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

diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
index 7c3590f..017c4cd 100644 (file)
--- a/drivers/base/firmware_loader/main.c
+++ b/drivers/base/firmware_loader/main.c
@@ -821,7 +821,7 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
         * called by a driver when serving an unrelated request from userland, we use
         * the kernel credentials to read the file.
         */
-       kern_cred = prepare_kernel_cred(NULL);
+       kern_cred = prepare_kernel_cred(&init_task);
        if (!kern_cred) {
                ret = -ENOMEM;
                goto out;

diff --git a/fs/cifs/cifs_spnego.c b/fs/cifs/cifs_spnego.c
index 342717b..6f3285f 100644 (file)
--- 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 fa480d6..574de2b 100644 (file)
--- 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 d96da87..2a4fbbd 100644 (file)
--- 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 1ec79cc..7deb3cd 100644 (file)
--- 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 e3fdd2f..25a7c77 100644 (file)
--- 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 f0e69ed..4a9e8d1 100644 (file)
--- 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;
 

diff --git a/kernel/cred.c b/kernel/cred.c
index e10c15f..811ad65 100644 (file)
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -701,9 +701,9 @@ void __init cred_init(void)
  * override a task's own credentials so that work can be done on behalf of that
  * task that requires a different subjective context.
  *
- * @daemon is used to provide a base for the security record, but can be NULL.
- * If @daemon is supplied, then the security data will be derived from that;
- * otherwise they'll be set to 0 and no groups, full capabilities and no keys.
+ * @daemon is used to provide a base cred, with the security data derived from
+ * that; if this is "&init_task", they'll be set to 0, no groups, full
+ * capabilities, and no keys.
  *
  * The caller may change these controls afterwards if desired.
  *
@@ -714,17 +714,16 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
        const struct cred *old;
        struct cred *new;
 
+       if (WARN_ON_ONCE(!daemon))
+               return NULL;
+
        new = kmem_cache_alloc(cred_jar, GFP_KERNEL);
        if (!new)
                return NULL;
 
        kdebug("prepare_kernel_cred() alloc %p", new);
 
-       if (daemon)
-               old = get_task_cred(daemon);
-       else
-               old = get_cred(&init_cred);
-
+       old = get_task_cred(daemon);
        validate_creds(old);
 
        *new = *old;

diff --git a/net/dns_resolver/dns_key.c b/net/dns_resolver/dns_key.c
index 3aced95..01e54b4 100644 (file)
--- a/net/dns_resolver/dns_key.c
+++ b/net/dns_resolver/dns_key.c
@@ -337,7 +337,7 @@ static int __init init_dns_resolver(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;