Re: 2.6.23 regression: accessing invalid mmap'ed memory from gdb causes unkillable spinning

Linus Torvalds
2007-11-01T15:15:52Z

On Thu, 1 Nov 2007, Nick Piggin wrote:

> On Wed, Oct 31, 2007 at 04:08:21PM -0700, Linus Torvalds wrote:
> >
> > We made much bigger changes to ptrace support when we disallowed writing
> > to read-only shared memory areas (we used to do the magic per-page COW
> > thing).
>
> Really? No, we still do that magic COW thing which creates anonymous
> pages in MAP_SHARED vmas, don't we?

No, we don't. I'm pretty sure. It didn't work with the VM cleanups, since
the MAP_SHARED vma's won't be on the anonymous list any more, and cannot
be swapped out.

So now, if you try to write to a read-only shared page through ptrace,
you'll get "Unable to access".

Of course, I didn't really look closely, so maybe I just don't remember
things right..

> > access_vm_pages() (things like core-dumping comes to mind - although I
> > think we don't dump pure file mappings at all, do we?) it would certainly
> > be good to run any such tests on the current -git tree...
>
> We do for MAP_SHARED|MAP_ANONYMOUS, by the looks.

Well, as we should. There's no way for a debugger to get those pages back.
So that all looks sane.

> - vm_flags |= VM_SHARED | VM_MAYSHARE;
> - if (!(file->f_mode & FMODE_WRITE))
> - vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
> + vm_flags |= VM_MAYSHARE;
> + if (file->f_mode & FMODE_WRITE)
> + vm_flags |= VM_SHARED;
> + if (!(vm_flags & VM_WRITE))
> + vm_flags &= ~VM_MAYWRITE;

This looks totally bogus. What was the intent of this patch?

The VM_MAYWRITE flag is *not* supposed to track the VM_WRITE flag: that
would defeat the whole purpose of it! The whole point of that flag is to
say whether mprotect() could turn it into a VM_WRITE mapping, and it
depends on the file mode, not VM_WRITE!

> vm_flags |= VM_SHARED | VM_MAYSHARE;
> + if (!(vm_flags & VM_WRITE))
> + vm_flags &= ~VM_MAYWRITE;

More apparent bogosities.

Linus