cred: Do not default to init_cred in prepare_kernel_cred()

A common exploit pattern for ROP attacks is to abuse prepare_kernel_cred()
in order to construct escalated privileges[1]. Instead of providing a
short-hand argument (NULL) to the "daemon" argument to indicate using
init_cred as the base cred, require that "daemon" is always set to
an actual task. Replace all existing callers that were passing NULL
with &init_task.

Future attacks will need to have sufficiently powerful read/write
primitives to have found an appropriately privileged task and written it
to the ROP stack as an argument to succeed, which is similarly difficult
to the prior effort needed to escalate privileges before struct cred
existed: locate the current cred and overwrite the uid member.

This has the added benefit of meaning that prepare_kernel_cred() can no
longer exceed the privileges of the init task, which may have changed from
the original init_cred (e.g. dropping capabilities from the bounding set).

[1] https://google.com/search?q=commit_creds(prepare_kernel_cred(0))

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: David Howells <dhowells@redhat.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Steve French <sfrench@samba.org>
Cc: Ronnie Sahlberg <lsahlber@redhat.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: "Michal Koutný" <mkoutny@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Cc: linux-nfs@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Acked-by: Russ Weight <russell.h.weight@intel.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Link: https://lore.kernel.org/r/20221026232943.never.775-kees@kernel.org

6 files changed, 7 insertions, 7 deletions

diff --git a/fs/cifs/cifs_spnego.c b/fs/cifs/cifs_spnego.c
index 342717bf1dc2..6f3285f1dfee 100644
--- a/fs/cifs/cifs_spnego.c
+++ b/fs/cifs/cifs_spnego.c
@@ -189,7 +189,7 @@ init_cifs_spnego(void)
 	 * spnego upcalls.
 	 */
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 
diff --git a/fs/cifs/cifsacl.c b/fs/cifs/cifsacl.c
index fa480d62f313..574de2b225ae 100644
--- a/fs/cifs/cifsacl.c
+++ b/fs/cifs/cifsacl.c
@@ -465,7 +465,7 @@ init_cifs_idmap(void)
 	 * this is used to prevent malicious redirections from being installed
 	 * with add_key().
 	 */
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 
diff --git a/fs/ksmbd/smb_common.c b/fs/ksmbd/smb_common.c
index d96da872d70a..2a4fbbd55b91 100644
--- a/fs/ksmbd/smb_common.c
+++ b/fs/ksmbd/smb_common.c
@@ -623,7 +623,7 @@ int ksmbd_override_fsids(struct ksmbd_work *work)
 	if (share->force_gid != KSMBD_SHARE_INVALID_GID)
 		gid = share->force_gid;
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 
diff --git a/fs/nfs/flexfilelayout/flexfilelayout.c b/fs/nfs/flexfilelayout/flexfilelayout.c
index 1ec79ccf89ad..7deb3cd76abe 100644
--- a/fs/nfs/flexfilelayout/flexfilelayout.c
+++ b/fs/nfs/flexfilelayout/flexfilelayout.c
@@ -493,10 +493,10 @@ ff_layout_alloc_lseg(struct pnfs_layout_hdr *lh,
 		gid = make_kgid(&init_user_ns, id);
 
 		if (gfp_flags & __GFP_FS)
-			kcred = prepare_kernel_cred(NULL);
+			kcred = prepare_kernel_cred(&init_task);
 		else {
 			unsigned int nofs_flags = memalloc_nofs_save();
-			kcred = prepare_kernel_cred(NULL);
+			kcred = prepare_kernel_cred(&init_task);
 			memalloc_nofs_restore(nofs_flags);
 		}
 		rc = -ENOMEM;
diff --git a/fs/nfs/nfs4idmap.c b/fs/nfs/nfs4idmap.c
index e3fdd2f45b01..25a7c771cfd8 100644
--- a/fs/nfs/nfs4idmap.c
+++ b/fs/nfs/nfs4idmap.c
@@ -203,7 +203,7 @@ int nfs_idmap_init(void)
 	printk(KERN_NOTICE "NFS: Registering the %s key type\n",
 		key_type_id_resolver.name);
 
-	cred = prepare_kernel_cred(NULL);
+	cred = prepare_kernel_cred(&init_task);
 	if (!cred)
 		return -ENOMEM;
 
diff --git a/fs/nfsd/nfs4callback.c b/fs/nfsd/nfs4callback.c
index f0e69edf5f0f..4a9e8d17e56a 100644
--- a/fs/nfsd/nfs4callback.c
+++ b/fs/nfsd/nfs4callback.c
@@ -870,7 +870,7 @@ static const struct cred *get_backchannel_cred(struct nfs4_client *clp, struct r
 	} else {
 		struct cred *kcred;
 
-		kcred = prepare_kernel_cred(NULL);
+		kcred = prepare_kernel_cred(&init_task);
 		if (!kcred)
 			return NULL;