[PATCH 05/14] fs: don't allow kernel reads and writes without iter ops

[Alexander Viro]

On Fri, Oct 09, 2020 at 06:29:13PM -0700, Linus Torvalds wrote:
> On Fri, Oct 9, 2020 at 6:19 PM Eric Biggers <ebiggers at kernel.org> wrote:
> >
> > Okay, that makes more sense.  So the patchset from Matthew
> > https://lkml.kernel.org/linux-fsdevel/20201003025534.21045-1-willy@infradead.org/T/#u
> > isn't what you had in mind.
> 
> No.
> 
> That first patch makes sense - it's just the "ppos can be NULL" patch.
> 
> But as mentioned, NULL isn't "shorthand for zero". It's just "pipes
> don't _have_ a pos, trying to pass in some explicit position is
> crazy".
> 
> So no, the other patches in that set are a bit odd, I think.
> 
> SOME of them look potentially fine - the bpfilter one seems to be
> valid, for example, because it's literally about reading/writing a
> pipe. And maybe the sysctl one is similarly sensible - I didn't check
> the context of that one.

FWIW, I hadn't pushed that branch out (or merged it into #for-next yet);
for one thing, uml part (mconsole) is simply broken, for another...
IMO ##5--8 are asking for kernel_pread() and if you look at binfmt_elf.c,
you'll see elf_read() being pretty much that.  acct.c, keys and usermode
parts are asking for kernel_pwrite() as well.

I've got stuck looking through the drivers/target stuff - it would've
been another kernel_pwrite() candidate, but it smells like its use of
filp_open() is really asking for trouble, starting with symlink attacks.
Not sure - I'm not familiar with the area, but...