context.
>
> While at it, remove the pointless special case for the stack pointer.
>

As it turns out, this 'pointless special case' was not so pointless in
v4.4, and removing it breaks the build.

Given that the only in-module user of adr_l is the new scalar AES
code, which is not backported to v4.4, there is really no point in
backporting this to v4.4.

--
Ard.

> Acked-by: Mark Rutland <mark.rutland@arm.com>
> Acked-by: Will Deacon <will.deacon@arm.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
> Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> arch/arm64/include/asm/assembler.h | 36 +++++++++++++++++++++++++++---------
> 1 file changed, 27 insertions(+), 9 deletions(-)
>
> --- a/arch/arm64/include/asm/assembler.h
> +++ b/arch/arm64/include/asm/assembler.h
> @@ -147,22 +147,25 @@ lr .req x30 // link register
>
> /*
> * Pseudo-ops for PC-relative adr/ldr/str <reg>, <symbol> where
> - * <symbol> is within the range +/- 4 GB of the PC.
> + * <symbol> is within the range +/- 4 GB of the PC when running
> + * in core kernel context. In module context, a movz/movk sequence
> + * is used, since modules may be loaded far away from the kernel
> + * when KASLR is in effect.
> */
> /*
> * @dst: destination register (64 bit wide)
> * @sym: name of the symbol
> - * @tmp: optional scratch register to be used if <dst> == sp, which
> - * is not allowed in an adrp instruction
> */
> - .macro adr_l, dst, sym, tmp=
> - .ifb \tmp
> + .macro adr_l, dst, sym
> +#ifndef MODULE
> adrp \dst, \sym
> add \dst, \dst, :lo12:\sym
> - .else
> - adrp \tmp, \sym
> - add \dst, \tmp, :lo12:\sym
> - .endif
> +#else
> + movz \dst, #:abs_g3:\sym
> + movk \dst, #:abs_g2_nc:\sym
> + movk \dst, #:abs_g1_nc:\sym
> + movk \dst, #:abs_g0_nc:\sym
> +#endif
> .endm
>
> /*
> @@ -173,6 +176,7 @@ lr .req x30 // link register
> * the address
> */
> .macro ldr_l, dst, sym, tmp=
> +#ifndef MODULE
> .ifb \tmp
> adrp \dst, \sym
> ldr \dst, [\dst, :lo12:\sym]
> @@ -180,6 +184,15 @@ lr .req x30 // link register
> adrp \tmp, \sym
> ldr \dst, [\tmp, :lo12:\sym]
> .endif
> +#else
> + .ifb \tmp
> + adr_l \dst, \sym
> + ldr \dst, [\dst]
> + .else
> + adr_l \tmp, \sym
> + ldr \dst, [\tmp]
> + .endif
> +#endif
> .endm
>
> /*
> @@ -189,8 +202,13 @@ lr .req x30 // link register
> * while <src> needs to be preserved.
> */
> .macro str_l, src, sym, tmp
> +#ifndef MODULE
> adrp \tmp, \sym
> str \src, [\tmp, :lo12:\sym]
> +#else
> + adr_l \tmp, \sym
> + str \src, [\tmp]
> +#endif
> .endm
>
> /*
>
>