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Message-ID: <87a84cr7oc.fsf@xmission.com>
Date: Mon, 10 Jul 2017 03:57:07 -0500
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Kees Cook <keescook@...omium.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
David Howells <dhowells@...hat.com>,
Serge Hallyn <serge@...lyn.com>,
John Johansen <john.johansen@...onical.com>,
Casey Schaufler <casey@...aufler-ca.com>,
Alexander Viro <viro@...iv.linux.org.uk>,
Michal Hocko <mhocko@...nel.org>,
Ben Hutchings <ben@...adent.org.uk>,
Hugh Dickins <hughd@...gle.com>,
Oleg Nesterov <oleg@...hat.com>,
"Jason A. Donenfeld" <Jason@...c4.com>,
Rik van Riel <riel@...hat.com>,
James Morris <james.l.morris@...cle.com>,
Greg Ungerer <gerg@...ux-m68k.org>,
Ingo Molnar <mingo@...nel.org>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
Stephen Smalley <sds@...ho.nsa.gov>,
Paul Moore <paul@...l-moore.com>,
Vivek Goyal <vgoyal@...hat.com>,
Mickaël Salaün <mic@...ikod.net>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org, selinux@...ho.nsa.gov
Subject: Re: [PATCH v2 2/8] exec: Move security_bprm_secureexec() earlier
Kees Cook <keescook@...omium.org> writes:
> There are several places where exec needs to know if a privilege-gain has
> happened. These should be using the results of security_bprm_secureexec()
> but it is getting (needlessly) called very late.
It is hard to tell at a glance but I believe this introduces a
regression.
cap_bprm_set_creds is currently called before cap_bprm_secureexec and
it has a number of cases such as no_new_privs and ptrace that can result
in some of the precomputed credential changes not happening.
Without accounting for that I believe your cap_bprm_securexec now
returns a postive value too early.
> Instead, move this earlier in the exec code, to the start of the point
> of no return in setup_new_exec(). Here, the new creds have already
> been calculated (and stored in bprm->cred), which is normally what
> security_bprm_secureexec() wants to examine. Since it's moved earlier,
> LSMs hooking bprm_secureexec() need to be adjusted to use the creds in
> bprm:
>
> $ git grep LSM_HOOK_INIT.*bprm_secureexec
> apparmor/lsm.c: LSM_HOOK_INIT(bprm_secureexec, apparmor_bprm_secureexec),
> commoncap.c: LSM_HOOK_INIT(bprm_secureexec, cap_bprm_secureexec),
> selinux/hooks.c: LSM_HOOK_INIT(bprm_secureexec, selinux_bprm_secureexec),
> smack/smack_lsm.c: LSM_HOOK_INIT(bprm_secureexec, smack_bprm_secureexec),
>
> AppArmor does not access creds in apparmor_bprm_secureexec.
>
> Capabilities needed to be adjusted to use bprm creds.
>
> SELinux needed to be adjusted to use bprm creds for the security structure.
>
> Smack needed to be adjusted to use bprm creds for the security structure.
>
> The result of the bprm_secureexec() hook is saved in a new bprm field
> "secureexec" so it can be queried later (just AT_SECURE currently).
>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> fs/binfmt_elf.c | 2 +-
> fs/binfmt_elf_fdpic.c | 2 +-
> fs/exec.c | 5 +++++
> include/linux/binfmts.h | 3 ++-
> include/linux/lsm_hooks.h | 3 ++-
> security/commoncap.c | 4 +---
> security/selinux/hooks.c | 2 +-
> security/smack/smack_lsm.c | 2 +-
> 8 files changed, 14 insertions(+), 9 deletions(-)
>
> 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 7842ae661e34..b92e37fb53aa 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1337,6 +1337,11 @@ EXPORT_SYMBOL(would_dump);
>
> void setup_new_exec(struct linux_binprm * bprm)
> {
> + if (security_bprm_secureexec(bprm)) {
> + /* Record for AT_SECURE. */
> + bprm->secureexec = 1;
> + }
> +
> 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 05488da3aee9..1afaa303cad0 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -27,9 +27,10 @@ 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 */
> + secureexec:1; /* true when gaining privileges */
> #ifdef __alpha__
> unsigned int taso:1;
> #endif
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 080f34e66017..d1bd24fb4a33 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -72,7 +72,8 @@
> * Return a boolean value (0 or 1) indicating whether a "secure exec"
> * is required. The flag is passed in the auxiliary table
> * on the initial stack to the ELF interpreter to indicate whether libc
> - * should enable secure mode.
> + * should enable secure mode. Called before bprm_committing_creds(),
> + * so pending credentials are in @bprm->cred.
> * @bprm contains the linux_binprm structure.
> *
> * Security hooks for filesystem operations.
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 7abebd782d5e..482d3aac2fc6 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -624,12 +624,10 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
> * Determine whether a secure execution is required, return 1 if it is, and 0
> * if it is not.
> *
> - * The credentials have been committed by this point, and so are no longer
> - * available through @bprm->cred.
> */
> int cap_bprm_secureexec(struct linux_binprm *bprm)
> {
> - const struct cred *cred = current_cred();
> + const struct cred *cred = bprm->cred;
> kuid_t root_uid = make_kuid(cred->user_ns, 0);
>
> if (!uid_eq(cred->uid, root_uid)) {
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 819fd6858b49..9381c8474cf4 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2420,7 +2420,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
>
> static int selinux_bprm_secureexec(struct linux_binprm *bprm)
> {
> - const struct task_security_struct *tsec = current_security();
> + const struct task_security_struct *tsec = bprm->cred->security;
> u32 sid, osid;
> int atsecure = 0;
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 658f5d8c7e76..13cf9e66d5fe 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -975,7 +975,7 @@ static void smack_bprm_committing_creds(struct linux_binprm *bprm)
> */
> static int smack_bprm_secureexec(struct linux_binprm *bprm)
> {
> - struct task_smack *tsp = current_security();
> + struct task_smack *tsp = bprm->cred->security;
>
> if (tsp->smk_task != tsp->smk_forked)
> return 1;
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