On 3/23/23 11:08, David Hildenbrand wrote:
> On 23.03.23 10:52, Vlastimil Babka wrote:
>> On 2/3/22 19:26, Yang Shi wrote:
>>> --- a/fs/proc/task_mmu.c
>>> +++ b/fs/proc/task_mmu.c
>>> @@ -440,7 +440,8 @@ static void smaps_page_accumulate(struct mem_size_stats *mss,
>>> }
>>>
>>> static void smaps_account(struct mem_size_stats *mss, struct page *page,
>>> - bool compound, bool young, bool dirty, bool locked)
>>> + bool compound, bool young, bool dirty, bool locked,
>>> + bool migration)
>>> {
>>> int i, nr = compound ? compound_nr(page) : 1;
>>> unsigned long size = nr * PAGE_SIZE;
>>> @@ -467,8 +468,15 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>>> * page_count(page) == 1 guarantees the page is mapped exactly once.
>>> * If any subpage of the compound page mapped with PTE it would elevate
>>> * page_count().
>>> + *
>>> + * The page_mapcount() is called to get a snapshot of the mapcount.
>>> + * Without holding the page lock this snapshot can be slightly wrong as
>>> + * we cannot always read the mapcount atomically. It is not safe to
>>> + * call page_mapcount() even with PTL held if the page is not mapped,
>>> + * especially for migration entries. Treat regular migration entries
>>> + * as mapcount == 1.
>>> */
>>> - if (page_count(page) == 1) {
>>> + if ((page_count(page) == 1) || migration) {
>>
>> Since this is now apparently a CVE-2023-1582 for whatever RHeasons...
>>
>> wonder if the patch actually works as intended when
>> (page_count() || migration) is in this particular order and not the other one?
>
> Only the page_mapcount() call to a page that should be problematic, not
> the page_count() call. There might be the rare chance of the page
Oh right, page_mapcount() vs page_count(), I need more coffee.
> getting remove due to memory offlining... but we're still holding the
> page table lock with the migration entry, so we should be protected
> against that.
>
> Regarding the CVE, IIUC the main reason for the CVE should be
> RHEL-specific -- which behaves differently than other code bases; for
> other code bases, it's just a way to trigger a BUG_ON as described here.
That's good to know so at least my bogus mail was useful for that, thanks!
