Messages in this thread | | | Subject | Re: [RFC] Clean up MNT_NOSUID handling in exec | From | Stephen Smalley <> | Date | Wed, 21 Apr 2010 08:25:44 -0400 |
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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@us.ibm.com> wrote: > > Quoting Andrew Lutomirski (luto@mit.edu): > >> On Tue, Apr 20, 2010 at 8:37 AM, Stephen Smalley <sds@tycho.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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