Subject: Re: [PATCH v1 1/2] elf: Allow architectures to parse properties on the main executable

From: Dave Martin <[email-masked]>
Date: Thu, 3 Jun 2021 16:40:24 +0100

On Fri, May 21, 2021 at 03:46:20PM +0100, Mark Brown wrote:
> Currently the ELF code only attempts to parse properties on the image
> that will start execution, either the interpreter or for statically linked
> executables the main executable. The expectation is that any property
> handling for the main executable will be done by the interpreter. This is
> a bit inconsistent since we do map the executable and is causing problems
> for the arm64 BTI support when used in conjunction with systemd's use of
> seccomp to implement MemoryDenyWriteExecute which stops the dynamic linker
> adjusting the permissions of executable segments.
> 
> Allow architectures to handle properties for both the dynamic linker and
> main executable, adjusting arch_parse_elf_properties() to have an is_interp
> flag as with arch_elf_adjust_prot() and calling it for both the main
> executable and any intepreter.
>
> Signed-off-by: Mark Brown <[email-masked]>
> ---
>  arch/arm64/include/asm/elf.h |  3 ++-
>  fs/binfmt_elf.c              | 25 +++++++++++++++++--------
>  include/linux/elf.h          |  4 +++-
>  3 files changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 8d1c8dcb87fd..c8678a8c36d5 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -261,6 +261,7 @@ struct arch_elf_state {
>  
>  static inline int arch_parse_elf_property(u32 type, const void *data,
>  					  size_t datasz, bool compat,
> +					  bool is_interp,
>  					  struct arch_elf_state *arch)
>  {
>  	/* No known properties for AArch32 yet */
> @@ -273,7 +274,7 @@ static inline int arch_parse_elf_property(u32 type, const void *data,
>  		if (datasz != sizeof(*p))
>  			return -ENOEXEC;
>  
> -		if (system_supports_bti() &&
> +		if (system_supports_bti() && is_interp &&

Won't this cause BTI to be forced off for static binaries?

Perhaps this should be (has_interp == is_interp), as for
arch_elf_adjust_prot().  Seems gross though, since has_interp would
become useless after the next patch.  If there's no sensible way to
keep this bisectable, perhaps the patches can be merged instead.

(has_interp should probably also go away for arch_elf_adjust_prot() --
see my comments on the next patch).

>  		    (*p & GNU_PROPERTY_AARCH64_FEATURE_1_BTI))
>  			arch->flags |= ARM64_ELF_BTI;
>  	}
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 187b3f2b9202..c8397664af39 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -716,7 +716,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
>   */
>  
>  static int parse_elf_property(const char *data, size_t *off, size_t datasz,
> -			      struct arch_elf_state *arch,
> +			      struct arch_elf_state *arch, bool is_interp,
>  			      bool have_prev_type, u32 *prev_type)
>  {
>  	size_t o, step;
> @@ -751,7 +751,8 @@ static int parse_elf_property(const char *data, size_t *off, size_t datasz,
>  	*prev_type = pr->pr_type;
>  
>  	ret = arch_parse_elf_property(pr->pr_type, data + o,
> -				      pr->pr_datasz, ELF_COMPAT, arch);
> +				      pr->pr_datasz, ELF_COMPAT, is_interp,
> +				      arch);
>  	if (ret)
>  		return ret;
>  
> @@ -764,7 +765,7 @@ static int parse_elf_property(const char *data, size_t *off, size_t datasz,
>  #define NOTE_NAME_SZ (sizeof(GNU_PROPERTY_TYPE_0_NAME))
>  
>  static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
> -				struct arch_elf_state *arch)
> +				bool is_interp, struct arch_elf_state *arch)
>  {
>  	union {
>  		struct elf_note nhdr;
> @@ -813,7 +814,8 @@ static int parse_elf_properties(struct file *f, const struct elf_phdr *phdr,
>  	have_prev_type = false;
>  	do {
>  		ret = parse_elf_property(note.data, &off, datasz, arch,
> -					 have_prev_type, &prev_type);
> +					 is_interp, have_prev_type,
> +					 &prev_type);
>  		have_prev_type = true;
>  	} while (!ret);
>  
> @@ -828,6 +830,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	unsigned long error;
>  	struct elf_phdr *elf_ppnt, *elf_phdata, *interp_elf_phdata = NULL;
>  	struct elf_phdr *elf_property_phdata = NULL;
> +	struct elf_phdr *interp_elf_property_phdata = NULL;
>  	unsigned long elf_bss, elf_brk;
>  	int bss_prot = 0;
>  	int retval, i;
> @@ -963,12 +966,11 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  			goto out_free_dentry;
>  
>  		/* Pass PT_LOPROC..PT_HIPROC headers to arch code */
> -		elf_property_phdata = NULL;
>  		elf_ppnt = interp_elf_phdata;
>  		for (i = 0; i < interp_elf_ex->e_phnum; i++, elf_ppnt++)
>  			switch (elf_ppnt->p_type) {
>  			case PT_GNU_PROPERTY:
> -				elf_property_phdata = elf_ppnt;
> +				interp_elf_property_phdata = elf_ppnt;

(Hmm, this actually looks a bit cleaner than just clobbering
elf_property_phdata with the interpreter properties as was done
previously.)

>  				break;
>  
>  			case PT_LOPROC ... PT_HIPROC:
> @@ -979,10 +981,17 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  					goto out_free_dentry;
>  				break;
>  			}
> +
> +		retval = parse_elf_properties(interpreter,
> +					      interp_elf_property_phdata,
> +					      true, &arch_state);
> +		if (retval)
> +			goto out_free_dentry;
> +
>  	}
>  
> -	retval = parse_elf_properties(interpreter ?: bprm->file,
> -				      elf_property_phdata, &arch_state);
> +	retval = parse_elf_properties(bprm->file, elf_property_phdata,
> +				      false, &arch_state);
>  	if (retval)
>  		goto out_free_dentry;
>  
> diff --git a/include/linux/elf.h b/include/linux/elf.h
> index c9a46c4e183b..a20dcdcd86c5 100644
> --- a/include/linux/elf.h
> +++ b/include/linux/elf.h
> @@ -88,13 +88,15 @@ struct arch_elf_state;
>  #ifndef CONFIG_ARCH_USE_GNU_PROPERTY
>  static inline int arch_parse_elf_property(u32 type, const void *data,
>  					  size_t datasz, bool compat,
> +					  bool is_interp,
>  					  struct arch_elf_state *arch)
>  {
>  	return 0;
>  }
>  #else
>  extern int arch_parse_elf_property(u32 type, const void *data, size_t datasz,
> -				   bool compat, struct arch_elf_state *arch);
> +				   bool compat, bool is_interp,
> +				   struct arch_elf_state *arch);
>  #endif

Looks like a sensible change, modulo my comments above.

You may want to Cc Yu-cheng Yu <[email-masked]> when reposting,
since this would affect his patches (trivially though).

Cheers
---Dave

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