[PATCH v1] keys: Restrict KEYCTL_SESSION_TO_PARENT according to ptrace_may_access()

["Mickaël Salaün" <mic@digikod.net>]

A process can modify its parent's credentials with
KEYCTL_SESSION_TO_PARENT when their EUID and EGID are the same.  This
doesn't take into account all possible access controls.

Enforce the same access checks as for impersonating a process.

The current credentials checks are untouch because they check against
EUID and EGID, whereas ptrace_may_access() checks against UID and GID.

Cc: David Howells <dhowells@redhat.com>
Cc: Günther Noack <gnoack@google.com>
Cc: James Morris <jmorris@namei.org>
Cc: Jann Horn <jannh@google.com>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Fixes: ee18d64c1f63 ("KEYS: Add a keyctl to install a process's session keyring on its parent [try #6]")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20240729125846.1043211-1-mic@digikod.net
---
 security/keys/keyctl.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/security/keys/keyctl.c b/security/keys/keyctl.c
index ab927a142f51..511bf79fa14c 100644
--- a/security/keys/keyctl.c
+++ b/security/keys/keyctl.c
@@ -21,6 +21,7 @@
 #include <linux/security.h>
 #include <linux/uio.h>
 #include <linux/uaccess.h>
+#include <linux/ptrace.h>
 #include <keys/request_key_auth-type.h>
 #include "internal.h"
 
@@ -1687,6 +1688,10 @@ long keyctl_session_to_parent(void)
 	    !gid_eq(pcred->sgid, mycred->egid))
 		goto unlock;
 
+	/* The child must be allowed to impersonate its parent process. */
+	if (!ptrace_may_access(parent, PTRACE_MODE_ATTACH_REALCREDS))
+		goto unlock;
+
 	/* the keyrings must have the same UID */
 	if ((pcred->session_keyring &&
 	     !uid_eq(pcred->session_keyring->uid, mycred->euid)) ||

base-commit: 8400291e289ee6b2bf9779ff1c83a291501f017b
-- 
2.45.2