linux-kernel - Re: [PATCH 05/20] tomoyo_write_control(): get rid of pointless access

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Date:   Sat, 9 May 2020 17:57:56 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 05/20] tomoyo_write_control(): get rid of pointless access_ok()

On Sat, May 9, 2020 at 5:51 PM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> I think that this access_ok() check helps reducing partial writes (either
> "whole amount was processed" or "not processed at all" unless -ENOMEM).

No it doesn't.

"access_ok()" only checks the range being a valid user address range.

It doesn't actually help at all if the worry is "what if we take a
page fault in the middle".  Because it simply doesn't check those
kinds of things.

Now, if somebody passes actual invalid ranges (ie kernel addresses or
other crazy stuff), they only have themselves to blame. The invalid
range will be noticed when actually doing the user copy, and then
you'll get EFAULT there. But there's no point in trying to figure that
out early - it's only adding overhead, and it doesn't help any normal
case.

                  Linus