Re: [PATCH] cred: Do not default to init_cred in prepare_kernel_cred()

[Russ Weight]

On 10/26/22 16:31, Kees Cook wrote:
> 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@xxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
> Cc: Russ Weight <russell.h.weight@xxxxxxxxx>
> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Cc: Steve French <sfrench@xxxxxxxxx>
> Cc: Paulo Alcantara <pc@xxxxxx>
> Cc: Ronnie Sahlberg <lsahlber@xxxxxxxxxx>
> Cc: Shyam Prasad N <sprasad@xxxxxxxxxxxxx>
> Cc: Tom Talpey <tom@xxxxxxxxxx>
> Cc: Namjae Jeon <linkinjeon@xxxxxxxxxx>
> Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
> Cc: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> Cc: Anna Schumaker <anna@xxxxxxxxxx>
> Cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
> Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> Cc: "Michal Koutný" <mkoutny@xxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: linux-cifs@xxxxxxxxxxxxxxx
> Cc: samba-technical@xxxxxxxxxxxxxxx
> Cc: linux-nfs@xxxxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  drivers/base/firmware_loader/main.c    |  2 +-
>  fs/cifs/cifs_spnego.c                  |  2 +-
>  fs/cifs/cifsacl.c                      |  2 +-
>  fs/ksmbd/smb_common.c                  |  2 +-
>  fs/nfs/flexfilelayout/flexfilelayout.c |  4 ++--
>  fs/nfs/nfs4idmap.c                     |  2 +-
>  fs/nfsd/nfs4callback.c                 |  2 +-
>  kernel/cred.c                          | 15 +++++++--------
>  net/dns_resolver/dns_key.c             |  2 +-
>  9 files changed, 16 insertions(+), 17 deletions(-)
>
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index 7c3590fd97c2..017c4cdb219e 100644
> --- 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 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;
>  
> diff --git a/kernel/cred.c b/kernel/cred.c
> index e10c15f51c1f..811ad654abd1 100644
> --- 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 3aced951d5ab..01e54b46ae0b 100644
> --- 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;
>  
Acked-by: Russ Weight <russell.h.weight@xxxxxxxxx>

- Russ