linux-kernel - Re: [PATCH v3 3/3] module: Make __tracepoints

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Date:   Wed, 17 Apr 2019 08:36:24 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, keescook@...omium.org,
        mathieu.desnoyers@...icios.com,
        kernel-hardening@...ts.openwall.com, kernel-team@...roid.com,
        rcu@...r.kernel.org
Subject: Re: [PATCH v3 3/3] module: Make __tracepoints_ptrs as read-only

On Wed, Apr 17, 2019 at 05:16:18PM +0200, Jessica Yu wrote:
> +++ Steven Rostedt [10/04/19 20:44 -0400]:
> >On Wed, 10 Apr 2019 16:29:02 -0400
> >Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> >>The srcu structure pointer array is modified at module load time because the
> >>array is fixed up by the module loader at load-time with the final locations
> >>of the tracepoints right?  Basically relocation fixups. At compile time, I
> >>believe it is not know what the values in the ptr array are. I believe same
> >>is true for the tracepoint ptrs array.
> >>
> >>Also it needs to be in a separate __tracepoint_ptrs so that this code works:
> >>
> >>
> >>#ifdef CONFIG_TRACEPOINTS
> >>	mod->tracepoints_ptrs = section_objs(info, "__tracepoints_ptrs",
> >>					     sizeof(*mod->tracepoints_ptrs),
> >>					     &mod->num_tracepoints);
> >>#endif
> >>
> >>Did I  miss some point? Thanks,
> >
> >But there's a lot of others too. Hmm, does this mean that the RO data
> >sections that are in modules are not set to RO?
> >
> >There's a bunch of separate sections that are RO. Just look in
> >include/asm-generic/vmlinux.lds.h under the RO_DATA_SECTION() macro.
> >
> >A lot of the sections saved in module.c:find_module_sections() are in
> >that RO_DATA when compiled as a builtin. Are they not RO when loaded via
> >a module?
> 
> Unlike the kernel, the module loader does not rely on a linker script
> to determine which sections get what protections. On module load, all
> sections in a module are looped through and those sections without the
> SHF_WRITE flag will be set to RO. For example, when there is a section
> filled with structs declared as const or if the section was explicitly
> given only the SHF_ALLOC attribute, those will be read-only. As long
> as the sections were given the correct section attributes for
> read-only, it'll have read-only protection. I see this is already the
> case for __param and  __ksymtab*/__kcrctab* sections, but I agree that
> a full audit would be useful to be consistent with builtin RO
> protections.

Thank you very much for the explanation!

							Thanx, Paul