CRED: Use RCU to access another task's creds and to release a task's own creds

Use RCU to access another task's creds and to release a task's own creds.
This means that it will be possible for the credentials of a task to be
replaced without another task (a) requiring a full lock to read them, and (b)
seeing deallocated memory.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

2 files changed, 45 insertions, 19 deletions

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 62fe9b2..7e4877d 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -159,6 +159,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 	struct group_info *group_info;
 	int g;
 	struct fdtable *fdt = NULL;
+	const struct cred *cred;
 	pid_t ppid, tpid;
 
 	rcu_read_lock();
@@ -170,6 +171,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		if (tracer)
 			tpid = task_pid_nr_ns(tracer, ns);
 	}
+	cred = get_cred((struct cred *) __task_cred(p));
 	seq_printf(m,
 		"State:\t%s\n"
 		"Tgid:\t%d\n"
@@ -182,8 +184,8 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		task_tgid_nr_ns(p, ns),
 		pid_nr_ns(pid, ns),
 		ppid, tpid,
-		p->cred->uid, p->cred->euid, p->cred->suid, p->cred->fsuid,
-		p->cred->gid, p->cred->egid, p->cred->sgid, p->cred->fsgid);
+		cred->uid, cred->euid, cred->suid, cred->fsuid,
+		cred->gid, cred->egid, cred->sgid, cred->fsgid);
 
 	task_lock(p);
 	if (p->files)
@@ -194,13 +196,12 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		fdt ? fdt->max_fds : 0);
 	rcu_read_unlock();
 
-	group_info = p->cred->group_info;
-	get_group_info(group_info);
+	group_info = cred->group_info;
 	task_unlock(p);
 
 	for (g = 0; g < min(group_info->ngroups, NGROUPS_SMALL); g++)
 		seq_printf(m, "%d ", GROUP_AT(group_info, g));
-	put_group_info(group_info);
+	put_cred(cred);
 
 	seq_printf(m, "\n");
 }
@@ -262,7 +263,7 @@ static inline void task_sig(struct seq_file *m, struct task_struct *p)
 		blocked = p->blocked;
 		collect_sigign_sigcatch(p, &ignored, &caught);
 		num_threads = atomic_read(&p->signal->count);
-		qsize = atomic_read(&p->cred->user->sigpending);
+		qsize = atomic_read(&__task_cred(p)->user->sigpending);
 		qlim = p->signal->rlim[RLIMIT_SIGPENDING].rlim_cur;
 		unlock_task_sighand(p, &flags);
 	}
@@ -293,12 +294,21 @@ static void render_cap_t(struct seq_file *m, const char *header,
 
 static inline void task_cap(struct seq_file *m, struct task_struct *p)
 {
-	struct cred *cred = p->cred;
+	const struct cred *cred;
+	kernel_cap_t cap_inheritable, cap_permitted, cap_effective, cap_bset;
 
-	render_cap_t(m, "CapInh:\t", &cred->cap_inheritable);
-	render_cap_t(m, "CapPrm:\t", &cred->cap_permitted);
-	render_cap_t(m, "CapEff:\t", &cred->cap_effective);
-	render_cap_t(m, "CapBnd:\t", &cred->cap_bset);
+	rcu_read_lock();
+	cred = __task_cred(p);
+	cap_inheritable	= cred->cap_inheritable;
+	cap_permitted	= cred->cap_permitted;
+	cap_effective	= cred->cap_effective;
+	cap_bset	= cred->cap_bset;
+	rcu_read_unlock();
+
+	render_cap_t(m, "CapInh:\t", &cap_inheritable);
+	render_cap_t(m, "CapPrm:\t", &cap_permitted);
+	render_cap_t(m, "CapEff:\t", &cap_effective);
+	render_cap_t(m, "CapBnd:\t", &cap_bset);
 }
 
 static inline void task_context_switch_counts(struct seq_file *m,
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 6862b36..cf42c42 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1406,6 +1406,7 @@ static struct inode *proc_pid_make_inode(struct super_block * sb, struct task_st
 {
 	struct inode * inode;
 	struct proc_inode *ei;
+	const struct cred *cred;
 
 	/* We need a new inode */
 
@@ -1428,8 +1429,11 @@ static struct inode *proc_pid_make_inode(struct super_block * sb, struct task_st
 	inode->i_uid = 0;
 	inode->i_gid = 0;
 	if (task_dumpable(task)) {
-		inode->i_uid = task->cred->euid;
-		inode->i_gid = task->cred->egid;
+		rcu_read_lock();
+		cred = __task_cred(task);
+		inode->i_uid = cred->euid;
+		inode->i_gid = cred->egid;
+		rcu_read_unlock();
 	}
 	security_task_to_inode(task, inode);
 
@@ -1445,6 +1449,8 @@ static int pid_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat
 {
 	struct inode *inode = dentry->d_inode;
 	struct task_struct *task;
+	const struct cred *cred;
+
 	generic_fillattr(inode, stat);
 
 	rcu_read_lock();
@@ -1454,8 +1460,9 @@ static int pid_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat
 	if (task) {
 		if ((inode->i_mode == (S_IFDIR|S_IRUGO|S_IXUGO)) ||
 		    task_dumpable(task)) {
-			stat->uid = task->cred->euid;
-			stat->gid = task->cred->egid;
+			cred = __task_cred(task);
+			stat->uid = cred->euid;
+			stat->gid = cred->egid;
 		}
 	}
 	rcu_read_unlock();
@@ -1483,11 +1490,16 @@ static int pid_revalidate(struct dentry *dentry, struct nameidata *nd)
 {
 	struct inode *inode = dentry->d_inode;
 	struct task_struct *task = get_proc_task(inode);
+	const struct cred *cred;
+
 	if (task) {
 		if ((inode->i_mode == (S_IFDIR|S_IRUGO|S_IXUGO)) ||
 		    task_dumpable(task)) {
-			inode->i_uid = task->cred->euid;
-			inode->i_gid = task->cred->egid;
+			rcu_read_lock();
+			cred = __task_cred(task);
+			inode->i_uid = cred->euid;
+			inode->i_gid = cred->egid;
+			rcu_read_unlock();
 		} else {
 			inode->i_uid = 0;
 			inode->i_gid = 0;
@@ -1649,6 +1661,7 @@ static int tid_fd_revalidate(struct dentry *dentry, struct nameidata *nd)
 	struct task_struct *task = get_proc_task(inode);
 	int fd = proc_fd(inode);
 	struct files_struct *files;
+	const struct cred *cred;
 
 	if (task) {
 		files = get_files_struct(task);
@@ -1658,8 +1671,11 @@ static int tid_fd_revalidate(struct dentry *dentry, struct nameidata *nd)
 				rcu_read_unlock();
 				put_files_struct(files);
 				if (task_dumpable(task)) {
-					inode->i_uid = task->cred->euid;
-					inode->i_gid = task->cred->egid;
+					rcu_read_lock();
+					cred = __task_cred(task);
+					inode->i_uid = cred->euid;
+					inode->i_gid = cred->egid;
+					rcu_read_unlock();
 				} else {
 					inode->i_uid = 0;
 					inode->i_gid = 0;