Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

[Linus Torvalds]

On Fri, Apr 24, 2015 at 9:10 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
> Even though I've been accused (correctly?) of suggesting that, I'm not
> sure I like it anymore.  Suppose I map some anonymous memory, learn
> its (scrambled) pfn, then unmap it and remap a setuid file.  Now I can
> tell whether I've mapped the setuid file at the same pfn that was
> mapped as my anonymous memory.  IIRC that's sufficient for one of the
> variants of Mark's attack.

Ack. So we really do have to zero out the pfn entirely for security
reasons, and not just because it's less effort.

                         Linus
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