Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

[Vlastimil Babka]

On 12/5/24 17:05, Zi Yan wrote:
> On 5 Dec 2024, at 3:10, David Hildenbrand wrote:
>>>
>>> Kernel log confirms it's enabled:
>>> -mem auto-init: stack:off, heap alloc:off, heap free:off
>>> +mem auto-init: stack:off, heap alloc:on, heap free:off
>>
>> If I'm not wrong that's expected ... because we'll be double-zeroing that memory, clearing the cache :)
>>
>> I guess the question is, how *effective* is CONFIG_INIT_ON_ALLOC_DEFAULT_ON on systems to prevent exposing un-zeroed data to userspace, when it doesn't end up doing the flush we really need.
> 
> Hi Geert,
> 
> Is it possible to run a 32bit kernel with HIGHMEM and
> CONFIG_INIT_ON_ALLOC_DEFAULT_ON on the machine (of course with my patch
> reverted)? Just to check my reasoning below.
> 
> Thanks.
> 
> 
> Yes, it should work, since I forgot the actual issue is HIGHMEM+cache flush, not just cache flush is needed after clearing user page.
> 
> For arch which needs to flush cache after clearing user page, with HIGHMEM,
> init_on_alloc first clears the page using kmap_addr0 without flushing
> the cache, then clear_user_page() clears the page using kmap_addr1
> with cache flush. After returning to userspace, the cache lines of
> kmap_addr0 will be evicted and written back to RAM eventually, corrupting
> user data with 0s, because no one flushes them before returning to userspace.
> 
> For a proper fix, I will add ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE and
> make mips, sh, sparc, arm, xtensa, nios2, m68k, parisc, csky, arc, and powerpc
> select it, then make alloc_zeroed() returns false if
> ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE is enabled.
> 
> If my reasoning above is verified to be true, I will send a separate patch
> to disable CONFIG_INIT_ON_ALLOC_DEFAULT_ON if HIGHMEM &&
> ARCH_HAS_OPS_AFTER_CLEAR_USER_PAGE.

If your reasoning is true, wouldn't any other user of kmap_local_page() of a
highpage on such system also leave the cache unflushed in case the page is
ever reused as a userspace page?

> Best Regards,
> Yan, Zi