linux-kernel - Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call

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Message-ID: <CAHse=S-x6NKEn9EGFos6bS=asK-1mWCr0r9aji2a97_fgxU+Sg@mail.gmail.com>
Date:	Mon, 27 Oct 2014 18:03:43 +0000
From:	David Drysdale <drysdale@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Meredydd Luff <meredydd@...atehouse.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCHv5 1/3] syscalls,x86: implement execveat() system call

On Wed, Oct 22, 2014 at 7:44 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Wed, Oct 22, 2014 at 4:44 AM, David Drysdale <drysdale@...gle.com> wrote:
>> Add a new system execveat(2) syscall. execveat() is to execve() as
>> openat() is to open(): it takes a file descriptor that refers to a
>> directory, and resolves the filename relative to that.
>>
>
>>         bprm->file = file;
>> -       bprm->filename = bprm->interp = filename->name;
>> +       if (fd == AT_FDCWD || filename->name[0] == '/') {
>> +               bprm->filename = filename->name;
>> +       } else {
>> +               /*
>> +                * Build a pathname that reflects how we got to the file,
>> +                * either "/dev/fd/<fd>" (for an empty filename) or
>> +                * "/dev/fd/<fd>/<filename>".
>> +                */
>> +               pathbuf = kmalloc(PATH_MAX, GFP_TEMPORARY);
>> +               if (!pathbuf) {
>> +                       retval = -ENOMEM;
>> +                       goto out_unmark;
>> +               }
>> +               bprm->filename = pathbuf;
>> +               if (filename->name[0] == '\0')
>> +                       sprintf(pathbuf, "/dev/fd/%d", fd);
>
> If the fd is O_CLOEXEC, then this will result in a confused child
> process.  Should we fail exec attempts like that for non-static
> programs?  (E.g. set filename to "" or something and fix up the binfmt
> drivers to handle that?)

Isn't it just scripts that get confused here (as normal executables don't
get to see brpm->filename)?

Given that we don't know which we have at this point, I'd suggest
carrying on regardless.  Or we could fall back to use the previous
best-effort d_path() code for O_CLOEXEC fds.  Thoughts?

>> +               else
>> +                       snprintf(pathbuf, PATH_MAX,
>> +                                "/dev/fd/%d/%s", fd, filename->name);
>
> Does this need to handle the case where the result exceeds PATH_MAX?

I guess we could kmalloc(strlen(filename->name) + 19) to avoid the
possibility of failure, but that just defers the inevitable -- the interpreter
won't be able to open the script file anyway.  But it would at least then
generate the appropriate error (ENAMETOOLONG rather than ENOENT).
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