[PATCH v3 10/16] exec: Remove do_execve_file

[Luis Chamberlain]

On Wed, Jul 08, 2020 at 06:35:25AM +0000, Luis Chamberlain wrote:
> On Thu, Jul 02, 2020 at 11:41:34AM -0500, Eric W. Biederman wrote:
> > Now that the last callser has been removed remove this code from exec.
> > 
> > For anyone thinking of resurrecing do_execve_file please note that
> > the code was buggy in several fundamental ways.
> > 
> > - It did not ensure the file it was passed was read-only and that
> >   deny_write_access had been called on it.  Which subtlely breaks
> >   invaniants in exec.
> > 
> > - The caller of do_execve_file was expected to hold and put a
> >   reference to the file, but an extra reference for use by exec was
> >   not taken so that when exec put it's reference to the file an
> >   underflow occured on the file reference count.
> 
> Maybe its my growing love with testing, but I'm going to have to partly
> blame here that we added a new API without any respective testing.
> Granted, I recall this this patch set could have used more wider review
> and a bit more patience... but just mentioning this so we try to avoid
> new api-without-testing with more reason in the future.
> 
> But more importantly, *how* could we have caught this? Or how can we
> catch this sort of stuff better in the future?

Of all the issues you pointed out with do_execve_file(), since upon
review the assumption *by design* was that LSMs/etc would pick up issues
with the file *prior* to processing, I think that this file reference
count issue comes to my attention as the more serious issue which I
wish we could address *first* before this crusade.

So I have to ask, has anyone *really tried* to give a crack at fixing
this refcount issue in a smaller way first? Alexei?

I'm not opposed to the removal of do_execve_file(), however if there
is a reproducible crash / issue with the existing user, this sledge
hammer seems a bit overkill for older kernels.

  Luis