linux-kernel - Re: + syscallsx86-implement-execveat-system-call.patch added to -mm tree

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Date:	Fri, 14 Nov 2014 14:55:41 +0000
From:	David Drysdale <drysdale@...gle.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Shuah Khan <shuahkh@....samsung.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andy Lutomirski <luto@...capital.net>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>, Rich Felker <dalias@...ifal.cx>,
	Christoph Hellwig <hch@...radead.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: + syscallsx86-implement-execveat-system-call.patch added to -mm tree

On Fri, Nov 14, 2014 at 12:11 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> @@ -1479,7 +1489,26 @@ static int do_execve_common(struct filen
>>
>>       bprm->file = file;
>> -     bprm->filename = bprm->interp = filename->name;
>> +     if (fd == AT_FDCWD || filename->name[0] == '/') {
>> +             bprm->filename = filename->name;
>> +     } else {
>> +             if (filename->name[0] == '\0')
>> +                     pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d", fd);
>> +             else
>> +                     pathbuf = kasprintf(GFP_TEMPORARY, "/dev/fd/%d/%s",
>> +                                         fd, filename->name);
>> +             if (!pathbuf) {
>> +                     retval = -ENOMEM;
>> +                     goto out_unmark;
>> +             }
>> +             /* Record that a name derived from an O_CLOEXEC fd will be
>> +              * inaccessible after exec. Relies on having exclusive access to
>> +              * current->files (due to unshare_files above). */
>> +             if (close_on_exec(fd, current->files->fdt))
>> +                     bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
>> +             bprm->filename = pathbuf;
> +       }
> +       bprm->interp = bprm->filename;
>
> Not sure I understand this patch, will try to read later...
>
> Just once question, don't we leak pathbuf if exec() succeeds?

Doh, yes.  I was sure I'd run this through kmemleak too, although
the evidence in front of me now clearly implies I didn't ...

> OTOH, if it fails,
>
>>  out_free:
>>       free_bprm(bprm);
>> +     kfree(pathbuf);
>
> Is it correct if we fail after bprm_change_interp() was called? It seems
> that we can free interp == pathbuf twice?

I think this is OK -- bprm_change_interp() changes bprm->interp to point to a
newly kstrdup'ed string, but leaves brpm->filename as pathbuf.  The former
then gets freed in free_bprm() (because it differs from filename == pathbuf),
and pathbuf is freed on the line afterwards.

> Oleg.
>
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