linux-kernel - Re: [PATCH 5/7] exec: Factor bprm_execve out of do_execve

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Date:   Wed, 15 Jul 2020 07:36:29 +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 5/7] exec: Factor bprm_execve out of do_execve_common

On Tue, Jul 14, 2020 at 08:30:30AM -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.  Factor bprm_execve
> out of do_execve_common to separate out the copying of arguments
> to the newe stack, and the rest of exec.
> 
> In separating bprm_execve from do_execve_common the copying
> of the arguments onto the new stack happens earlier.
> 
> As the copying of the arguments does not depend any security hooks,
> files, the file table, current->in_execve, current->fs->in_exec,
> bprm->unsafe, or creds this is safe.
> 
> Likewise the security hook security_creds_for_exec does not depend upon
> preventing the argument copying from happening.
> 
> In addition to making it possible to implement kernel_execve that
> performs the copying differently, this separation of bprm_execve from
> do_execve_common makes for a nice separation of responsibilities making
> the exec code easier to navigate.
> 
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> ---
>  fs/exec.c | 108 +++++++++++++++++++++++++++++-------------------------
>  1 file changed, 58 insertions(+), 50 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index afb168bf5e23..50508892fa71 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1856,44 +1856,16 @@ static int exec_binprm(struct linux_binprm *bprm)
>  /*
>   * sys_execve() executes a new program.
>   */
> -static int do_execveat_common(int fd, struct filename *filename,
> -			      struct user_arg_ptr argv,
> -			      struct user_arg_ptr envp,
> -			      int flags)
> +static int bprm_execve(struct linux_binprm *bprm,
> +		       int fd, struct filename *filename, int flags)

int fd easily fits onto the previous line.

Otherwise looks good:

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