[PATCH] ptrace: being capable wrt a process requires mapped uids/gids

[Jann Horn]

ptrace_has_cap() checks whether the current process should be
treated as having a certain capability for ptrace checks
against another process. Until now, this was equivalent to
has_ns_capability(current, target_ns, CAP_SYS_PTRACE).

However, if a root-owned process wants to enter a user
namespace for some reason without knowing who owns it and
therefore can't change to the namespace owner's uid and gid
before entering, as soon as it has entered the namespace,
the namespace owner can attach to it via ptrace and thereby
gain access to its uid and gid.

While it is possible for the entering process to switch to
the uid of a claimed namespace owner before entering,
causing the attempt to enter to fail if the claimed uid is
wrong, this doesn't solve the problem of determining an
appropriate gid.

With this change, the entering process can first enter the
namespace and then safely inspect the namespace's
properties, e.g. through /proc/self/{uid_map,gid_map},
assuming that the namespace owner doesn't have access to
uid 0.

Changed in v2: The caller needs to be capable in the
namespace into which tcred's uids/gids can be mapped.

Signed-off-by: Jann Horn <jann@xxxxxxxxx>
---
 kernel/ptrace.c | 33 ++++++++++++++++++++++++++++-----
 1 file changed, 28 insertions(+), 5 deletions(-)

diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index b760bae..260a08d 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -20,6 +20,7 @@
 #include <linux/uio.h>
 #include <linux/audit.h>
 #include <linux/pid_namespace.h>
+#include <linux/user_namespace.h>
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
 #include <linux/regset.h>
@@ -207,12 +208,34 @@ static int ptrace_check_attach(struct task_struct *child, bool ignore_state)
 	return ret;
 }
 
-static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
+static bool ptrace_has_cap(const struct cred *tcred, unsigned int mode)
 {
+	struct user_namespace *tns = tcred->user_ns;
+
+	/* When a root-owned process enters a user namespace created by a
+	 * malicious user, the user shouldn't be able to execute code under
+	 * uid 0 by attaching to the root-owned process via ptrace.
+	 * Therefore, similar to the capable_wrt_inode_uidgid() check,
+	 * verify that all the uids and gids of the target process are
+	 * mapped into a namespace below the current one in which the caller
+	 * is capable.
+	 * No fsuid/fsgid check because __ptrace_may_access doesn't do it
+	 * either.
+	 */
+	while (
+	    !kuid_has_mapping(tns, tcred->euid) ||
+	    !kuid_has_mapping(tns, tcred->suid) ||
+	    !kuid_has_mapping(tns, tcred->uid)  ||
+	    !kgid_has_mapping(tns, tcred->egid) ||
+	    !kgid_has_mapping(tns, tcred->sgid) ||
+	    !kgid_has_mapping(tns, tcred->gid)) {
+		tns = tns->parent;
+	}
+
 	if (mode & PTRACE_MODE_NOAUDIT)
-		return has_ns_capability_noaudit(current, ns, CAP_SYS_PTRACE);
+		return has_ns_capability_noaudit(current, tns, CAP_SYS_PTRACE);
 	else
-		return has_ns_capability(current, ns, CAP_SYS_PTRACE);
+		return has_ns_capability(current, tns, CAP_SYS_PTRACE);
 }
 
 /* Returns 0 on success, -errno on denial. */
@@ -241,7 +264,7 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	    gid_eq(cred->gid, tcred->sgid) &&
 	    gid_eq(cred->gid, tcred->gid))
 		goto ok;
-	if (ptrace_has_cap(tcred->user_ns, mode))
+	if (ptrace_has_cap(tcred, mode))
 		goto ok;
 	rcu_read_unlock();
 	return -EPERM;
@@ -252,7 +275,7 @@ ok:
 		dumpable = get_dumpable(task->mm);
 	rcu_read_lock();
 	if (dumpable != SUID_DUMP_USER &&
-	    !ptrace_has_cap(__task_cred(task)->user_ns, mode)) {
+	    !ptrace_has_cap(__task_cred(task), mode)) {
 		rcu_read_unlock();
 		return -EPERM;
 	}
-- 
2.1.4

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