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Date: Tue, 20 Apr 2010 22:49:39 -0400
From: Andrew Lutomirski <luto@....edu>
To: "Serge E. Hallyn" <serue@...ibm.com>
Cc: Stephen Smalley <sds@...ho.nsa.gov>, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org,
Eric Biederman <ebiederm@...ssion.com>,
"Andrew G. Morgan" <morgan@...nel.org>
Subject: [RFC] Clean up MNT_NOSUID handling in exec
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.
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.
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;
--
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