Re: [PATCH v3 3/3] module: Make __tracepoints_ptrs as read-only

From: Joel Fernandes <joel@joelfernandes.org>
To: Jessica Yu <jeyu@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com,
	keescook@chromium.org, mathieu.desnoyers@efficios.com,
	kernel-hardening@lists.openwall.com, kernel-team@android.com,
	rcu@vger.kernel.org
Subject: Re: [PATCH v3 3/3] module: Make __tracepoints_ptrs as read-only
Date: Sat, 20 Apr 2019 11:38:58 +0000	[thread overview]
Message-ID: <20190420113858.GA1058@localhost> (raw)
In-Reply-To: <20190417151618.GD17099@linux-8ccs>

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@joelfernandes.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.

Thanks a lot for the explanations. Yes we dropped the patches because const
worked. This is good to know for future such ventures as well ;-)

Best,

- Joel

> Jessica