[PATCH v11 2/2] arm64: Enable BTI for main executable as well as the interpreter

[Jeremy Linton]

Hi,

On 3/8/22 07:22, 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 adding a sysctl abi.bti_main which causes the kernel to
> checking the BTI property for the main executable and enable BTI if it
> is present when doing the initial mapping. This sysctl is disabled by
> default.

This seems less than ideal, maybe the default can be flipped with a 
CONFIG option?

> 
> Signed-off-by: Mark Brown <broonie at kernel.org>
> Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>
> ---
>   arch/arm64/include/asm/elf.h | 15 ++++++++---
>   arch/arm64/kernel/process.c  | 52 +++++++++++++++++++++++++++---------
>   2 files changed, 51 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 5cc002376abe..c4aa60db76a4 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -251,12 +251,21 @@ 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,				\
>   }
>   
> +static inline int arm64_elf_bti_flag(bool is_interp)
> +{
> +	if (is_interp)
> +		return ARM64_ELF_INTERP_BTI;
> +	else
> +		return ARM64_ELF_EXEC_BTI;
> +}
> +
>   static inline int arch_parse_elf_property(u32 type, const void *data,
>   					  size_t datasz, bool compat,
>   					  bool has_interp, bool is_interp,
> @@ -272,9 +281,9 @@ 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 &&
> +		if (system_supports_bti() &&
>   		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
> -			arch->flags |= ARM64_ELF_BTI;
> +			arch->flags |= arm64_elf_bti_flag(is_interp);
>   	}
>   
>   	return 0;
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 5369e649fa79..82aaf361fa17 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -701,23 +701,49 @@ core_initcall(tagged_addr_init);
>   #endif	/* CONFIG_ARM64_TAGGED_ADDR_ABI */
>   
>   #ifdef CONFIG_BINFMT_ELF
> +static unsigned int bti_main;
> +
>   int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
>   			 bool has_interp, 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 (!(state->flags & ARM64_ELF_BTI))
> -		return prot;
> -
> -	if (prot & PROT_EXEC)
> +	if ((prot & PROT_EXEC) &&
> +	    (is_interp || !has_interp || bti_main) &&
> +	    (state->flags & arm64_elf_bti_flag(is_interp)))
>   		prot |= PROT_BTI;
>   
>   	return prot;
>   }
> -#endif
> +
> +#ifdef CONFIG_ARM64_BTI
> +/*
> + * If this sysctl is enabled then we will apply PROT_BTI to the main
> + * executable as well as the dynamic linker if it has the appropriate
> + * ELF note.  It is disabled by default, in which case we will only
> + * apply PROT_BTI to the dynamic linker or static binaries.
> + */
> +static struct ctl_table bti_main_sysctl_table[] = {
> +	{
> +		.procname	= "bti_main",
> +		.mode		= 0644,
> +		.data		= &bti_main,
> +		.maxlen		= sizeof(int),
> +		.proc_handler	= proc_dointvec_minmax,
> +		.extra1		= SYSCTL_ZERO,
> +		.extra2		= SYSCTL_ONE,
> +	},
> +	{ }
> +};
> +
> +static int __init bti_main_init(void)
> +{
> +	if (!system_supports_bti())
> +		return 0;
> +
> +	if (!register_sysctl("abi", bti_main_sysctl_table))
> +		return -EINVAL;
> +	return 0;
> +}
> +core_initcall(bti_main_init);
> +#endif /* CONFIG_ARM64_BTI */
> +
> +#endif /* CONFIG_BINFMT_ELF */