[PATCH v2 3/3] selinux: require EXECMEM for forced ptrace poke

Jann Horn
2016-09-28T22:55:22Z

This is a breaking change for SELinux users that restrict EXECMEM: It might
break gdb if gdb is executed in a domain that does not have EXECMEM
privilege over the debuggee domain.

Unlike most other SELinux hooks, this one takes the subject credentials as
an argument instead of looking up current_cred(). This is done because the
security_forced_write() LSM hook can be invoked from within the write
handler of /proc/$pid/mem, where current_cred() is pretty useless.

Signed-off-by: Jann Horn <jann@thejh.net>
Reviewed-by: Janis Danisevskis <jdanis@android.com>
---
security/selinux/hooks.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 13185a6..e36682a 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2149,6 +2149,20 @@ static int selinux_ptrace_traceme(struct task_struct *parent)
return task_has_perm(parent, current, PROCESS__PTRACE);
}

+static int selinux_forced_write(struct vm_area_struct *vma,
+ const struct cred *subject_cred,
+ const struct cred *object_cred)
+{
+ /* Permitting a write to readonly memory is fine - making the readonly
+ * memory executable afterwards would require EXECMOD permission because
+ * anon_vma would be non-NULL.
+ */
+ if ((vma->vm_flags & VM_EXEC) == 0)
+ return 0;
+
+ return cred_has_perm(subject_cred, object_cred, PROCESS__EXECMEM);
+}
+
static int selinux_capget(struct task_struct *target, kernel_cap_t *effective,
kernel_cap_t *inheritable, kernel_cap_t *permitted)
{
@@ -6033,6 +6047,7 @@ static struct security_hook_list selinux_hooks[] = {

LSM_HOOK_INIT(ptrace_access_check, selinux_ptrace_access_check),
LSM_HOOK_INIT(ptrace_traceme, selinux_ptrace_traceme),
+ LSM_HOOK_INIT(forced_write, selinux_forced_write),
LSM_HOOK_INIT(capget, selinux_capget),
LSM_HOOK_INIT(capset, selinux_capset),
LSM_HOOK_INIT(capable, selinux_capable),
--
2.1.4