On Tue, 10 Dec 2013, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:19 PM, Linus Torvalds
> <torva...@linux-foundation.org> wrote:
> >
> > Shouldn't we do something like the attached?
>
> So I think that kernel/futex.c part of the patch might be a good idea,
> but on x86-64 (which is what Dave is running), the
>
> if (end >> __VIRTUAL_MASK_SHIFT)
>
> test in get_user_pages_fast() should have been equivalent. And even on
> 32-bit, we do check the _PAGE_USER bits in the page tables, so I guess
> it's all good on a get_user_pages_fast() side.
>
> So never mind. It's not the address checking.
>
> And I think I see what's up.
>
> I think what happens is:
> - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only)
> - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page
> - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only).
>
> so what triggers this is likely that Dave now does large-pages, and
> one of them is a read-only mapping.
>
> So I would suggest replacing the second "1" in the
> __get_user_pages_fast() call with a "!ro" instead. So how about this
> second patch instead (the access_ok() move remains).
Yes, that's what I decoded as well.
But how does the access_ok() move do anything helpful here?
We really need it for the fastpath !fshared case, but for the fshared
case you actively break working code, because you force a VERIFY_WRITE
check into it. The VERIFY_WRITE is necessary for !fshared, because
there is no way that one thread can map the futex RO and the other RW,
right?
But for fshared it's legitimate to have a RO mapping if you just wait
for the futex. Note, that futexes are (ab)used as user space
waitqueues, so RO makes sense. And your move would break them.
If [__]get_user_pages_fast() does not do the right checks, then we
need to fix that and not add random access_ok() checks into the call
sites.
Thanks,
tglx
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
