linux-kernel - Re: [PATCH 3/7] exec: Move initialization of bprm->filename into alloc

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Date:   Tue, 14 Jul 2020 14:38:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        linux-fsdevel@...r.kernel.org,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-security-module@...r.kernel.org,
        "Serge E. Hallyn" <serge@...lyn.com>,
        James Morris <jmorris@...ei.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Casey Schaufler <casey@...aufler-ca.com>,
        John Johansen <john.johansen@...onical.com>,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 3/7] exec: Move initialization of bprm->filename into
 alloc_bprm

On Tue, Jul 14, 2020 at 08:29:36AM -0500, Eric W. Biederman wrote:
> 
> Currently it is necessary for the usermode helper code and the code
> that launches init to use set_fs so that pages coming from the kernel
> look like they are coming from userspace.
> 
> To allow that usage of set_fs to be removed cleanly the argument
> copying from userspace needs to happen earlier.  Move the computation
> of bprm->filename and possible allocation of a name in the case
> of execveat into alloc_bprm to make that possible.
> 
> The exectuable name, the arguments, and the environment are
> copied into the new usermode stack which is stored in bprm
> until exec passes the point of no return.
> 
> As the executable name is copied first onto the usermode stack
> it needs to be known.  As there are no dependencies to computing
> the executable name, compute it early in alloc_bprm.
> 
> As an implementation detail if the filename needs to be generated
> because it embeds a file descriptor store that filename in a new field
> bprm->fdpath, and free it in free_bprm.  Previously this was done in
> an independent variable pathbuf.  I have renamed pathbuf fdpath
> because fdpath is more suggestive of what kind of path is in the
> variable.  I moved fdpath into struct linux_binprm because it is
> tightly tied to the other variables in struct linux_binprm, and as
> such is needed to allow the call alloc_binprm to move.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook