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SubjectRe: [PATCH v4] livepatch: introduce shadow variable API
On Fri 2017-08-18 09:46:08, Joe Lawrence wrote:
> On 08/17/2017 10:05 AM, Petr Mladek wrote:
> > On Mon 2017-08-14 16:02:43, Joe Lawrence wrote:
> >> [ ... snip ... ]
> >> diff --git a/samples/livepatch/livepatch-shadow-fix1.c b/samples/livepatch/livepatch-shadow-fix1.c
> >> new file mode 100644
> >> index 000000000000..5acc838463d1
> >> --- /dev/null
> >> +++ b/samples/livepatch/livepatch-shadow-fix1.c
> >> +void livepatch_fix1_dummy_free(struct dummy *d)
> >> +{
> >> + void **shadow_leak;
> >> +
> >> + /*
> >> + * Patch: fetch the saved SV_LEAK shadow variable, detach and
> >> + * free it. Note: handle cases where this shadow variable does
> >> + * not exist (ie, dummy structures allocated before this livepatch
> >> + * was loaded.)
> >> + */
> >> + shadow_leak = klp_shadow_get(d, SV_LEAK);
> >> + if (shadow_leak) {
> >> + klp_shadow_detach(d, SV_LEAK);
> >> + kfree(*shadow_leak);
> >
> > This should get removed. The buffer used for the shadow variable
> > is freed by kfree_rcu() called from klp_shadow_detach().
> >
> > Same problem is also in the other livepatch.
> >
> >> + pr_info("%s: dummy @ %p, prevented leak @ %p\n",
> >> + __func__, d, *shadow_leak);
> >
> > This might access shadow_leak after it was (double) freed.
> >
> >> + } else {
> >> + pr_info("%s: dummy @ %p leaked!\n", __func__, d);
> >> + }
> >> +
> >> + kfree(d);
> >> +}
>
> Hi Petr,
>
> I think you're half correct.
>
> The kfree is the crux of the memory leak patch, so it needs to stay.
> However, the shadow variable is holding a copy of the pointer to the
> memory leak area, so you're right that it can't be safely dereferenced
> after the shadow variable is detached*.

Ah, I see. The extra kftree does not free the shadow->data but
it frees the data that the shadow variable points to.

> The code should to be rearranged like:
>
> void livepatch_fix1_dummy_free(struct dummy *d)
> {
> void **p_shadow_leak, *shadow_leak;
>
> p_shadow_leak = klp_shadow_get(d, SV_LEAK);
> if (p_shadow_leak) {
> shadow_leak = *p_shadow_leak; << deref before detach

I would rename shadow_leak -> leak. It will make it more clear
that it is the original leak pointer.

Well, we could actually free the data before we detach/destroy
the shadow variable. But then it might deserve a comment to
avoid confusion that I had. I mean:

shadow_leak = klp_shadow_get(d, SV_LEAK);
if (shadow_leak) {
pr_info("%s: dummy @ %p, prevented leak @ %p\n",
__func__, d, *shadow_leak);
/* Free the previously leaked data */
kfree(*shadow_leak);
/* Free the shadow variable */
klp_shadow_detach(d, SV_LEAK);

> klp_shadow_detach(d, SV_LEAK);
> kfree(shadow_leak);
> ...
>
> * Aside: I usually develop with slub_debug=FZPU set to catch silly
> use-after-frees like this.

Sounds like a good practice.

> released via kfree_rcu(), I think there was a window before the grace
> period where this one worked out okay... once I added a
> synchronize_rcu() call in between the klp_shadow_detch() and kfree()
> calls, I did see the poison pattern. This is my first time using
> kfree_rcu(), so it was interesting to dig into.

Yup.

Best Regards,
Petr

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