[v4,2/4] arm64: Enable BTI for main executable as well as the interpreter

  Currently for dynamically linked ELF executables we only enable BTI for
the interpreter, expecting the interpreter to do this for the main
executable. This is a bit inconsistent since we do map main executable and
is causing issues with systemd's MemoryDenyWriteExecute feature which is
implemented using a seccomp filter which prevents setting PROT_EXEC on
already mapped memory and lacks the context to be able to detect that
memory is already mapped with PROT_EXEC.

Resolve this by checking the BTI property for the main executable and
enabling BTI if it is present when doing the initial mapping. This does
mean that we may get more code with BTI enabled if running on a system
without BTI support in the dynamic linker, this is expected to be a safe
configuration and testing seems to confirm that. It also reduces the
flexibility userspace has to disable BTI but it is expected that for cases
where there are problems which require BTI to be disabled it is more likely
that it will need to be disabled on a system level.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
---
 arch/arm64/include/asm/elf.h | 14 ++++++++++----
 arch/arm64/kernel/process.c  | 23 +++++++++++------------
 2 files changed, 21 insertions(+), 16 deletions(-)
  

On Mon, Jul 12, 2021 at 12:52:57PM +0100, Mark Brown wrote:
> Currently for dynamically linked ELF executables we only enable BTI for
> the interpreter, expecting the interpreter to do this for the main
> executable. This is a bit inconsistent since we do map main executable and
> is causing issues with systemd's MemoryDenyWriteExecute feature which is
> implemented using a seccomp filter which prevents setting PROT_EXEC on
> already mapped memory and lacks the context to be able to detect that
> memory is already mapped with PROT_EXEC.
> 
> Resolve this by checking the BTI property for the main executable and
> enabling BTI if it is present when doing the initial mapping. This does
> mean that we may get more code with BTI enabled if running on a system
> without BTI support in the dynamic linker, this is expected to be a safe
> configuration and testing seems to confirm that. It also reduces the
> flexibility userspace has to disable BTI but it is expected that for cases
> where there are problems which require BTI to be disabled it is more likely
> that it will need to be disabled on a system level.
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> Reviewed-by: Dave Martin <Dave.Martin@arm.com>
> Tested-by: Jeremy Linton <jeremy.linton@arm.com>
> ---
>  arch/arm64/include/asm/elf.h | 14 ++++++++++----
>  arch/arm64/kernel/process.c  | 23 +++++++++++------------
>  2 files changed, 21 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index a488a1329b16..9f86dbce2680 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -253,7 +253,8 @@ struct arch_elf_state {
>  	int flags;
>  };
>  
> -#define ARM64_ELF_BTI		(1 << 0)
> +#define ARM64_ELF_INTERP_BTI		(1 << 0)
> +#define ARM64_ELF_EXEC_BTI		(1 << 1)
>  
>  #define INIT_ARCH_ELF_STATE {			\
>  	.flags = 0,				\
> @@ -274,9 +275,14 @@ static inline int arch_parse_elf_property(u32 type, const void *data,
>  		if (datasz != sizeof(*p))
>  			return -ENOEXEC;
>  
> -		if (system_supports_bti() && has_interp == is_interp &&
> -		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
> -			arch->flags |= ARM64_ELF_BTI;
> +		if (system_supports_bti() &&
> +		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI)) {
> +			if (is_interp) {
> +				arch->flags |= ARM64_ELF_INTERP_BTI;
> +			} else {
> +				arch->flags |= ARM64_ELF_EXEC_BTI;
> +			}

Can this just be

	arch->flags |= arm64_elf_bti_flag(is_interp);

if the helper is moved to this header?


> +		}
>  	}
>  
>  	return 0;
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index c8989b999250..5a6c3b198bd3 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -683,21 +683,20 @@ core_initcall(tagged_addr_init);
>  #endif	/* CONFIG_ARM64_TAGGED_ADDR_ABI */
>  
>  #ifdef CONFIG_BINFMT_ELF
> -int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> -			 bool has_interp, bool is_interp)
> +static inline int arm64_elf_bti_flag(bool is_interp)
>  {
> -	/*
> -	 * For dynamically linked executables the interpreter is
> -	 * responsible for setting PROT_BTI on everything except
> -	 * itself.
> -	 */
> -	if (is_interp != has_interp)
> -		return prot;
> +	if (is_interp)
> +		return ARM64_ELF_INTERP_BTI;
> +	else
> +		return ARM64_ELF_EXEC_BTI;
> +}
>  
> -	if (!(state->flags & ARM64_ELF_BTI))
> -		return prot;
>  
> -	if (prot & PROT_EXEC)
> +int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> +			 bool has_interp, bool is_interp)
> +{
> +	if ((prot & PROT_EXEC) &&
> +	    (state->flags & arm64_elf_bti_flag(is_interp)))

Preferably with the above change (but if not, I'll live without):

Reviewed-by: Dave Martin <Dave.Martin@arm.com>

>  		prot |= PROT_BTI;
>  
>  	return prot;
> --