[PATCH 3/3] selinux: require EXECMEM or 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 one
of:

 - EXECMEM (like for other methods of creating anonymous executable pages)
 - PTRACE (like when using /proc/$pid/mem of another process)

This closes a hole in EXECMEM 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
permission to overwrite executable code via /proc/self/mem, which made it
possible to load and run shellcode with a kernel exploit.

Signed-off-by: Jann Horn <jannh at google.com>
---
 security/selinux/hooks.c | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 18dd28b2bb13..905137c47321 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2157,6 +2157,32 @@ static int selinux_ptrace_traceme(struct task_struct *parent)
 			    SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
 }
 
+/*
+ * Decide whether it should be possible to read non-readable VMAs and write
+ * non-writable VMAs via /proc/self/mem.
+ * This only applies to systems configured with PROC_MEM_FORCE_ALWAYS, and only
+ * triggers on accesses that are not visible to selinux_ptrace_access_check().
+ *
+ * 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_introspect_mem_foll_force(const struct cred *subject)
+{
+	struct av_decision avd;
+	int rc;
+	u32 sid = cred_sid(subject);
+
+	/* Allow if the process is generally allowed to have executable anonymous memory. */
+	rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__EXECMEM, 0, &avd);
+
+	/* Also allow if selinux_ptrace_access_check() would allow it. */
+	if (rc)
+		rc = avc_has_perm_noaudit(sid, sid, SECCLASS_PROCESS, PROCESS__PTRACE, 0, &avd);
+	return rc;
+}
+
 static int selinux_capget(const struct task_struct *target, kernel_cap_t *effective,
 			  kernel_cap_t *inheritable, kernel_cap_t *permitted)
 {
@@ -7558,6 +7584,7 @@ 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(introspect_mem_foll_force, selinux_introspect_mem_foll_force),
 	LSM_HOOK_INIT(capget, selinux_capget),
 	LSM_HOOK_INIT(capset, selinux_capset),
 	LSM_HOOK_INIT(capable, selinux_capable),

-- 
2.55.0.737.g08866a6d13-goog