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Date:   Wed, 29 Mar 2023 19:58:51 +0200
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
Message-ID: <20230329175850.GA8425@redhat.com>
References: <20230328164811.2451-1-gregory.price@memverge.com>
 <20230328164811.2451-2-gregory.price@memverge.com>
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 <9a456346-e207-44e1-873e-40d21334e01b@app.fastmail.com>
 <20230329160322.GA4477@redhat.com>
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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.