[PATCH 1/3] p9auth: split core function out of some set*{u,g}id functions

["Serge E. Hallyn" <serue@us.ibm.com>]

Break the core functionality of set{fs,res}{u,g}id into cred_setX
which performs the access checks based on current_cred(), but performs
the requested change on a passed-in cred.

Export the helpers, since p9auth can be compiled as a module.  It
might be worth not allowing modular p9auth to avoid having to export
them.

Really the setfs{u,g}id helper isn't needed, but move it as
well to keep the code consistent.

This patch also changes set_user() to use new->user->user_ns.  While
technically not needed as all callers should have new->user->user_ns
equal to current_userns(), it is more correct and may prevent surprises
in the future.

Changelog:
	Apr 24: (David Howells) make cred_setresuid etc extern, and
		document the helpers in Documentation/credentials.txt.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Cc: David Howells <dhowells@redhat.com>
---
 Documentation/credentials.txt |   18 ++++++
 include/linux/cred.h          |   12 ++++
 kernel/cred.c                 |  119 +++++++++++++++++++++++++++++++++++++
 kernel/sys.c                  |  131 ++++++----------------------------------
 4 files changed, 169 insertions(+), 111 deletions(-)

diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt
index df03169..7da876b 100644
--- a/Documentation/credentials.txt
+++ b/Documentation/credentials.txt
@@ -529,6 +529,24 @@ A typical credentials alteration function would look something like this:
 		return commit_creds(new);
 	}
 
+SETUID/SETGID HELPERS
+---------------------
+
+Helpers exist to perform the core of uid and gid alterations:
+
+cred_setresuid(struct cred *new, uid_t ruid, uid_t euid, uid_t suid,
+                int force);
+cred_setresgid(struct cred *new, gid_t rgid, gid_t egid, gid_t sgid,
+                int force);
+cred_setfsuid(struct cred *new, uid_t uid, uid_t *old_fsuid);
+cred_setfsgid(struct cred *new, gid_t gid, gid_t *old_fsgid);
+
+The force argument means that while the caller does not have CAP_SETUID
+or CAP_SETUID, the credentials were received from a task with CAP_GRANT_ID.
+
+These helpers are used in kernel/sys.c for the analogous syscalls.
+As can be seen in those examples, these helpers are to be wrapped
+between calls to prepare_creds() and commit_creds() or abort_creds().
 
 MANAGING CREDENTIALS
 --------------------
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 52507c3..8034e22 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -22,6 +22,9 @@ struct user_struct;
 struct cred;
 struct inode;
 
+/* defined in sys.c, used in cred_setresuid */
+extern int set_user(struct cred *new);
+
 /*
  * COW Supplementary groups list
  */
@@ -396,4 +399,13 @@ do {						\
 	*(_fsgid) = __cred->fsgid;		\
 } while(0)
 
+#define CRED_SETID_NOFORCE 0
+#define CRED_SETID_FORCE 1
+extern int cred_setresuid(struct cred *new, uid_t ruid, uid_t euid, uid_t suid,
+		   int force);
+extern int cred_setresgid(struct cred *new, gid_t rgid, gid_t egid, gid_t sgid,
+		   int force);
+extern int cred_setfsuid(struct cred *new, uid_t uid, uid_t *old_fsuid);
+extern int cred_setfsgid(struct cred *new, gid_t gid, gid_t *old_fsgid);
+
 #endif /* _LINUX_CRED_H */
diff --git a/kernel/cred.c b/kernel/cred.c
index e1dbe9e..4fc3284 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -785,6 +785,125 @@ int set_create_files_as(struct cred *new, struct inode *inode)
 }
 EXPORT_SYMBOL(set_create_files_as);
 
+int cred_setresuid(struct cred *new, uid_t ruid, uid_t euid, uid_t suid,
+		   int force)
+{
+	int retval;
+	const struct cred *old;
+
+	retval = security_task_setuid(ruid, euid, suid, LSM_SETID_RES);
+	if (retval)
+		return retval;
+	old = current_cred();
+
+	if (!force && !capable(CAP_SETUID)) {
+		if (ruid != (uid_t) -1 && ruid != old->uid &&
+		    ruid != old->euid  && ruid != old->suid)
+			return -EPERM;
+		if (euid != (uid_t) -1 && euid != old->uid &&
+		    euid != old->euid  && euid != old->suid)
+			return -EPERM;
+		if (suid != (uid_t) -1 && suid != old->uid &&
+		    suid != old->euid  && suid != old->suid)
+			return -EPERM;
+	}
+
+	if (ruid != (uid_t) -1) {
+		new->uid = ruid;
+		if (ruid != old->uid) {
+			retval = set_user(new);
+			if (retval < 0)
+				return retval;
+		}
+	}
+	if (euid != (uid_t) -1)
+		new->euid = euid;
+	if (suid != (uid_t) -1)
+		new->suid = suid;
+	new->fsuid = new->euid;
+
+	return security_task_fix_setuid(new, old, LSM_SETID_RES);
+}
+EXPORT_SYMBOL_GPL(cred_setresuid);
+
+int cred_setresgid(struct cred *new, gid_t rgid, gid_t egid, gid_t sgid,
+		   int force)
+{
+	const struct cred *old = current_cred();
+	int retval;
+
+	retval = security_task_setgid(rgid, egid, sgid, LSM_SETID_RES);
+	if (retval)
+		return retval;
+
+	if (!force && !capable(CAP_SETGID)) {
+		if (rgid != (gid_t) -1 && rgid != old->gid &&
+		    rgid != old->egid  && rgid != old->sgid)
+			return -EPERM;
+		if (egid != (gid_t) -1 && egid != old->gid &&
+		    egid != old->egid  && egid != old->sgid)
+			return -EPERM;
+		if (sgid != (gid_t) -1 && sgid != old->gid &&
+		    sgid != old->egid  && sgid != old->sgid)
+			return -EPERM;
+	}
+
+	if (rgid != (gid_t) -1)
+		new->gid = rgid;
+	if (egid != (gid_t) -1)
+		new->egid = egid;
+	if (sgid != (gid_t) -1)
+		new->sgid = sgid;
+	new->fsgid = new->egid;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(cred_setresgid);
+
+int cred_setfsuid(struct cred *new, uid_t uid, uid_t *old_fsuid)
+{
+	const struct cred *old;
+
+	old = current_cred();
+	*old_fsuid = old->fsuid;
+
+	if (security_task_setuid(uid, (uid_t)-1, (uid_t)-1, LSM_SETID_FS) < 0)
+		return -EPERM;
+
+	if (uid == old->uid  || uid == old->euid  ||
+	    uid == old->suid || uid == old->fsuid ||
+	    capable(CAP_SETUID)) {
+		if (uid != *old_fsuid) {
+			new->fsuid = uid;
+			if (security_task_fix_setuid(new, old, LSM_SETID_FS) == 0)
+				return 0;
+		}
+	}
+	return -EPERM;
+}
+EXPORT_SYMBOL_GPL(cred_setfsuid);
+
+int cred_setfsgid(struct cred *new, gid_t gid, gid_t *old_fsgid)
+{
+	const struct cred *old;
+
+	old = current_cred();
+	*old_fsgid = old->fsgid;
+
+	if (security_task_setgid(gid, (gid_t)-1, (gid_t)-1, LSM_SETID_FS))
+		return -EPERM;
+
+	if (gid == old->gid  || gid == old->egid  ||
+	    gid == old->sgid || gid == old->fsgid ||
+	    capable(CAP_SETGID)) {
+		if (gid != *old_fsgid) {
+			new->fsgid = gid;
+			return 0;
+		}
+	}
+	return -EPERM;
+}
+EXPORT_SYMBOL_GPL(cred_setfsgid);
+
 #ifdef CONFIG_DEBUG_CREDENTIALS
 
 bool creds_are_invalid(const struct cred *cred)
diff --git a/kernel/sys.c b/kernel/sys.c
index 6d1a7e0..78f32eb 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -565,11 +565,11 @@ error:
 /*
  * change the user struct in a credentials set to match the new UID
  */
-static int set_user(struct cred *new)
+int set_user(struct cred *new)
 {
 	struct user_struct *new_user;
 
-	new_user = alloc_uid(current_user_ns(), new->uid);
+	new_user = alloc_uid(new->user->user_ns, new->uid);
 	if (!new_user)
 		return -EAGAIN;
 
@@ -711,7 +711,6 @@ error:
  */
 SYSCALL_DEFINE3(setresuid, uid_t, ruid, uid_t, euid, uid_t, suid)
 {
-	const struct cred *old;
 	struct cred *new;
 	int retval;
 
@@ -719,45 +718,10 @@ SYSCALL_DEFINE3(setresuid, uid_t, ruid, uid_t, euid, uid_t, suid)
 	if (!new)
 		return -ENOMEM;
 
-	retval = security_task_setuid(ruid, euid, suid, LSM_SETID_RES);
-	if (retval)
-		goto error;
-	old = current_cred();
+	retval = cred_setresuid(new, ruid, euid, suid, CRED_SETID_NOFORCE);
+	if (retval == 0)
+		return commit_creds(new);
 
-	retval = -EPERM;
-	if (!capable(CAP_SETUID)) {
-		if (ruid != (uid_t) -1 && ruid != old->uid &&
-		    ruid != old->euid  && ruid != old->suid)
-			goto error;
-		if (euid != (uid_t) -1 && euid != old->uid &&
-		    euid != old->euid  && euid != old->suid)
-			goto error;
-		if (suid != (uid_t) -1 && suid != old->uid &&
-		    suid != old->euid  && suid != old->suid)
-			goto error;
-	}
-
-	if (ruid != (uid_t) -1) {
-		new->uid = ruid;
-		if (ruid != old->uid) {
-			retval = set_user(new);
-			if (retval < 0)
-				goto error;
-		}
-	}
-	if (euid != (uid_t) -1)
-		new->euid = euid;
-	if (suid != (uid_t) -1)
-		new->suid = suid;
-	new->fsuid = new->euid;
-
-	retval = security_task_fix_setuid(new, old, LSM_SETID_RES);
-	if (retval < 0)
-		goto error;
-
-	return commit_creds(new);
-
-error:
 	abort_creds(new);
 	return retval;
 }
@@ -779,43 +743,17 @@ SYSCALL_DEFINE3(getresuid, uid_t __user *, ruid, uid_t __user *, euid, uid_t __u
  */
 SYSCALL_DEFINE3(setresgid, gid_t, rgid, gid_t, egid, gid_t, sgid)
 {
-	const struct cred *old;
 	struct cred *new;
 	int retval;
 
 	new = prepare_creds();
 	if (!new)
 		return -ENOMEM;
-	old = current_cred();
 
-	retval = security_task_setgid(rgid, egid, sgid, LSM_SETID_RES);
-	if (retval)
-		goto error;
+	retval = cred_setresgid(new, rgid, egid, sgid, CRED_SETID_NOFORCE);
+	if (retval == 0)
+		return commit_creds(new);
 
-	retval = -EPERM;
-	if (!capable(CAP_SETGID)) {
-		if (rgid != (gid_t) -1 && rgid != old->gid &&
-		    rgid != old->egid  && rgid != old->sgid)
-			goto error;
-		if (egid != (gid_t) -1 && egid != old->gid &&
-		    egid != old->egid  && egid != old->sgid)
-			goto error;
-		if (sgid != (gid_t) -1 && sgid != old->gid &&
-		    sgid != old->egid  && sgid != old->sgid)
-			goto error;
-	}
-
-	if (rgid != (gid_t) -1)
-		new->gid = rgid;
-	if (egid != (gid_t) -1)
-		new->egid = egid;
-	if (sgid != (gid_t) -1)
-		new->sgid = sgid;
-	new->fsgid = new->egid;
-
-	return commit_creds(new);
-
-error:
 	abort_creds(new);
 	return retval;
 }
@@ -841,35 +779,20 @@ SYSCALL_DEFINE3(getresgid, gid_t __user *, rgid, gid_t __user *, egid, gid_t __u
  */
 SYSCALL_DEFINE1(setfsuid, uid_t, uid)
 {
-	const struct cred *old;
 	struct cred *new;
 	uid_t old_fsuid;
+	int retval;
 
 	new = prepare_creds();
 	if (!new)
 		return current_fsuid();
-	old = current_cred();
-	old_fsuid = old->fsuid;
-
-	if (security_task_setuid(uid, (uid_t)-1, (uid_t)-1, LSM_SETID_FS) < 0)
-		goto error;
-
-	if (uid == old->uid  || uid == old->euid  ||
-	    uid == old->suid || uid == old->fsuid ||
-	    capable(CAP_SETUID)) {
-		if (uid != old_fsuid) {
-			new->fsuid = uid;
-			if (security_task_fix_setuid(new, old, LSM_SETID_FS) == 0)
-				goto change_okay;
-		}
-	}
 
-error:
-	abort_creds(new);
-	return old_fsuid;
+	retval = cred_setfsuid(new, uid, &old_fsuid);
+	if (retval == 0)
+		commit_creds(new);
+	else
+		abort_creds(new);
 
-change_okay:
-	commit_creds(new);
 	return old_fsuid;
 }
 
@@ -878,34 +801,20 @@ change_okay:
  */
 SYSCALL_DEFINE1(setfsgid, gid_t, gid)
 {
-	const struct cred *old;
 	struct cred *new;
 	gid_t old_fsgid;
+	int retval;
 
 	new = prepare_creds();
 	if (!new)
 		return current_fsgid();
-	old = current_cred();
-	old_fsgid = old->fsgid;
-
-	if (security_task_setgid(gid, (gid_t)-1, (gid_t)-1, LSM_SETID_FS))
-		goto error;
-
-	if (gid == old->gid  || gid == old->egid  ||
-	    gid == old->sgid || gid == old->fsgid ||
-	    capable(CAP_SETGID)) {
-		if (gid != old_fsgid) {
-			new->fsgid = gid;
-			goto change_okay;
-		}
-	}
 
-error:
-	abort_creds(new);
-	return old_fsgid;
+	retval = cred_setfsgid(new, gid, &old_fsgid);
+	if (retval == 0)
+		commit_creds(new);
+	else
+		abort_creds(new);
 
-change_okay:
-	commit_creds(new);
 	return old_fsgid;
 }
 
-- 
1.7.0.4