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Date:   Wed, 29 Mar 2023 19:13:22 +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: <20230329171322.GB4477@redhat.com>
References: <20230328164811.2451-1-gregory.price@memverge.com>
 <20230328164811.2451-2-gregory.price@memverge.com>
 <20230329151515.GA913@redhat.com>
 <9a456346-e207-44e1-873e-40d21334e01b@app.fastmail.com>
 <20230329160322.GA4477@redhat.com>
 <ZCO20bzX/IB8J6Gp@memverge.com>
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On 03/28, Gregory Price wrote:
>
> Not sure how I should proceed here,

Can't we just kill this access_ok() in set_syscall_user_dispatch() ?
I don't think it buys too much.

Oleg.

diff --git a/kernel/entry/syscall_user_dispatch.c b/kernel/entry/syscall_user_dispatch.c
index 0b6379adff6b..d2e516ece52b 100644
--- a/kernel/entry/syscall_user_dispatch.c
+++ b/kernel/entry/syscall_user_dispatch.c
@@ -43,11 +43,7 @@ bool syscall_user_dispatch(struct pt_regs *regs)
 		return false;
 
 	if (likely(sd->selector)) {
-		/*
-		 * access_ok() is performed once, at prctl time, when
-		 * the selector is loaded by userspace.
-		 */
-		if (unlikely(__get_user(state, sd->selector))) {
+		if (unlikely(get_user(state, sd->selector))) {
 			force_exit_sig(SIGSEGV);
 			return true;
 		}
@@ -86,9 +82,6 @@ int set_syscall_user_dispatch(unsigned long mode, unsigned long offset,
 		if (offset && offset + len <= offset)
 			return -EINVAL;
 
-		if (selector && !access_ok(selector, sizeof(*selector)))
-			return -EFAULT;
-
 		break;
 	default:
 		return -EINVAL;