Re: [PATCH] module: Harden STRICT_MODULE_RWX

Jessica Yu
2020-04-06T10:46:24Z

+++ 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..

>Looking at layout_sections()... a section with
>SHF_EXECINSTR|SHF_WRITE|SHF_ALLOC would not be counted at all.

Also correct, a section with SHF_EXECINSTR|SHF_WRITE|SHF_ALLOC would
be ignored as it matches none of the masks listed in
layout_sections() - its section->sh_entsize will stay ~0UL.

>However,
>move_module() later copies everything with SHF_ALLOC flag to the final
>module. If there is WXA section, there would be a bug because the
>allocation there would not get the correct size. In that case it is
>important to error out early as you're proposing.

That would be a bug indeed, - we'd get a completely wrong offset to
copy into since sh_entsize was never initialized. Actually, there
should probably be a check for that in move_module() :-/

>Am I missing something?

Nope, thanks for double checking everything!

Jessica