Re: [PATCH 26/26] x86, pkeys: Documentation

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, Oct 1, 2015 at 6:56 PM, Dave Hansen <dave@sr71.net> wrote:
>
> Also, a quick ftrace showed that most mmap() callers that set PROT_EXEC
> also set PROT_READ.  I'm just assuming that folks are setting PROT_READ
> but aren't _really_ going to read it, so we can safely deny them all
> access other than exec.

That's a completely insane assumption. There are tons of reasons to
have code and read-only data in the same segment, and it's very
traditional. Just assuming that you only execute out of something that
has PROT_EXEC | PROT_READ is insane.

No, what you *should* look at is to use the protection keys to
actually enforce a plain PROT_EXEC. That has never worked before
(because traditionally R implies X, and then we got NX).

That would at least allow people who know they don't intersperse
read-only constants in the code to use PROT_EXE only.

Of course, there may well be users who use PROT_EXE that actually *do*
do reads, and just relied on the old hardware behavior. So it's not
guaranteed to work either without any extra flags. But at least it's
worth a try, unlike the "yeah, the user asked for read, but the user
doesn't know what he's doing" thinking that is just crazy talk.

           Linus

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