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Date:   Sun, 17 Feb 2019 00:02:37 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>, Jann Horn <jannh@google.com>,
        baloo@gandi.net, the arch/x86 maintainers <x86@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        kernel list <linux-kernel@vger.kernel.org>,
        Pascal Bouchareine <pascal@gandi.net>
Subject: Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user
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On Sat, Feb 16, 2019 at 11:47:02PM +0000, Al Viro 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.
> 
> 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?

... the fact that read_info() might skip copying if everything it
wants is already in the buffer passed by decompressor ;-/  It's
been a while since I looked into that code...