linux-kernel - Re: [PATCHv14 01/17] x86/mm: Rework address range check in get_user() and put

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Message-ID: <CAHk-=wjtUnjYSttxsYtXu2VtsTeBjoaL8Je8cjWqFMJzw1-MhQ@mail.gmail.com>
Date:   Wed, 18 Jan 2023 09:01:01 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>, x86@...nel.org,
        Kostya Serebryany <kcc@...gle.com>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Taras Madan <tarasmadan@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "H . J . Lu" <hjl.tools@...il.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Bharata B Rao <bharata@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv14 01/17] x86/mm: Rework address range check in get_user()
 and put_user()

On Wed, Jan 18, 2023 at 8:49 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> > We'll still return -EFAULT, of course, we're just getting rid of the
> >
> >         WARN_ONCE(trapnr == X86_TRAP_GP,
> >                 "General protection fault in user access.
> > Non-canonical address?");
> >
> > issue that comes from not being so exact about the address limit any more.
>
> Ah indeed, so for !LAM we'd now print the message were we would not
> before (the whole TASK_SIZE_MAX+ range).

Yeah.

We could just remove that warning entirely, but it has been useful for
syzbot catching random user addresses that weren't caught by
"access_ok()" when people did bad bad things (ie using the
non-checking "__copy_from_user()" and friends).

I'm not sure how much that warning is worth any more - and for
get_user() and put_user() itself it buys us nothing, since by
definition _those_ do the range checking. Christoph getting rid of the
set_fs() model simplified a lot of our user address checking.

But I think it's easier to just keep that existing warning about "how
did you get a non-canonical address here" for other user accesses, and
just make get/put_user() use that _ASM_EXTABLE() version that doesn't
do it.

               Linus