Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of access_ok

From: Oleg Nesterov <oleg@redhat.com>
To: Gregory Price <gregory.price@memverge.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	Gregory Price <gourry.memverge@gmail.com>,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	Linux-Arch <linux-arch@vger.kernel.org>,
	avagin@gmail.com, Peter Zijlstra <peterz@infradead.org>,
	Andy Lutomirski <luto@kernel.org>,
	krisman@collabora.com, Thomas Gleixner <tglx@linutronix.de>,
	Jonathan Corbet <corbet@lwn.net>, shuah <shuah@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	tongtiangen@huawei.com, Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v14 1/4] asm-generic,arm64: create task variant of access_ok
Date: Wed, 29 Mar 2023 19:58:51 +0200	[thread overview]
Message-ID: <20230329175850.GA8425@redhat.com> (raw)
In-Reply-To: <ZCPOpClZ3hOQCs7a@memverge.com>

On 03/29, Gregory Price wrote:
>
> On Wed, Mar 29, 2023 at 07:13:22PM +0200, Oleg Nesterov wrote:
> >
> > -		if (selector && !access_ok(selector, sizeof(*selector)))
> > -			return -EFAULT;
> > -
> >  		break;
> >  	default:
> >  		return -EINVAL;
> >
>
> The result of this would be either a task calling via prctl or a tracer
> calling via ptrace would be capable of setting selector to a bad pointer
> and producing a SIGSEGV on the next system call.

Yes,

> It's a pretty small footgun, but maybe that's reasonable?

I hope this is reasonable,

> From a user perspective, debugging this behavior would be nightmarish.
> Your call to prctl/ptrace would succeed and the process would continue
> to execute until the next syscall - at which point you incur a SIGSEGV,

Yes. But how does this differ from the case when, for example, user
does prtcl(PR_SET_SYSCALL_USER_DISPATCH, selector = 1) ? Or another
bad address < TASK_SIZE?

access_ok() will happily succeed, then later syscall_user_dispatch()
will equally trigger SIGSEGV.

Oleg.