Re: [PATCH v2 01/17] add support for Clang CFI

From: Peter Zijlstra <peterz@infradead.org>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Kees Cook <keescook@chromium.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Will Deacon <will@kernel.org>, Jessica Yu <jeyu@kernel.org>,
	Arnd Bergmann <arnd@arndb.de>, Tejun Heo <tj@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Christoph Hellwig <hch@infradead.org>,
	bpf@vger.kernel.org, linux-hardening@vger.kernel.org,
	linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
	linux-kbuild@vger.kernel.org, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 01/17] add support for Clang CFI
Date: Thu, 18 Mar 2021 23:29:10 +0100	[thread overview]
Message-ID: <YFPUNlOomp173o5B@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210318171111.706303-2-samitolvanen@google.com>

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?

> +
> +	if (prev) {
> +		set_memory_rw((unsigned long)prev, SHADOW_PAGES);
> +		vfree(prev);
> +	}
> +}