On 4/4/19 1:56 AM, Peter Zijlstra wrote:
> On Wed, Apr 03, 2019 at 11:34:12AM -0600, Khalid Aziz wrote:
>> From: Julian Stecklina <jsteckli@xxxxxxxxx>
>>
>> Only the xpfo_kunmap call that needs to actually unmap the page
>> needs to be serialized. We need to be careful to handle the case,
>> where after the atomic decrement of the mapcount, a xpfo_kmap
>> increased the mapcount again. In this case, we can safely skip
>> modifying the page table.
>>
>> Model-checked with up to 4 concurrent callers with Spin.
>>
>> Signed-off-by: Julian Stecklina <jsteckli@xxxxxxxxx>
>> Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
>> Cc: Khalid Aziz <khalid@xxxxxxxxxxxxxx>
>> Cc: x86@xxxxxxxxxx
>> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx
>> Cc: Vasileios P. Kemerlis <vpk@xxxxxxxxxxxxxxx>
>> Cc: Juerg Haefliger <juerg.haefliger@xxxxxxxxxxxxx>
>> Cc: Tycho Andersen <tycho@xxxxxxxx>
>> Cc: Marco Benatto <marco.antonio.780@xxxxxxxxx>
>> Cc: David Woodhouse <dwmw2@xxxxxxxxxxxxx>
>> ---
>> include/linux/xpfo.h | 24 +++++++++++++++---------
>> 1 file changed, 15 insertions(+), 9 deletions(-)
>>
>> diff --git a/include/linux/xpfo.h b/include/linux/xpfo.h
>> index 2318c7eb5fb7..37e7f52fa6ce 100644
>> --- a/include/linux/xpfo.h
>> +++ b/include/linux/xpfo.h
>> @@ -61,6 +61,7 @@ static inline void xpfo_kmap(void *kaddr, struct page *page)
>> static inline void xpfo_kunmap(void *kaddr, struct page *page)
>> {
>> unsigned long flags;
>> + bool flush_tlb = false;
>>
>> if (!static_branch_unlikely(&xpfo_inited))
>> return;
>> @@ -72,18 +73,23 @@ static inline void xpfo_kunmap(void *kaddr, struct page *page)
>> * The page is to be allocated back to user space, so unmap it from
>> * the kernel, flush the TLB and tag it as a user page.
>> */
>> - spin_lock_irqsave(&page->xpfo_lock, flags);
>> -
>> if (atomic_dec_return(&page->xpfo_mapcount) == 0) {
>> -#ifdef CONFIG_XPFO_DEBUG
>> - WARN_ON(PageXpfoUnmapped(page));
>> -#endif
>> - SetPageXpfoUnmapped(page);
>> - set_kpte(kaddr, page, __pgprot(0));
>> - xpfo_flush_kernel_tlb(page, 0);
>> + spin_lock_irqsave(&page->xpfo_lock, flags);
>> +
>> + /*
>> + * In the case, where we raced with kmap after the
>> + * atomic_dec_return, we must not nuke the mapping.
>> + */
>> + if (atomic_read(&page->xpfo_mapcount) == 0) {
>> + SetPageXpfoUnmapped(page);
>> + set_kpte(kaddr, page, __pgprot(0));
>> + flush_tlb = true;
>> + }
>> + spin_unlock_irqrestore(&page->xpfo_lock, flags);
>> }
>>
>> - spin_unlock_irqrestore(&page->xpfo_lock, flags);
>> + if (flush_tlb)
>> + xpfo_flush_kernel_tlb(page, 0);
>> }
>
> This doesn't help with the TLB invalidation issue, AFAICT this is still
> completely buggered. kunmap_atomic() can be called from IRQ context.
>
OK. xpfo_kmap/xpfo_kunmap need redesign.
--
Khalid
