linux-kernel - Re: [PATCH 4/7] exec: Move bprm_mm_init into alloc

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Date:   Wed, 15 Jul 2020 07:35:15 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.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 4/7] exec: Move bprm_mm_init into alloc_bprm

On Tue, Jul 14, 2020 at 08:30:02AM -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 allocation and
> initialization of bprm->mm into alloc_bprm so that the bprm->mm is
> available early to store the new user stack into.  This is a prerequisite
> for copying argv and envp into the new user stack early before ther rest of
> exec.
> 
> To keep the things consistent the cleanup of bprm->mm is moved into
> free_bprm.  So that bprm->mm will be cleaned up whenever bprm->mm is
> allocated and free_bprm are called.
> 
> Moving bprm_mm_init earlier is safe as it does not depend on any files,
> current->in_execve, current->fs->in_exec, bprm->unsafe, or the if the file
> table is shared. (AKA bprm_mm_init does not depend on any of the code that
> happens between alloc_bprm and where it was previously called.)
> 
> This moves bprm->mm cleanup after current->fs->in_exec is set to 0.  This
> is safe because current->fs->in_exec is only used to preventy taking an
> additional reference on the fs_struct.
> 
> This moves bprm->mm cleanup after current->in_execve is set to 0.  This is
> safe because current->in_execve is only used by the lsms (apparmor and
> tomoyou) and always for LSM specific functions, never for anything to do
> with the mm.
> 
> This adds bprm->mm cleanup into the successful return path.  This is safe
> because being on the successful return path implies that begin_new_exec
> succeeded and set brpm->mm to NULL.  As bprm->mm is NULL bprm cleanup I am
> moving into free_bprm will do nothing.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>

Looks good,

Reviewed-by: Christoph Hellwig <hch@....de>