[PATCH v2 3/3] selinux: require PROCESS__PTRACE for FOLL_FORCE introspection

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.

Signed-off-by: Jann Horn <[email protected]>
---
 security/selinux/hooks.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 18dd28b2bb13..2c2c60e9e0fe 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2157,6 +2157,27 @@ 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()
+ * 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_opened_by_owner(const struct cred *subject)
+{
+	struct av_decision avd;
+	u32 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 +7579,7 @@ 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_opened_by_owner, selinux_mem_foll_force_opened_by_owner),
 	LSM_HOOK_INIT(capget, selinux_capget),
 	LSM_HOOK_INIT(capset, selinux_capset),
 	LSM_HOOK_INIT(capable, selinux_capable),

-- 
2.55.0.860.g4b6b3295ed-goog