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    SubjectRe: [PATCH v2] module: Harden STRICT_MODULE_RWX
    +++ Ard Biesheuvel [22/08/20 15:47 +0200]:
    >(+ Masahiro)
    >
    >On Fri, 21 Aug 2020 at 14:30, Will Deacon <will@kernel.org> wrote:
    >>
    >> On Fri, Aug 21, 2020 at 02:27:05PM +0200, Ard Biesheuvel wrote:
    >> > On Fri, 21 Aug 2020 at 14:20, Will Deacon <will@kernel.org> wrote:
    >> > >
    >> > > On Thu, Aug 13, 2020 at 03:07:13PM +0200, Ard Biesheuvel wrote:
    >> > > > On Thu, 13 Aug 2020 at 15:04, Jessica Yu <jeyu@kernel.org> wrote:
    >> > > > >
    >> > > > > +++ Ard Biesheuvel [13/08/20 10:36 +0200]:
    >> > > > > >On Wed, 12 Aug 2020 at 22:00, Peter Zijlstra <peterz@infradead.org> wrote:
    >> > > > > >>
    >> > > > > >> On Wed, Aug 12, 2020 at 06:37:57PM +0200, Ard Biesheuvel wrote:
    >> > > > > >> > I know there is little we can do at this point, apart from ignoring
    >> > > > > >> > the permissions - perhaps we should just defer the w^x check until
    >> > > > > >> > after calling module_frob_arch_sections()?
    >> > > > > >>
    >> > > > > >> My earlier suggestion was to ignore it for 0-sized sections.
    >> > > > > >
    >> > > > > >Only they are 1 byte sections in this case.
    >> > > > > >
    >> > > > > >We override the sh_type and sh_flags explicitly for these sections at
    >> > > > > >module load time, so deferring the check seems like a reasonable
    >> > > > > >alternative to me.
    >> > > > >
    >> > > > > So module_enforce_rwx_sections() is already called after
    >> > > > > module_frob_arch_sections() - which really baffled me at first, since
    >> > > > > sh_type and sh_flags should have been set already in
    >> > > > > module_frob_arch_sections().
    >> > > > >
    >> > > > > I added some debug prints to see which section the module code was
    >> > > > > tripping on, and it was .text.ftrace_trampoline. See this snippet from
    >> > > > > arm64's module_frob_arch_sections():
    >> > > > >
    >> > > > > else if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE) &&
    >> > > > > !strcmp(secstrings + sechdrs[i].sh_name,
    >> > > > > ".text.ftrace_trampoline"))
    >> > > > > tramp = sechdrs + i;
    >> > > > >
    >> > > > > Since Mauro's config doesn't have CONFIG_DYNAMIC_FTRACE enabled, tramp
    >> > > > > is never set here and the if (tramp) check at the end of the function
    >> > > > > fails, so its section flags are never set, so they remain WAX and fail
    >> > > > > the rwx check.
    >> > > >
    >> > > > Right. Our module.lds does not go through the preprocessor, so we
    >> > > > cannot add the #ifdef check there currently. So we should either drop
    >> > > > the IS_ENABLED() check here, or simply rename the section, dropping
    >> > > > the .text prefix (which doesn't seem to have any significance outside
    >> > > > this context)
    >> > > >
    >> > > > I'll leave it to Will to make the final call here.
    >> > >
    >> > > Why don't we just preprocess the linker script, like we do for the main
    >> > > kernel?
    >> > >
    >> >
    >> > That should work as well, I just haven't checked how straight-forward
    >> > it is to change that.
    >>
    >> Ok, if it's _not_ straightforward, then let's just drop the IS_ENABLED()
    >> altogether.
    >>
    >
    >I played around with this for a while, but failed to get Kbuild to
    >instantiate $(objtree)/arch/arm64/kernel/module.lds based on
    >$(srctree)/arch/arm64/kernel/module.lds.S and the cpp_lds_S rule.
    >Perhaps Masahiro has any suggestions here? Otherwise, let's just drop
    >the IS_ENABLED() check for now.

    I also tinkered around a bit and was able to generate
    $(objtree)/arch/arm64/kernel/module.lds based on
    $(srctree)/arch/arm64/kernel/module.lds.S only if I specified the
    former as the make target directly. Correct me if I'm wrong, but I
    guess this might be because the single build targets would utilize
    scripts/Makefile.build (where the cpp_lds_S rule is defined) while the
    module-related Makefiles don't seem to support .lds.S -> .lds in
    general.. Masahiro, how easy would it be to extend .lds.S -> .lds
    support to module linker scripts as well?

    Thanks,

    Jessica

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