On Thu, Nov 10, 2022 at 11:27:14AM +0000, Joey Gouly wrote:

On Fri, Oct 28, 2022 at 11:51:00AM -0700, Kees Cook wrote:

quoted

On Wed, Oct 26, 2022 at 04:04:56PM +0100, Joey Gouly wrote:

quoted

diff --git a/mm/mmap.c b/mm/mmap.c
index 099468aee4d8..42eaf6683216 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c

@@ -1409,6 +1409,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			vm_flags |= VM_NORESERVE;
 	}
 
+	if (map_deny_write_exec(NULL, vm_flags))
+		return -EACCES;
+

This seems like the wrong place to do the check -- that the vma argument
is a hard-coded "NULL" is evidence that something is wrong. Shouldn't
it live in mmap_region()? What happens with MAP_FIXED, when there is
an underlying vma? i.e. an MAP_FIXED will, I think, bypass the intended
check. For example, we had "c" above:

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

But this would allow another case:

     e)	addr = mmap(..., PROT_READ, ...);
	mmap(addr, ..., PROT_READ | PROT_EXEC, MAP_FIXED, ...);	// passes

I can move the check into mmap_region() but it won't fix the MAP_FIXED
example that you showed here.

mmap_region() calls do_mas_munmap(..) which will unmap overlapping regions.
However the `vma` for the 'old' region is not kept around, and a new vma will
be allocated later on "vma = vm_area_alloc(mm);", and the vm_flags are just set
to what is passed into mmap_region(), so map_deny_write_exec(vma, vm_flags)
will just be as good as passing NULL.

It's possible to save the vm_flags from the region that is unmapped, but Catalin
suggested it might be better if that is part of a later extension, what do you
think? 

I thought initially we should keep the behaviour close to what systemd
achieves via SECCOMP while only relaxing an mprotect(PROT_EXEC) if the
vma is already executable (i.e. check actual permission change not just
the PROT_* flags).

We could pass the old vm_flags for that region (and maybe drop the vma
pointer entirely, just check old and new vm_flags). But this feels like
tightening slightly systemd's MDWE approach. If user-space doesn't get
confused by this, I'm fine to go with it. Otherwise we can add a new
flag later for this behaviour

I guess that's more of a question for Topi on whether point tightening
point (e) is feasible/desirable.

-- 
Catalin

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