ptrace: Capture the ptracer's creds not PT_PTRACE_CAP

commit 64b875f7ac8a5d60a4e191479299e931ee949b67 upstream.

When the flag PT_PTRACE_CAP was added the PTRACE_TRACEME path was
overlooked.  This can result in incorrect behavior when an application
like strace traces an exec of a setuid executable.

Further PT_PTRACE_CAP does not have enough information for making good
security decisions as it does not report which user namespace the
capability is in.  This has already allowed one mistake through
insufficient granulariy.

I found this issue when I was testing another corner case of exec and
discovered that I could not get strace to set PT_PTRACE_CAP even when
running strace as root with a full set of caps.

This change fixes the above issue with strace allowing stracing as
root a setuid executable without disabling setuid.  More fundamentaly
this change allows what is allowable at all times, by using the correct
information in it's decision.

Fixes: 4214e42f96d4 ("v2.4.9.11 -> v2.4.9.12")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

6 files changed, 30 insertions, 7 deletions

diff --git a/fs/exec.c b/fs/exec.c
index a9dec2f540dc..67e86571685a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1425,7 +1425,7 @@ static void check_unsafe_exec(struct linux_binprm *bprm)
         unsigned n_fs;
 
         if (p->ptrace) {
-                if (p->ptrace & PT_PTRACE_CAP)
+                if (ptracer_capable(p, current_user_ns()))
                         bprm->unsafe |= LSM_UNSAFE_PTRACE_CAP;
                 else
                         bprm->unsafe |= LSM_UNSAFE_PTRACE;
diff --git a/include/linux/capability.h b/include/linux/capability.h
index 6ce3eefe2769..6ffb67e10c06 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -243,6 +243,7 @@ static inline bool ns_capable_noaudit(struct user_namespace *ns, int cap)
 extern bool privileged_wrt_inode_uidgid(struct user_namespace *ns, const struct inode *inode);
 extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
 extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
+extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);
 
 /* audit system wants to get cap info from files as well */
 extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 504c98a278d4..e13bfdf7f314 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -19,7 +19,6 @@
 #define PT_SEIZED       0x00010000      /* SEIZE used, enable new behavior */
 #define PT_PTRACED      0x00000001
 #define PT_DTRACE       0x00000002      /* delayed trace (used on m68k, i386) */
-#define PT_PTRACE_CAP   0x00000004      /* ptracer can follow suid-exec */
 
 #define PT_OPT_FLAG_SHIFT       3
 /* PT_TRACE_* event enable flags */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index e9c009dc3a4a..75d9a57e212e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1656,6 +1656,7 @@ struct task_struct {
         struct list_head cpu_timers[3];
 
 /* process credentials */
+        const struct cred __rcu *ptracer_cred; /* Tracer's credentials at attach */
         const struct cred __rcu *real_cred; /* objective and real subjective task
                                          * credentials (COW) */
         const struct cred __rcu *cred;  /* effective (overridable) subjective task
diff --git a/kernel/capability.c b/kernel/capability.c
index c020c0047983..4984e1f552eb 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -485,3 +485,23 @@ bool capable_wrt_inode_uidgid(const struct inode *inode, int cap)
         return ns_capable(ns, cap) && privileged_wrt_inode_uidgid(ns, inode);
 }
 EXPORT_SYMBOL(capable_wrt_inode_uidgid);
+
+/**
+ * ptracer_capable - Determine if the ptracer holds CAP_SYS_PTRACE in the namespace
+ * @tsk: The task that may be ptraced
+ * @ns: The user namespace to search for CAP_SYS_PTRACE in
+ *
+ * Return true if the task that is ptracing the current task had CAP_SYS_PTRACE
+ * in the specified user namespace.
+ */
+bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns)
+{
+        int ret = 0;  /* An absent tracer adds no restrictions */
+        const struct cred *cred;
+        rcu_read_lock();
+        cred = rcu_dereference(tsk->ptracer_cred);
+        if (cred)
+                ret = security_capable_noaudit(cred, ns, CAP_SYS_PTRACE);
+        rcu_read_unlock();
+        return (ret == 0);
+}
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 282821557183..e82c15cadd6d 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -39,6 +39,9 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
         BUG_ON(!list_empty(&child->ptrace_entry));
         list_add(&child->ptrace_entry, &new_parent->ptraced);
         child->parent = new_parent;
+        rcu_read_lock();
+        child->ptracer_cred = get_cred(__task_cred(new_parent));
+        rcu_read_unlock();
 }
 
 /**
@@ -71,12 +74,16 @@ void __ptrace_link(struct task_struct *child, struct task_struct *new_parent)
  */
 void __ptrace_unlink(struct task_struct *child)
 {
+        const struct cred *old_cred;
         BUG_ON(!child->ptrace);
 
         clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 
         child->parent = child->real_parent;
         list_del_init(&child->ptrace_entry);
+        old_cred = child->ptracer_cred;
+        child->ptracer_cred = NULL;
+        put_cred(old_cred);
 
         spin_lock(&child->sighand->siglock);
         child->ptrace = 0;
@@ -326,11 +333,6 @@ static int ptrace_attach(struct task_struct *task, long request,
 
         task_lock(task);
         retval = __ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS);
-        if (!retval) {
-                struct mm_struct *mm = task->mm;
-                if (mm && ns_capable(mm->user_ns, CAP_SYS_PTRACE))
-                        flags |= PT_PTRACE_CAP;
-        }
         task_unlock(task);
         if (retval)
                 goto unlock_creds;