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Date:   Thu, 18 Mar 2021 16:48:43 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Will Deacon <will@...nel.org>, Jessica Yu <jeyu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        bpf <bpf@...r.kernel.org>, linux-hardening@...r.kernel.org,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kbuild <linux-kbuild@...r.kernel.org>,
        PCI <linux-pci@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 01/17] add support for Clang CFI

On Thu, Mar 18, 2021 at 3:29 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Thu, Mar 18, 2021 at 10:10:55AM -0700, Sami Tolvanen wrote:
> > +static void update_shadow(struct module *mod, unsigned long base_addr,
> > +             update_shadow_fn fn)
> > +{
> > +     struct cfi_shadow *prev;
> > +     struct cfi_shadow *next;
> > +     unsigned long min_addr, max_addr;
> > +
> > +     next = vmalloc(SHADOW_SIZE);
> > +
> > +     mutex_lock(&shadow_update_lock);
> > +     prev = rcu_dereference_protected(cfi_shadow,
> > +                                      mutex_is_locked(&shadow_update_lock));
> > +
> > +     if (next) {
> > +             next->base = base_addr >> PAGE_SHIFT;
> > +             prepare_next_shadow(prev, next);
> > +
> > +             min_addr = (unsigned long)mod->core_layout.base;
> > +             max_addr = min_addr + mod->core_layout.text_size;
> > +             fn(next, mod, min_addr & PAGE_MASK, max_addr & PAGE_MASK);
> > +
> > +             set_memory_ro((unsigned long)next, SHADOW_PAGES);
> > +     }
> > +
> > +     rcu_assign_pointer(cfi_shadow, next);
> > +     mutex_unlock(&shadow_update_lock);
> > +     synchronize_rcu_expedited();
>
> expedited is BAD(tm), why is it required and why doesn't it have a
> comment?

Ah, this uses synchronize_rcu_expedited() because we have a case where
synchronize_rcu() hangs here with a specific SoC family after the
vendor's cpu_pm driver powers down CPU cores. Would you say expedited
is bad enough that we should avoid it here? The function is called
only when kernel modules are loaded or unloaded, so not very
frequently.

Sami

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