Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Tue, 5 Mar 2019 15:53:06 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>,
        g@hirez.programming.kicks-ass.net
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        kernel test robot <lkp@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Shuah Khan <shuah@kernel.org>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Changbin Du <changbin.du@gmail.com>,
        Jann Horn <jannh@google.com>,
        Kees Cook <keescook@chromium.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        Nadav Amit <namit@vmware.com>,
        Joel Fernandes <joel@joelfernandes.org>, yhs@fb.com, lkp@01.org
Subject: Re: [uaccess] 780464aed0:
 WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x
Message-ID: <20190305145306.GL32477@hirez.programming.kicks-ass.net>
References: <155136980507.2968.15165201054223875356.stgit@devbox>
 <20190303173954.kliegojbuigqi5tn@inn2.lkp.intel.com>
 <CAHk-=wij8VLupHh1U-fFhFBjin9Fch3mG2_2=aL9h1L+WBvm2g@mail.gmail.com>
 <20190304101434.8429ffffb17813c0e7930130@kernel.org>
 <CAHk-=whYMvevgF4yh7uhQug=Wt1A1Eg7abyM=RCi=bQiqiMBvQ@mail.gmail.com>
 <20190304180610.2d4f6f08d9ad89d6abae3597@kernel.org>
 <CAHk-=wiQM84VsAiw92UWyQZTQxr+F032Gm5BiFBXk7tC+LQohQ@mail.gmail.com>
 <20190305113635.18f80ea3b1f4fca54b9d21e4@kernel.org>
 <20190305090729.GF32477@hirez.programming.kicks-ass.net>
 <20190305225801.a63ac8712105ab2e673be1bc@kernel.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190305225801.a63ac8712105ab2e673be1bc@kernel.org>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Tue, Mar 05, 2019 at 10:58:01PM +0900, Masami Hiramatsu wrote:

> Could you tell me why WARN_ON_ONCE(!in_task()) is needed in access_ok()?

That came from here:

  lkml.kernel.org/r/20190225145240.GB32534@hirez.programming.kicks-ass.net

Because in-irq usage is dodgy, since we don't actually know what mm or
ds it loaded.

> > I dislike that whole KERNEL_DS thing, but obviously that's not something
> > that's going away.
> > 
> > Would something like:
> > 
> > 	WARN_ON_ONCE(!(in_task || segment_eq(get_fs(), USER_DS)))
> > 
> > Work? Then we allow KERNEL_DS in task context, but for interrupt and
> > others require USER_DS.
> 
> But what would this mean? I can't understand why we limit using
> access_ok() so strictly and narrow the cases.

Because it's been a source of bugs. Any sanity checking we can put in
seems like a good thing at this point.