[kernel-hardening] Re: [PATCH] shm: fix a race between shm_exit() and shm_init()

Andrew Morton
2011-08-02T20:55:12Z

On Tue, 2 Aug 2011 16:45:30 +0400
Vasiliy Kulikov <segoon@openwall.com> wrote:

> On thread exit shm_exit() is called, it uses shm_ids(ns).rw_mutex.
> It is initialized in shm_init(), but it is not called yet at the moment
> of kernel threads exit. Some kernel threads are created in
> do_pre_smp_initcalls(), and shm_init() is called in do_initcalls().
>
> Static initialization of shm_ids(init_ipc_ns).rw_mutex fixes the race.
>
> It fixes a kernel oops:
>
> Unable to handle kernel NULL pointer dereference at virtual address 00000000
> ...
> [<c0320090>] (__down_write_nested+0x88/0xe0) from [<c015da08>] (exit_shm+0x28/0x48)
> [<c015da08>] (exit_shm+0x28/0x48) from [<c002e550>] (do_exit+0x59c/0x750)
> [<c002e550>] (do_exit+0x59c/0x750) from [<c003eaac>] (____call_usermodehelper+0x13c/0x154)
> [<c003eaac>] (____call_usermodehelper+0x13c/0x154) from [<c000f630>] (kernel_thread_exit+0x0/0x8)

erm, wait. There's no reason I can think of why a kernel thread needs
to call shm_exit() at all?

Is that a regular kernel thread exiting, or is it a
call_usermodehelper() worker thread? It *looks* like
____call_usermodehelper()'s kernel_execve() failed, so
____call_usermodehelper() directly called do_exit().

Something's still screwed up here - we shouldn't be trying to run
usermode helper applications before shm_init() has been run - usermode
helpers can use ipc!

Can someone who can reproduce this please work out if and why we're
calling call_usermodehelper() under do_pre_smp_initcalls()? Something
like this...

--- a/init/main.c~a
+++ a/init/main.c
@@ -722,12 +722,16 @@ static void __init do_basic_setup(void)
do_initcalls();
}

+int in_do_pre_smp_initcalls;
+
static void __init do_pre_smp_initcalls(void)
{
initcall_t *fn;

+ in_do_pre_smp_initcalls = 1;
for (fn = __initcall_start; fn < __early_initcall_end; fn++)
do_one_initcall(*fn);
+ in_do_pre_smp_initcalls = 0;
}

static void run_init_process(const char *init_filename)
--- a/kernel/kmod.c~a
+++ a/kernel/kmod.c
@@ -412,12 +412,17 @@ EXPORT_SYMBOL(call_usermodehelper_setfns
* asynchronously if wait is not set, and runs as a child of keventd.
* (ie. it runs with full root capabilities).
*/
+
+extern int in_do_pre_smp_initcalls;
+
int call_usermodehelper_exec(struct subprocess_info *sub_info,
enum umh_wait wait)
{
DECLARE_COMPLETION_ONSTACK(done);
int retval = 0;

+ if (in_do_pre_smp_initcalls)
+ dump_stack();
helper_lock();
if (sub_info->path[0] == '\0')
goto out;
_