linux-kernel - Re: [PATCH -next] x86/efi_64: fix a user-memory-access in runtime

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Date:   Sat, 18 Jan 2020 14:34:55 +0100
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Qian Cai <cai@....pw>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] x86/efi_64: fix a user-memory-access in runtime

On Sat, 18 Jan 2020 at 12:04, Qian Cai <cai@....pw> wrote:
>
>
>
> > On Jan 18, 2020, at 3:00 AM, Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> >
> > Can't we just use READ_ONCE_NOCHECK() instead?
>
> My understanding is that KASAN actually want to make sure there is a no dereference of user memory because it has security implications. Does that make no sense here?

Not really. This code runs extremely early in the boot, with a
temporary 1:1 memory mapping installed so that the EFI firmware can
transition into virtually remapped mode.

Furthermore, the same issue exists for mixed mode, so we'll need to
fix that as well. I'll spin a patch and credit you as the reporter.