Subject: Re: [PATCH v3 06/12] elf: Allow arch to tweak initial mmap prot flags

From: Kees Cook <[email-masked]>
Date: Tue, 29 Oct 2019 16:19:50 -0700

On Fri, Oct 18, 2019 at 06:25:39PM +0100, Dave Martin wrote:
> An arch may want to tweak the mmap prot flags for an
> ELFexecutable's initial mappings.  For example, arm64 is going to
> need to add PROT_BTI for executable pages in an ELF process whose
> executable is marked as using Branch Target Identification (an
> ARMv8.5-A control flow integrity feature).
> 
> So that this can be done in a generic way, add a hook
> arch_elf_adjust_prot() to modify the prot flags as desired: arches
> can select CONFIG_HAVE_ELF_PROT and implement their own backend
> where necessary.
> 
> By default, leave the prot flags unchanged.
> 
> Signed-off-by: Dave Martin <[email-masked]>
> ---
>  fs/Kconfig.binfmt   |  3 +++
>  fs/binfmt_elf.c     | 18 ++++++++++++------
>  include/linux/elf.h | 12 ++++++++++++
>  3 files changed, 27 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/Kconfig.binfmt b/fs/Kconfig.binfmt
> index d2cfe07..2358368 100644
> --- a/fs/Kconfig.binfmt
> +++ b/fs/Kconfig.binfmt
> @@ -36,6 +36,9 @@ config COMPAT_BINFMT_ELF
>  config ARCH_BINFMT_ELF_STATE
>  	bool
>  
> +config ARCH_HAVE_ELF_PROT
> +	bool
> +
>  config ARCH_USE_GNU_PROPERTY
>  	bool
>  
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index ae345f6..dbfab2e 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -531,7 +531,8 @@ static inline int arch_check_elf(struct elfhdr *ehdr, bool has_interp,
>  
>  #endif /* !CONFIG_ARCH_BINFMT_ELF_STATE */
>  
> -static inline int make_prot(u32 p_flags)
> +static inline int make_prot(u32 p_flags, struct arch_elf_state *arch_state,
> +			    bool has_interp, bool is_interp)
>  {
>  	int prot = 0;
>  
> @@ -541,7 +542,8 @@ static inline int make_prot(u32 p_flags)
>  		prot |= PROT_WRITE;
>  	if (p_flags & PF_X)
>  		prot |= PROT_EXEC;
> -	return prot;
> +
> +	return arch_elf_adjust_prot(prot, arch_state, has_interp, is_interp);
>  }

Random thought: there is already an 'exec-state' structure: bprm. I
wonder if arch_state should be attached there instead of passed around
here? It'd require almost the same amount of changes, though, so my idea
might not gain much in the diffstat, but maybe it's better
organizationally? I'm not sure.

Otherwise, this all looks fine to me.

Reviewed-by: Kees Cook <[email-masked]>

-Kees

>  
>  /* This is much more generalized than the library routine read function,
> @@ -551,7 +553,8 @@ static inline int make_prot(u32 p_flags)
>  
>  static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>  		struct file *interpreter, unsigned long *interp_map_addr,
> -		unsigned long no_base, struct elf_phdr *interp_elf_phdata)
> +		unsigned long no_base, struct elf_phdr *interp_elf_phdata,
> +		struct arch_elf_state *arch_state)
>  {
>  	struct elf_phdr *eppnt;
>  	unsigned long load_addr = 0;
> @@ -583,7 +586,8 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>  	for (i = 0; i < interp_elf_ex->e_phnum; i++, eppnt++) {
>  		if (eppnt->p_type == PT_LOAD) {
>  			int elf_type = MAP_PRIVATE | MAP_DENYWRITE;
> -			int elf_prot = make_prot(eppnt->p_flags);
> +			int elf_prot = make_prot(eppnt->p_flags, arch_state,
> +						 true, true);
>  			unsigned long vaddr = 0;
>  			unsigned long k, map_addr;
>  
> @@ -1040,7 +1044,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  			}
>  		}
>  
> -		elf_prot = make_prot(elf_ppnt->p_flags);
> +		elf_prot = make_prot(elf_ppnt->p_flags, &arch_state,
> +				     !!interpreter, false);
>  
>  		elf_flags = MAP_PRIVATE | MAP_DENYWRITE | MAP_EXECUTABLE;
>  
> @@ -1186,7 +1191,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		elf_entry = load_elf_interp(&loc->interp_elf_ex,
>  					    interpreter,
>  					    &interp_map_addr,
> -					    load_bias, interp_elf_phdata);
> +					    load_bias, interp_elf_phdata,
> +					    &arch_state);
>  		if (!IS_ERR((void *)elf_entry)) {
>  			/*
>  			 * load_elf_interp() returns relocation
> diff --git a/include/linux/elf.h b/include/linux/elf.h
> index 7bdc6da..1b6e895 100644
> --- a/include/linux/elf.h
> +++ b/include/linux/elf.h
> @@ -83,4 +83,16 @@ extern int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
>  				   bool compat, struct arch_elf_state *arch);
>  #endif
>  
> +#ifdef CONFIG_ARCH_HAVE_ELF_PROT
> +int arch_elf_adjust_prot(int prot, const struct arch_elf_state *state,
> +			 bool has_interp, bool is_interp);
> +#else
> +static inline int arch_elf_adjust_prot(int prot,
> +				       const struct arch_elf_state *state,
> +				       bool has_interp, bool is_interp)
> +{
> +	return prot;
> +}
> +#endif
> +
>  #endif /* _LINUX_ELF_H */
> -- 
> 2.1.4
> 

-- 
Kees Cook

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