linux-kernel - Re: [PATCH] x86: uaccess: fix regression in unsafe_get

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Message-Id: <FBED20DC-1F47-4038-81CC-8F5119785F8E@amacapital.net>
Date:   Sat, 16 Feb 2019 18:36:52 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Jann Horn <jannh@...gle.com>,
        baloo@...di.net, the arch/x86 maintainers <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        kernel list <linux-kernel@...r.kernel.org>,
        Pascal Bouchareine <pascal@...di.net>
Subject: Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user



> On Feb 16, 2019, at 3:47 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> 
>> On Sat, Feb 16, 2019 at 02:50:15PM -0800, Andy Lutomirski wrote:
>> 
>> What is the actual problem?  We’re not actually demand-faulting this data, are we?  Are we just overrunning the buffer because the from_user helpers are too clever?  Can we fix it for real by having the fancy helpers do *aligned* loads so that they don’t overrun the buffer?  Heck, this might be faster, too.
> 
> Unaligned _stores_ are not any cheaper, and you'd get one hell of
> extra arithmetics from trying to avoid both.  Check something
> like e.g. memcpy() on alpha, where you really have to keep all
> accesses aligned, both on load and on store side.

I think we should avoid unaligned loads and do unaligned stores instead.

I would general expect that unaligned stores are a bit cheaper since they don’t need to complete for the comparisons to happen.

But I maintain my claim that this code should not be overrunning its input buffer into the next page, since it could have observable side effects.

> 
> Can't we just pad the buffers a bit?  Making sure that name_buf
> and symlink_buf are _not_ followed by unmapped pages shouldn't
> be hard.  Both are allocated by kmalloc(), so...
> 
> What am I missing here?