[PATCH v2 4/8] futex: don't leak robust_list pointer

[Jann Horn <jann@xxxxxxxxx>]

This prevents an attacker from determining the robust_list or
compat_robust_list userspace pointer of a process created by executing
a setuid binary. Such an attack could be performed by racing
get_robust_list() with a setuid execution. The impact of this issue is that
an attacker could theoretically bypass ASLR when attacking setuid binaries.

changed in v2:
 - only get_task_struct(p) if p!=NULL (Ben Hutchings)
 - move the -ESRCH return check

Signed-off-by: Jann Horn <jann@xxxxxxxxx>
---
 kernel/futex.c        | 30 +++++++++++++++++++++---------
 kernel/futex_compat.c | 30 +++++++++++++++++++++---------
 2 files changed, 42 insertions(+), 18 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 46cb3a3..773806e 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3007,31 +3007,43 @@ SYSCALL_DEFINE3(get_robust_list, int, pid,
 	if (!futex_cmpxchg_enabled)
 		return -ENOSYS;
 
-	rcu_read_lock();
-
-	ret = -ESRCH;
-	if (!pid)
+	if (!pid) {
 		p = current;
-	else {
+		get_task_struct(p);
+	} else {
+		rcu_read_lock();
 		p = find_task_by_vpid(pid);
+		/* pin the task to permit dropping the RCU read lock before
+		 * acquiring the mutex
+		 */
+		if (p)
+			get_task_struct(p);
+		rcu_read_unlock();
 		if (!p)
-			goto err_unlock;
+			return -ESRCH;
 	}
 
+	ret = mutex_lock_killable(&p->signal->cred_guard_light);
+	if (ret)
+		goto err_put;
+
 	ret = -EPERM;
 	if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
 		goto err_unlock;
 
 	head = p->robust_list;
-	rcu_read_unlock();
+
+	mutex_unlock(&p->signal->cred_guard_light);
+	put_task_struct(p);
 
 	if (put_user(sizeof(*head), len_ptr))
 		return -EFAULT;
 	return put_user(head, head_ptr);
 
 err_unlock:
-	rcu_read_unlock();
-
+	mutex_unlock(&current->signal->cred_guard_light);
+err_put:
+	put_task_struct(p);
 	return ret;
 }
 
diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 4ae3232..92c350f 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -143,31 +143,43 @@ COMPAT_SYSCALL_DEFINE3(get_robust_list, int, pid,
 	if (!futex_cmpxchg_enabled)
 		return -ENOSYS;
 
-	rcu_read_lock();
-
-	ret = -ESRCH;
-	if (!pid)
+	if (!pid) {
 		p = current;
-	else {
+		get_task_struct(p);
+	} else {
+		rcu_read_lock();
 		p = find_task_by_vpid(pid);
+		/* pin the task to permit dropping the RCU read lock before
+		 * acquiring the mutex
+		 */
+		if (p)
+			get_task_struct(p);
+		rcu_read_unlock();
 		if (!p)
-			goto err_unlock;
+			return -ESRCH;
 	}
 
+	ret = mutex_lock_killable(&p->signal->cred_guard_light);
+	if (ret)
+		goto err_put;
+
 	ret = -EPERM;
 	if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS))
 		goto err_unlock;
 
 	head = p->compat_robust_list;
-	rcu_read_unlock();
+
+	mutex_unlock(&p->signal->cred_guard_light);
+	put_task_struct(p);
 
 	if (put_user(sizeof(*head), len_ptr))
 		return -EFAULT;
 	return put_user(ptr_to_compat(head), head_ptr);
 
 err_unlock:
-	rcu_read_unlock();
-
+	mutex_unlock(&current->signal->cred_guard_light);
+err_put:
+	put_task_struct(p);
 	return ret;
 }
 
-- 
2.1.4

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