Re: [PATCH v2] fs/exec: require argv[0] presence in do_execveat_common()

[Ariadne Conill]

Hi,

On Wed, 26 Jan 2022, Matthew Wilcox wrote:

> On Wed, Jan 26, 2022 at 11:44:47AM +0000, Ariadne Conill wrote:
> > In several other operating systems, it is a hard requirement that the
> > first argument to execve(2) be the name of a program, thus prohibiting
> > a scenario where argc < 1.  POSIX 2017 also recommends this behaviour,
> > but it is not an explicit requirement[0]:
> >
> >     The argument arg0 should point to a filename string that is
> >     associated with the process being started by one of the exec
> >     functions.
> >
> > To ensure that execve(2) with argc < 1 is not a useful gadget for
> > shellcode to use, we can validate this in do_execveat_common() and
> > fail for this scenario, effectively blocking successful exploitation
> > of CVE-2021-4034 and similar bugs which depend on this gadget.
> >
> > The use of -EFAULT for this case is similar to other systems, such
> > as FreeBSD, OpenBSD and Solaris.  QNX uses -EINVAL for this case.
> >
> > Interestingly, Michael Kerrisk opened an issue about this in 2008[1],
> > but there was no consensus to support fixing this issue then.
> > Hopefully now that CVE-2021-4034 shows practical exploitative use
> > of this bug in a shellcode, we can reconsider.
> >
> > [0]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html
> > [1]: https://bugzilla.kernel.org/show_bug.cgi?id=8408
> >
> > Changes from v1:
> > - Rework commit message significantly.
> > - Make the argv[0] check explicit rather than hijacking the error-check
> >   for count().
> >
> > Signed-off-by: Ariadne Conill <ariadne@xxxxxxxxxxxxxxxx>
> > ---
> >  fs/exec.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 79f2c9483302..e52c41991aab 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -1899,6 +1899,10 @@ static int do_execveat_common(int fd, struct filename *filename,
> >  	retval = count(argv, MAX_ARG_STRINGS);
> >  	if (retval < 0)
> >  		goto out_free;
> > +	if (retval == 0) {
> > +		retval = -EFAULT;
> > +		goto out_free;
> > +	}
>
> I don't object to the concept, but it's a more common pattern in Linux
> to do this:
>
> 	retval = count(argv, MAX_ARG_STRINGS);
> +	if (retval == 0)
> +		retval = -EFAULT;
> 	if (retval < 0)
> 		goto out_free;

Yeah, that seems fine.  We can of course do it that way, which I will 
revise the patch to do if we decide to stick with denial over making a 
"safe" argv instead.

> (aka I like my bikesheds painted in Toasty Eggshell)

Toasty Eggshell is a nice color for a bikeshed :)

Ariadne