linux-kernel - Re: [PATCH PTI v2 3/6] x86/vsyscall/64: Explicitly set _PAGE

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Message-ID: <20171211133945.pmsgwfj2b5py44gj@gmail.com>
Date:   Mon, 11 Dec 2017 14:39:45 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH PTI v2 3/6] x86/vsyscall/64: Explicitly set _PAGE_USER in
 the pagetable hierarchy

* Andy Lutomirski <luto@...nel.org> wrote:

> The kernel is very erratic as to which pagetables have _PAGE_USER
> set.  The vsyscall page gets lucky: it seems that all of the
> relevant pagetables are among the apparently arbitrary ones that set
> _PAGE_USER.  Rather than relying on chance, just explicitly set
> _PAGE_USER.
> 
> This will let us clean up pagetable setup to stop setting
> _PAGE_USER.  The added code can also be reused by pagetable
> isolation to manage the _PAGE_USER bit in the usermode tables.
> 
> Signed-off-by: Andy Lutomirski <luto@...nel.org>
> ---
>  arch/x86/entry/vsyscall/vsyscall_64.c | 33 ++++++++++++++++++++++++++++++++-
>  1 file changed, 32 insertions(+), 1 deletion(-)

Btw., would it make sense to clean up all this confusion?

In particular a 'KERNEL' pre of post fix is ambiguous in this context I think, and 
the PAGE_KERNEL_ prefix is actively harmful I think and is at the root of the 
confusion.

So if renamed it and used this nomenclature consistently instead:

  PAGE_USER_
  PAGE_SYSTEM_

... and got rid of PAGE_KERNEL uses in arch/x86/, then it would be obvious at 
first glance what kind of mapping is established in a particular place - and it 
would stay so in the future as well.

( There's some interaction with generic MM code which needs the original defines 
  like PAGE_KERNEL[_EXEC], but those generic masks could be defined as aliases, to 
  keep this cleanup within x86 for now. )

Thanks,

	Ingo