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

[Luis Chamberlain]

On Wed, Oct 26, 2022 at 04:31:11PM -0700, 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>

Acked-by: Luis Chamberlain <mcgrof@xxxxxxxxxx>

  Luis