[v4,03/15] binfmt: Introduce secureexec flag

Commit Message

Kees Cook July 31, 2017, 11:51 p.m. UTC

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->called_set_creds
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, and calls the secureexec hook during setup_new_exec(). This is
safe since all the cred work is finished (and past the point of no return).
This explicit call will be removed in later patches once the hook has been
removed.

Cc: David Howells <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Reviewed-by: John Johansen <[e-mail]>
Acked-by: Serge Hallyn <[e-mail]>
---
 fs/binfmt_elf.c         | 2 +-
 fs/binfmt_elf_fdpic.c   | 2 +-
 fs/exec.c               | 2 ++
 include/linux/binfmts.h | 6 ++++++
 4 files changed, 10 insertions(+), 2 deletions(-)

Comments

Kees Cook Aug. 1, 2017, 12:23 a.m. UTC | #1

On Mon, Jul 31, 2017 at 4:51 PM, Kees Cook <[e-mail]> wrote:
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index 3cd98e8bc9dc..6cfd36a27d4e 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -34,6 +34,12 @@ struct linux_binprm {
>                 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

Grrr. git rebase messed me up. (; vs , in variable list.) I will send
a v5 and double-check the per-patch builds. Bleh.

-Kees

James Morris Aug. 1, 2017, 12:44 a.m. UTC | #2

On Mon, 31 Jul 2017, Kees Cook wrote:

> Cc: David Howells <[e-mail]>
> Signed-off-by: Kees Cook <[e-mail]>
> Reviewed-by: John Johansen <[e-mail]>
> Acked-by: Serge Hallyn <[e-mail]>
> ---
>  fs/binfmt_elf.c         | 2 +-
>  fs/binfmt_elf_fdpic.c   | 2 +-
>  fs/exec.c               | 2 ++
>  include/linux/binfmts.h | 6 ++++++
>  4 files changed, 10 insertions(+), 2 deletions(-)
> 


Reviewed-by: James Morris <[e-mail]>

Patch

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 5075fd5c62c8..7f6ec4dac13d 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -254,7 +254,7 @@  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));
+	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..5aa9199dfb13 100644
--- a/fs/binfmt_elf_fdpic.c
+++ b/fs/binfmt_elf_fdpic.c
@@ -650,7 +650,7 @@  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));
+	NEW_AUX_ENT(AT_SECURE,	bprm->secureexec);
 	NEW_AUX_ENT(AT_EXECFN,	bprm->exec);
 
 #ifdef ARCH_DLINFO
diff --git a/fs/exec.c b/fs/exec.c
index 90bd5b85814f..77244367c773 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1322,6 +1322,8 @@  EXPORT_SYMBOL(would_dump);
 
 void setup_new_exec(struct linux_binprm * bprm)
 {
+	bprm->secureexec |= security_bprm_secureexec(bprm);
+
 	arch_pick_mmap_layout(current->mm);
 
 	current->sas_ss_sp = current->sas_ss_size = 0;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 3cd98e8bc9dc..6cfd36a27d4e 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -34,6 +34,12 @@  struct linux_binprm {
 		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