On Thu, Apr 21, 2022 at 10:41:43AM -0700, Kees Cook wrote:

On Thu, Apr 21, 2022 at 06:24:21PM +0100, Catalin Marinas wrote:

quoted

On Thu, Apr 21, 2022 at 09:42:23AM -0700, Kees Cook wrote:

quoted

On Thu, Apr 21, 2022 at 04:35:15PM +0100, Catalin Marinas wrote:

quoted

Do we want the "was PROT_WRITE" or we just reject mprotect(PROT_EXEC) if
the vma is not already PROT_EXEC? The latter is closer to the current
systemd approach. The former allows an mprotect(PROT_EXEC) if the
mapping was PROT_READ only for example.

I'd drop the "was PROT_WRITE" for now if the aim is a drop-in
replacement for BPF MDWE.

I think "was PROT_WRITE" is an important part of the defense that
couldn't be done with a simple seccomp filter (which is why the filter
ended up being a problem in the first place).

I would say "was PROT_WRITE" is slightly more relaxed than "is not
already PROT_EXEC". The seccomp filter can't do "is not already
PROT_EXEC" either since it only checks the mprotect() arguments, not the
current vma flags.

So we have (with sub-cases):

1. Current BPF filter:

   a)	mmap(PROT_READ|PROT_WRITE|PROT_EXEC);	// fails

   b)	mmap(PROT_READ|PROT_EXEC);
	mprotect(PROT_READ|PROT_EXEC|PROT_BTI);	// fails

   c)	mmap(PROT_READ);
	mprotect(PROT_READ|PROT_EXEC);		// fails

   d)	mmap(PROT_READ|PROT_WRITE);
	mprotect(PROT_READ);
	mprotect(PROT_READ|PROT_EXEC);		// fails

2. "is not already PROT_EXEC":

   a)	mmap(PROT_READ|PROT_WRITE|PROT_EXEC);	// fails

   b)	mmap(PROT_READ|PROT_EXEC);
	mprotect(PROT_READ|PROT_EXEC|PROT_BTI);	// passes

   c)	mmap(PROT_READ);
	mprotect(PROT_READ|PROT_EXEC);		// fails

   d)	mmap(PROT_READ|PROT_WRITE);
	mprotect(PROT_READ);
	mprotect(PROT_READ|PROT_EXEC);		// fails

3. "is or was not PROT_WRITE":

   a)	mmap(PROT_READ|PROT_WRITE|PROT_EXEC);	// fails

   b)	mmap(PROT_READ|PROT_EXEC);
	mprotect(PROT_READ|PROT_EXEC|PROT_BTI);	// passes

   c)	mmap(PROT_READ);
	mprotect(PROT_READ|PROT_EXEC);		// passes

   d)	mmap(PROT_READ|PROT_WRITE);
	mprotect(PROT_READ);
 	mprotect(PROT_READ|PROT_EXEC);		// fails

[edited above to show each case]

Thanks, I was in a rush to get home ;).

restated what was already summarized:
Problem is 1.b. 2 and 3 solve it. 3 is more relaxed (c passes).

quoted

If we don't care about 3.c, we might as well go for (2). I don't mind,
already went for (3) in this series. I think either of them would not be
a regression on MDWE, unless there is some test that attempts 3.c and
expects it to fail.

I should stop arguing for a less restrictive mode. ;) It just feels weird
that the combinations are API-mediated, rather than logically defined:
I can do PROT_READ|PROT_EXEC with mmap but not mprotect under 2. As
opposed to saying "the vma cannot be executable if it is or ever was
writable". I find the latter much easier to reason about as far as the
expectations of system state.

I had the same reasoning, hence option 3 in this series. I prefer to
treat mmap(PROT_READ|PROT_EXEC) and mprotect(PROT_READ|PROT_EXEC) in a
similar way.

-- 
Catalin

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