landlock: Don't lose track of restrictions on cred_transfer

When a process' cred struct is replaced, this _almost_ always invokes
the cred_prepare LSM hook; but in one special case (when
KEYCTL_SESSION_TO_PARENT updates the parent's credentials), the
cred_transfer LSM hook is used instead.  Landlock only implements the
cred_prepare hook, not cred_transfer, so KEYCTL_SESSION_TO_PARENT causes
all information on Landlock restrictions to be lost.

This basically means that a process with the ability to use the fork()
and keyctl() syscalls can get rid of all Landlock restrictions on
itself.

Fix it by adding a cred_transfer hook that does the same thing as the
existing cred_prepare hook. (Implemented by having hook_cred_prepare()
call hook_cred_transfer() so that the two functions are less likely to
accidentally diverge in the future.)

Cc: stable@kernel.org
Fixes: 385975dca5 ("landlock: Set up the security framework and manage credentials")
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://lore.kernel.org/r/20240724-landlock-houdini-fix-v1-1-df89a4560ca3@google.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

security/landlock/cred.c

@@ -14,8 +14,8 @@
 #include "ruleset.h"
 #include "setup.h"
 
-static int hook_cred_prepare(struct cred *const new,
-			     const struct cred *const old, const gfp_t gfp)
+static void hook_cred_transfer(struct cred *const new,
+			       const struct cred *const old)
 {
 	struct landlock_ruleset *const old_dom = landlock_cred(old)->domain;
 
@@ -23,6 +23,12 @@ static int hook_cred_prepare(struct cred *const new,
 		landlock_get_ruleset(old_dom);
 		landlock_cred(new)->domain = old_dom;
 	}
+}
+
+static int hook_cred_prepare(struct cred *const new,
+			     const struct cred *const old, const gfp_t gfp)
+{
+	hook_cred_transfer(new, old);
 	return 0;
 }
 
@@ -36,6 +42,7 @@ static void hook_cred_free(struct cred *const cred)
 
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(cred_prepare, hook_cred_prepare),
+	LSM_HOOK_INIT(cred_transfer, hook_cred_transfer),
 	LSM_HOOK_INIT(cred_free, hook_cred_free),
 };