linux-kernel - Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages

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Message-ID: <20240415104750.GJ40213@noisy.programming.kicks-ass.net>
Date: Mon, 15 Apr 2024 12:47:50 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Rapoport <rppt@...nel.org>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>,
	Catalin Marinas <catalin.marinas@....com>,
	Christoph Hellwig <hch@...radead.org>, Helge Deller <deller@....de>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Palmer Dabbelt <palmer@...belt.com>,
	Russell King <linux@...linux.org.uk>, Song Liu <song@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Uladzislau Rezki <urezki@...il.com>, Will Deacon <will@...nel.org>,
	bpf@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linux-modules@...r.kernel.org, linux-parisc@...r.kernel.org,
	linux-riscv@...ts.infradead.org, linux-trace-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [RFC PATCH 6/7] execmem: add support for cache of large ROX pages

On Thu, Apr 11, 2024 at 07:05:25PM +0300, Mike Rapoport wrote:

> To populate the cache, a writable large page is allocated from vmalloc with
> VM_ALLOW_HUGE_VMAP, filled with invalid instructions and then remapped as
> ROX.

> +static void execmem_invalidate(void *ptr, size_t size, bool writable)
> +{
> +	if (execmem_info->invalidate)
> +		execmem_info->invalidate(ptr, size, writable);
> +	else
> +		memset(ptr, 0, size);
> +}

+static void execmem_invalidate(void *ptr, size_t size, bool writeable)
+{
+       /* fill memory with INT3 instructions */
+       if (writeable)
+               memset(ptr, 0xcc, size);
+       else
+               text_poke_set(ptr, 0xcc, size);
+}

Thing is, 0xcc (aka INT3_INSN_OPCODE) is not an invalid instruction.
It raises #BP not #UD.