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SubjectRe: [PATCH 2/2] module: Make srcu_struct ptr array as read-only post init
On Tue, Apr 09, 2019 at 09:14:18PM -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.
>
> 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.
>
> Suggested-by: paulmck@linux.vnet.ibm.com
> Suggested-by: keescook@chromium.org
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Just wanted to mention, that I tested that the srcu_struct_ptrs array is not
writeable any more after init, but doing the following after
module_enable_ro() :

@@ -3513,6 +3523,14 @@ static noinline int do_init_module(struct module *mod)
rcu_assign_pointer(mod->kallsyms, &mod->core_kallsyms);
#endif
module_enable_ro(mod, true);
+
+ if (mod->srcu_struct_ptrs) {
+ // Check if SRCU 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);


thanks,

- Joel

>
> ---
> kernel/module.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index f9221381d076..ed1f2612aebc 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -3301,7 +3301,7 @@ static bool blacklisted(const char *module_name)
> core_param(module_blacklist, module_blacklist, charp, 0400);
>
> /*
> - * Mark ro_after_init section with SHF_RO_AFTER_INIT so that
> + * These are section names marked with SHF_RO_AFTER_INIT so that
> * layout_sections() can put it in the right place.
> * Note: ro_after_init sections also have SHF_{WRITE,ALLOC} set.
> */
> @@ -3314,6 +3314,13 @@ static char *ro_after_init_sections[] = {
> * annotated as such at module load time.
> */
> "__jump_table",
> +
> + /*
> + * Used for SRCU structures which need to be initialized/cleaned up
> + * by the SRCU notifiers
> + */
> + "___srcu_struct_ptrs",
> +
> NULL
> };
>
> --
> 2.21.0.392.gf8f6787159e-goog
>

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