[PATCH v3 3/3] selinux: require PROCESS__PTRACE for FOLL_FORCE introspection

[Jann Horn]

On systems configured with PROC_MEM_FORCE_ALWAYS, ensure that a process can
only create anonymous executable memory via /proc/self/mem if it has
PROCESS__PTRACE (like when using /proc/$pid/mem of another process).

This closes a hole in code integrity enforcement that Project Zero has used
in a remote Android exploit chain:
It was possible to use a memory corruption bug in a service without
EXECMEM/EXECMOD/PTRACE permission to overwrite executable code via
/proc/self/mem, which made it possible to load and run shellcode containing
a kernel exploit.

Acked-by: Stephen Smalley <stephen.smalley.work at gmail.com>
Acked-by: Lorenzo Stoakes (ARM) <ljs at kernel.org>
Signed-off-by: Jann Horn <jannh at google.com>
---
 security/selinux/hooks.c | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 18dd28b2bb13..45ead18c8f01 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2157,6 +2157,36 @@ static int selinux_ptrace_traceme(struct task_struct *parent)
 			    SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
 }
 
+/**
+ * selinux_mem_foll_force() - Determine whether /proc/$pid/mem can use FOLL_FORCE
+ * @subject: credentials using which /proc/$pid/mem was opened
+ * @opened_by_owner: whether checks on open() were bypassed because the opener
+ *                   has the same MM as the target
+ *
+ * Decide whether it should be possible to read non-readable VMAs and write
+ * non-writable VMAs via /proc/self/mem.
+ * The @opened_by_owner case only applies to systems configured with
+ * PROC_MEM_FORCE_ALWAYS, and only happens on accesses that are not visible to
+ * selinux_ptrace_access_check() because of the introspection exceptions in
+ * may_access_mm() and __ptrace_may_access().
+ *
+ * This allows a process to overwrite read-only code in its own address space.
+ *
+ * Creating an audit record on denial doesn't make sense here, since we can't
+ * tell whether FOLL_FORCE matters for the accessed VMAs.
+ */
+static int selinux_mem_foll_force(const struct cred *subject, bool opened_by_owner)
+{
+	struct av_decision avd;
+	u32 sid;
+
+	if (!opened_by_owner)
+		return 0;
+	sid = cred_sid(subject);
+
+	return avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__PTRACE, 0, &avd);
+}
+
 static int selinux_capget(const struct task_struct *target, kernel_cap_t *effective,
 			  kernel_cap_t *inheritable, kernel_cap_t *permitted)
 {
@@ -7558,6 +7588,7 @@ static struct security_hook_list selinux_hooks[] __ro_after_init = {
 
 	LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
+	LSM_HOOK_INIT(mem_foll_force, selinux_mem_foll_force),
 	LSM_HOOK_INIT(capget, selinux_capget),
 	LSM_HOOK_INIT(capset, selinux_capset),
 	LSM_HOOK_INIT(capable, selinux_capable),

-- 
2.55.0.1003.g10538fe699-goog