Re: [PATCH] module: Harden STRICT_MODULE_RWX

Jessica Yu
2020-04-06T12:53:43Z

+++ Peter Zijlstra [06/04/20 13:27 +0200]:
>On Mon, Apr 06, 2020 at 12:46:17PM +0200, Jessica Yu wrote:
>> +++ Miroslav Benes [06/04/20 11:55 +0200]:
>> > On Fri, 3 Apr 2020, Josh Poimboeuf wrote:
>> >
>> > > On Fri, Apr 03, 2020 at 06:37:16PM +0200, Peter Zijlstra wrote:
>> > > > +{
>> > > > + int i;
>> > > > +
>> > > > + for (i = 0; i < hdr->e_shnum; i++) {
>> > > > + if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE))
>> > > > + return -ENOEXEC;
>> > >
>> > > I think you only want the error when both are set?
>> > >
>> > > if (sechdrs[i].sh_flags & (SHF_EXECINSTR|SHF_WRITE) == (SHF_EXECINSTR|SHF_WRITE))
>> >
>> > A section with SHF_EXECINSTR and SHF_WRITE but without SHF_ALLOC would be
>> > strange though, no? It wouldn't be copied to the final module later
>> > anyway.
>>
>> That's right - move_module() ignores !SHF_ALLOC sections and does not
>> copy them over to their final location. So I think we want to look for
>> SHF_EXECINSTR|SHF_WRITE|SHF_ALLOC here..
>
>So I did notice that !SHF_ALLOC sections get ignored, but since this
>check is about W^X we don't strictly care about SHF_ALLOC. What we care
>about it never allowing a writable and executable map.
>
>Adding ALLOC to the test only allows for future mistakes and doesn't
>make the check any better.

Ugh sorry, my brain shorted out and for some reason I mistakenly
thought the check excluded SHF_WRITE|SHF_EXECINSTR|SHF_ALLOC sections.
It doesn't obviously. Sorry for the noise.