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Date: Wed, 21 Apr 2010 08:25:44 -0400
From: Stephen Smalley <sds@...ho.nsa.gov>
To: Andrew Lutomirski <luto@....edu>
Cc: "Serge E. Hallyn" <serue@...ibm.com>, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org,
Eric Biederman <ebiederm@...ssion.com>,
"Andrew G. Morgan" <morgan@...nel.org>
Subject: Re: [RFC] Clean up MNT_NOSUID handling in exec
On Tue, 2010-04-20 at 22:49 -0400, Andrew Lutomirski wrote:
> On Tue, Apr 20, 2010 at 10:25 PM, Serge E. Hallyn <serue@...ibm.com> wrote:
> > Quoting Andrew Lutomirski (luto@....edu):
> >> On Tue, Apr 20, 2010 at 8:37 AM, Stephen Smalley <sds@...ho.nsa.gov> wrote:
> >> >
> >> > At least in the case of SELinux, context transitions upon execve are
> >> > already disabled in the nosuid case, and Eric's patch updated the
> >> > SELinux test accordingly.
> >>
> >> I don't see that code in current -linus, nor do I see where SELinux
> >> affects dumpability. What's supposed to happen? I'm writing a patch
> >> right now to clean this stuff up.
> >
> > check out security/selinux/hooks.c:selinux_bprm_set_creds()
> >
> > if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> > new_tsec->sid = old_tsec->sid;
> >
> > I assume that's it?
>
> *sigh*, that's it, of course.
>
> But it seems odd that if you ask the system to change sid (i.e.
> exec_sid) and then you exec something that's nosuid, your request gets
> silently ignored. Presumably either the request should be respected
> or some error should occur.
>
> Also, the FILE__EXECUTE_NO_TRANS check looks bogus in the nosuid case
> -- you can't trust security labels if nosuid.
The current behavior, while possibly non-ideal, matches that of nosuid -
we suppress context transitions for files on that filesystem (without
preventing execution of files - for which there is a separate option,
noexec), while still accepting uids provided by the filesystem as
attributes for its files. It was only intended to prevent privilege
escalation via executables on such a nosuid filesystem. If you further
want to suppress all use of security contexts from the filesystem, then
you'd use the context= mount option that was later introduced.
In any event, you cannot simply change existing kernel behavior like
this, as it will break existing userspace+policy. We can evolve the
logic conditional on some policy setting, but we have to preserve the
existing behavior for existing userland/policy.
> Here's a compile-tested patch (against current -git) that might make
> this all it bit more sane, and as a bonus it makes it easier to reuse
> the NOSUID mechanism down the road for some prctl flag. Thoughts?
> SECINITSID_UNLABELED might want to be SECINITSID_FILE.
NAK for the changes to SELinux other than just changing the test to use
your new field (which btw looks just like a subset of Eric's patch aside
from naming it ->file_nosuid vs. just ->nosuid).
> diff --git a/fs/exec.c b/fs/exec.c
> index 49cdaa1..9b34234 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1151,11 +1151,14 @@ int prepare_binprm(struct linux_binprm *bprm)
> if (bprm->file->f_op == NULL)
> return -EACCES;
>
> + /* Check this exactly once to avoid races. */
> + bprm->file_nosuid = !!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID);
> +
> /* clear any previous set[ug]id data from a previous binary */
> bprm->cred->euid = current_euid();
> bprm->cred->egid = current_egid();
>
> - if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)) {
> + if (!bprm->file_nosuid) {
> /* Set-uid? */
> if (mode & S_ISUID) {
> bprm->per_clear |= PER_CLEAR_ON_SETID;
> diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
> index c809e28..ff6be04 100644
> --- a/include/linux/binfmts.h
> +++ b/include/linux/binfmts.h
> @@ -36,6 +36,8 @@ struct linux_binprm{
> struct mm_struct *mm;
> unsigned long p; /* current top of mem */
> unsigned int
> + file_nosuid:1, /* true if file's security state should
> + be ignored. */
> cred_prepared:1,/* true if creds already prepared (multiple
> * preps happen for interpreters) */
> cap_effective:1;/* true if has elevated effective capabilities,
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 6166973..4836913 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -390,7 +390,7 @@ static int get_file_caps(struct linux_binprm
> *bprm, bool *effective)
> if (!file_caps_enabled)
> return 0;
>
> - if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
> + if (bprm->file_nosuid)
> return 0;
>
> dentry = dget(bprm->file->f_dentry);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 5feecb4..762ac00 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2100,7 +2100,7 @@ static int selinux_bprm_set_creds(struct
> linux_binprm *bprm)
> {
> const struct task_security_struct *old_tsec;
> struct task_security_struct *new_tsec;
> - struct inode_security_struct *isec;
> + u32 isid;
> struct common_audit_data ad;
> struct inode *inode = bprm->file->f_path.dentry->d_inode;
> int rc;
> @@ -2116,7 +2116,9 @@ static int selinux_bprm_set_creds(struct
> linux_binprm *bprm)
>
> old_tsec = current_security();
> new_tsec = bprm->cred->security;
> - isec = inode->i_security;
> + isid = ((struct inode_security_struct*)inode->i_security)->sid;
> + if (bprm->file_nosuid)
> + isid = SECINITSID_UNLABELED;
>
> /* Default to the current task SID. */
> new_tsec->sid = old_tsec->sid;
> @@ -2133,7 +2135,7 @@ static int selinux_bprm_set_creds(struct
> linux_binprm *bprm)
> new_tsec->exec_sid = 0;
> } else {
> /* Check for a default transition on this program. */
> - rc = security_transition_sid(old_tsec->sid, isec->sid,
> + rc = security_transition_sid(old_tsec->sid, isid,
> SECCLASS_PROCESS, &new_tsec->sid);
> if (rc)
> return rc;
> @@ -2142,11 +2144,8 @@ static int selinux_bprm_set_creds(struct
> linux_binprm *bprm)
> COMMON_AUDIT_DATA_INIT(&ad, FS);
> ad.u.fs.path = bprm->file->f_path;
>
> - if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> - new_tsec->sid = old_tsec->sid;
> -
> if (new_tsec->sid == old_tsec->sid) {
> - rc = avc_has_perm(old_tsec->sid, isec->sid,
> + rc = avc_has_perm(old_tsec->sid, isid,
> SECCLASS_FILE, FILE__EXECUTE_NO_TRANS, &ad);
> if (rc)
> return rc;
> @@ -2157,7 +2156,7 @@ static int selinux_bprm_set_creds(struct
> linux_binprm *bprm)
> if (rc)
> return rc;
>
> - rc = avc_has_perm(new_tsec->sid, isec->sid,
> + rc = avc_has_perm(new_tsec->sid, isid,
> SECCLASS_FILE, FILE__ENTRYPOINT, &ad);
> if (rc)
> return rc;
--
Stephen Smalley
National Security Agency
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