On 8/8/20 5:17 PM, Pavel Machek wrote:

Hi!

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Thanks for the lively discussion. I have tried to answer some of the
comments below.

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There are options today, e.g.

a) If the restriction is only per-alias, you can have distinct aliases
   where one is writable and another is executable, and you can make it
   hard to find the relationship between the two.

b) If the restriction is only temporal, you can write instructions into
   an RW- buffer, transition the buffer to R--, verify the buffer
   contents, then transition it to --X.

c) You can have two processes A and B where A generates instrucitons into
   a buffer that (only) B can execute (where B may be restricted from
   making syscalls like write, mprotect, etc).

The general principle of the mitigation is W^X. I would argue that
the above options are violations of the W^X principle. If they are
allowed today, they must be fixed. And they will be. So, we cannot
rely on them.

Would you mind describing your threat model?

Because I believe you are using model different from everyone else.

In particular, I don't believe b) is a problem or should be fixed.

It is a problem because a kernel that implements W^X properly
will not allow it. It has no idea what has been done in userland.
It has no idea that the user has checked and verified the buffer
contents after transitioning the page to R--.

I'll add d), application mmaps a file(R--), and uses write syscall to change
trampolines in it.

No matter how you do it, these are all user-level methods that can be
hacked. The kernel cannot be sure that an attacker's code has
not found its way into the file.

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b) This is again a violation. The kernel should refuse to give execute
???????? permission to a page that was writeable in the past and refuse to
???????? give write permission to a page that was executable in the past.

Why?

I don't know about the latter part. I guess I need to think about it.
But the former is valid. When a page is RW-, a hacker could hack the
page. Then it does not matter that the page is transitioned to R--.
Again, the kernel cannot be sure that the user has verified the contents
after R--.

IMO, W^X needs to be enforced temporally as well.

Madhavan