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SubjectRe: [PATCH v4 3/3] vsprintf: Use hw RNG for ptr_key
On Tue, May 15, 2018 at 09:47:44AM -0400, Steven Rostedt wrote:
> On Tue, 15 May 2018 13:06:26 +1000
> "Tobin C. Harding" <me@tobin.cc> wrote:
>
> > Currently we must wait for enough entropy to become available before
> > hashed pointers can be printed. We can remove this wait by using the
> > hw RNG if available.
> >
> > Use hw RNG to get keying material.
> >
> > Suggested-by: Kees Cook <keescook@chromium.org>
> > Signed-off-by: Tobin C. Harding <me@tobin.cc>
> > ---
> > lib/vsprintf.c | 19 ++++++++++++++++---
> > 1 file changed, 16 insertions(+), 3 deletions(-)
> >
> > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > index b82f0c6c2aec..3697a19c2b25 100644
> > --- a/lib/vsprintf.c
> > +++ b/lib/vsprintf.c
> > @@ -1657,9 +1657,8 @@ char *device_node_string(char *buf, char *end, struct device_node *dn,
> > static bool have_filled_random_ptr_key __read_mostly;
> > static siphash_key_t ptr_key __read_mostly;
> >
> > -static void fill_random_ptr_key(struct random_ready_callback *unused)
> > +static void ptr_key_ready(void)
> > {
> > - get_random_bytes(&ptr_key, sizeof(ptr_key));
> > /*
> > * have_filled_random_ptr_key==true is dependent on get_random_bytes().
> > * ptr_to_id() needs to see have_filled_random_ptr_key==true
>
> Nothing to do with this patch, but I believe there's a missing memory
> barrier in the code.
>
> Right after this we have:
>
> smp_mb();
> WRITE_ONCE(have_filled_random_ptr_key, true);
>
> Where the comment says that have_filled_random_ptr_key must be set
> after ptr_key has been updated. But there's no memory barrier on the
> read side. In fact, I think this could be a smp_wmb() instead of a
> smp_mb(). The read side has:
>
> if (unlikely(!have_filled_random_ptr_key))
> return string(buf, end, "(ptrval)", spec);
>
> /* Missing memory barrier smp_rmb() here. */
>
> hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
>
> Thus we can have something like:
>
> CPU0 CPU1
> ---- ----
> load ptr_key = 0
> store ptr_key = random
> smp_mb()
> store have_filled_random_ptr_key
>
> load have_filled_random_ptr_key = true
>
> BAD BAD BAD!
>
> I'll send a patch.

Awesome reviewing. Thanks for catching this.


Tobin

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