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SubjectRe: [PATCH] module: Make srcu_struct ptr array as read-only
On Thu, Apr 11, 2019 at 04:24:21PM -0400, Joel Fernandes (Google) wrote:
> Since commit title ("srcu: Allocate per-CPU data for DEFINE_SRCU() in
> modules"), modules that call DEFINE_{STATIC,}SRCU will have a new array
> of srcu_struct pointers, which is used by srcu code to initialize and
> clean up these structures and save valuable per-cpu reserved space.
>
> There is no reason for this array of pointers to be writable, and can
> cause security or other hidden bugs. Mark these are read-only after the
> module init has completed.
>
> Tested with the following diff to ensure array not writable:
>
> (diff is a bit reduced to avoid patch command getting confused)
> a/kernel/module.c
> b/kernel/module.c
> -3506,6 +3506,14 static noinline int do_init_module [snip]
> rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
> #endif
> module_enable_ro(mod, true);
> +
> + if (mod->srcu_struct_ptrs) {
> + // Check if srcu_struct_ptrs access is possible
> + char x = *(char *)mod->srcu_struct_ptrs;
> + *(char *)mod->srcu_struct_ptrs = 0;
> + *(char *)mod->srcu_struct_ptrs = x;
> + }
> +
> mod_tree_remove_init(mod);
> disable_ro_nx(&mod->init_layout);
> module_arch_freeing_init(mod);
>
> Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Cc: paulmck@linux.vnet.ibm.com
> Cc: rostedt@goodmis.org
> Cc: mathieu.desnoyers@efficios.com
> Cc: rcu@vger.kernel.org
> Cc: kernel-hardening@lists.openwall.com
> Cc: kernel-team@android.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Queued for testing and further review, thank you, Joel!

Thanx, Paul

> ---
> This single patch superceded the patches at:
> https://lore.kernel.org/patchwork/patch/1060298/
> https://lore.kernel.org/patchwork/patch/1060298/
>
> include/linux/srcutree.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/srcutree.h b/include/linux/srcutree.h
> index 8af1824c46a8..9cfcc8a756ae 100644
> --- a/include/linux/srcutree.h
> +++ b/include/linux/srcutree.h
> @@ -123,7 +123,7 @@ struct srcu_struct {
> #ifdef MODULE
> # define __DEFINE_SRCU(name, is_static) \
> is_static struct srcu_struct name; \
> - struct srcu_struct *__srcu_struct_##name \
> + struct srcu_struct * const __srcu_struct_##name \
> __section("___srcu_struct_ptrs") = &name
> #else
> # define __DEFINE_SRCU(name, is_static) \
> --
> 2.21.0.392.gf8f6787159e-goog
>

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