Re: [PATCH v3 01/15] binfmt: Introduce secureexec flag

[John Johansen]

On 07/18/2017 03:25 PM, Kees Cook wrote:
> The bprm_secureexec hook can be moved earlier. Right now, it is called
> during create_elf_tables(), via load_binary(), via search_binary_handler(),
> via exec_binprm(). Nearly all (see exception below) state used by
> bprm_secureexec is created during the bprm_set_creds hook, called from
> prepare_binprm().
> 
> For all LSMs (except commoncaps described next), only the first execution
> of bprm_set_creds takes any effect (they all check bprm->cred_prepared which
> prepare_binprm() sets after the first call to the bprm_set_creds hook).
> However, all these LSMs also only do anything with bprm_secureexec when
> they detected a secure state during their first run of bprm_set_creds.
> Therefore, it is functionally identical to move the detection into
> bprm_set_creds, since the results from secureexec here only need to be
> based on the first call to the LSM's bprm_set_creds hook.
> 
> The single exception is that the commoncaps secureexec hook also examines
> euid/uid and egid/gid differences which are controlled by bprm_fill_uid(),
> via prepare_binprm(), which can be called multiple times (e.g.
> binfmt_script, binfmt_misc), and may clear the euid/egid for the final
> load (i.e. the script interpreter). However, while commoncaps specifically
> ignores bprm->cred_prepared, and runs its bprm_set_creds hook each time
> prepare_binprm() may get called, it needs to base the secureexec decision
> on the final call to bprm_set_creds. As a result, it will need special
> handling.
> 
> To begin this refactoring, this adds the secureexec flag to the bprm
> struct, which will eventually be used in place of the LSM hook.
> 
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>

looks good

Reviewed-by: John Johansen <john.johansen@xxxxxxxxxxxxx>

> ---
>  fs/binfmt_elf.c         | 3 ++-
>  fs/binfmt_elf_fdpic.c   | 3 ++-
>  include/linux/binfmts.h | 8 +++++++-
>  3 files changed, 11 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 5075fd5c62c8..991e4de3515f 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -254,7 +254,8 @@ create_elf_tables(struct linux_binprm *bprm, struct elfhdr *exec,
>  	NEW_AUX_ENT(AT_EUID, from_kuid_munged(cred->user_ns, cred->euid));
>  	NEW_AUX_ENT(AT_GID, from_kgid_munged(cred->user_ns, cred->gid));
>  	NEW_AUX_ENT(AT_EGID, from_kgid_munged(cred->user_ns, cred->egid));
> - 	NEW_AUX_ENT(AT_SECURE, security_bprm_secureexec(bprm));
> +	bprm->secureexec |= security_bprm_secureexec(bprm);
> +	NEW_AUX_ENT(AT_SECURE, bprm->secureexec);
>  	NEW_AUX_ENT(AT_RANDOM, (elf_addr_t)(unsigned long)u_rand_bytes);
>  #ifdef ELF_HWCAP2
>  	NEW_AUX_ENT(AT_HWCAP2, ELF_HWCAP2);
> diff --git a/fs/binfmt_elf_fdpic.c b/fs/binfmt_elf_fdpic.c
> index cf93a4fad012..c88b35d4a6b3 100644
> --- a/fs/binfmt_elf_fdpic.c
> +++ b/fs/binfmt_elf_fdpic.c
> @@ -650,7 +650,8 @@ static int create_elf_fdpic_tables(struct linux_binprm *bprm,
>  	NEW_AUX_ENT(AT_EUID,	(elf_addr_t) from_kuid_munged(cred->user_ns, cred->euid));
>  	NEW_AUX_ENT(AT_GID,	(elf_addr_t) from_kgid_munged(cred->user_ns, cred->gid));
>  	NEW_AUX_ENT(AT_EGID,	(elf_addr_t) from_kgid_munged(cred->user_ns, cred->egid));
> -	NEW_AUX_ENT(AT_SECURE,	security_bprm_secureexec(bprm));
> +	bprm->secureexec |= security_bprm_secureexec(bprm);
> +	NEW_AUX_ENT(AT_SECURE,	bprm->secureexec);
>  	NEW_AUX_ENT(AT_EXECFN,	bprm->exec);
>  
>  #ifdef ARCH_DLINFO
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index 05488da3aee9..9508b5f83c7e 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -27,9 +27,15 @@ struct linux_binprm {
>  	unsigned int
>  		cred_prepared:1,/* true if creds already prepared (multiple
>  				 * preps happen for interpreters) */
> -		cap_effective:1;/* true if has elevated effective capabilities,
> +		cap_effective:1,/* true if has elevated effective capabilities,
>  				 * false if not; except for init which inherits
>  				 * its parent's caps anyway */
> +		/*
> +		 * Set by bprm_set_creds hook to indicate a privilege-gaining
> +		 * exec has happened. Used to sanitize execution environment
> +		 * and to set AT_SECURE auxv for glibc.
> +		 */
> +		secureexec:1;
>  #ifdef __alpha__
>  	unsigned int taso:1;
>  #endif
>