[PATCH] keys: Check the authorization payload in process keyring searches

[PATCH] keys: Check the authorization payload in process keyring searches

[Cen Zhang]

An authorization payload and its saved credentials must remain valid
while search_process_keyrings_rcu() searches the requestor's keyrings.
The function validates the authorization key, then separately loads
payload.data[0] and unconditionally reads rka->cred. Validation does
not prevent request_key_auth_revoke() from clearing the payload.

A request-key helper can assume authority and fork a descendant that
inherits the authorization key. If the descendant misses in its own
keyrings, it falls back to the saved requestor credentials. Successful
target instantiation invalidates the authorization key without clearing
the payload. After the helper exits, UMH_WAIT_PROC returns to the
requester, which calls complete_request_key() and revokes that key.

With the descendant paused after validation, the paths can interleave
as follows:

  1. The descendant enters search_process_keyrings_rcu() under RCU and
     key_validate() succeeds.
  2. The helper instantiates the target, invalidates the authorization
     key, and exits.
  3. The requester resumes from UMH_WAIT_PROC, calls
     complete_request_key(), and revokes the authorization key.
     request_key_auth_revoke() sets its payload pointer to NULL.
  4. The descendant loads NULL and dereferences rka->cred.

This NULL dereference can oops the kernel. RCU delays payload disposal
but does not prevent the pointer from being cleared.

Load the payload with dereference_key_rcu() and require a non-NULL
result before validating the authorization key and searching its saved
credentials. A non-NULL payload and its credentials remain alive for
the caller's RCU read-side section; a cleared payload skips the fallback
search and follows the existing error selection.

KASAN report as below:

    Oops: general protection fault, probably for non-canonical address 0xdffffc0000000005: 0000 [#1] SMP KASAN NOPTI
    KASAN: null-ptr-deref in range [0x0000000000000028-0x000000000000002f]
    RIP: 0010:search_process_keyrings_rcu+0x32b/0x790

Fixes: e59428f721ee ("keys: Move the RCU locks outwards from the keyring search functions")
Assisted-by: LLM
Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---

diff --git a/security/keys/process_keys.c b/security/keys/process_keys.c
index a63c46bb2d148080add6a194579f7b1093c04d82..76da13fa07fab70115966e9ea8360af66390dd85 100644
--- a/security/keys/process_keys.c
+++ b/security/keys/process_keys.c
@@ -556,9 +556,8 @@ key_ref_t search_process_keyrings_rcu(struct keyring_search_context *ctx)
 	    ) {
 		const struct cred *cred = ctx->cred;
 
-		if (key_validate(cred->request_key_auth) == 0) {
-			rka = ctx->cred->request_key_auth->payload.data[0];
-
+		rka = dereference_key_rcu(cred->request_key_auth);
+		if (rka && key_validate(cred->request_key_auth) == 0) {
 			//// was search_process_keyrings() [ie. recursive]
 			ctx->cred = rka->cred;
 			key_ref = search_cred_keyrings_rcu(ctx);