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

[Dave Martin via Libc-alpha <libc-alpha@sourceware.org>]

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;
> --