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m7M1Rm8yNP3G0IcOMMkHsG/1oOyXlEaPi0ENtSG2u6bepOe9UepIyzMEnPA1SSoGUDEMCd19V0PF2CPkA526Tz/rRWN1eMK6O5Zcm59cykjA79nD1UkX6sIW+jizc24bTY1poFvL3lujl9zu5qgPnyn7KqclykfJ3Uf62qtW1105r4TgYRKBn0fkw6kS8NnhwyPTuM5BLMnMz9jcBmR3YLwJbHkaJLtt+Dca/+PisKMwTtnsHYy8HRdojSrv6UkVGBnVAhJC4rQuuUjuzWGk1KuQoMPx35UGOzdVU6OhpaPfqz6nCeHVVxIGyVI/HYn04vK5A3C9OmBVKxWY+LzpORpyZjioDSTCde/t8ZoAX/73Sq04FlnB9r5vXWfSve6gbTtEC2lmLkF38d93GuvtJ0M3FUQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5 Dec 2024, at 3:10, David Hildenbrand wrote: > On 05.12.24 09:04, Geert Uytterhoeven wrote: >> Hi Zi, >> >> On Wed, Dec 4, 2024 at 7:30=E2=80=AFPM Zi Yan wrote: >>> On 4 Dec 2024, at 12:33, Zi Yan wrote: >>>> On 4 Dec 2024, at 11:29, Matthew Wilcox wrote: >>>>> On Wed, Dec 04, 2024 at 11:16:51AM -0500, Zi Yan wrote: >>>>>>> So maybe the clearing done as part of page allocator isn't enough h= ere. >>>>>>> >>>>>> Basically, mips needs to flush data cache if kmap address is aliased= to >>>>> >>>>> People use "aliased" in contronym ways. Do you mean "has a >>>>> non-congruent alias" or "has a congruent alias"? >>>>> >>>>>> userspace address. This means when mips has THP on, the patch below >>>>>> is not enough to fix the issue. >>>>>> >>>>>> In post_alloc_hook(), it does not make sense to pass userspace addre= ss >>>>>> in to determine whether to flush dcache or not. >>>>>> >>>>>> One way to fix it is to add something like arch_userpage_post_alloc(= ) >>>>>> to flush dcache if kmap address is aliased to userspace address. >>>>>> But my questions are that >>>>>> 1) if kmap address will always be the same for two separate kmap_loc= al() calls, >>>>> >>>>> No. It just takes the next address in the stack. >>>> >>>> Hmm, if kmap_local() gives different addresses, wouldn=E2=80=99t init_= on_alloc be >>>> causing issues before my patch? In the page allocator, the page is zer= oed >>>> from one kmap address without flush, then clear_user_highpage() clears >>>> it again with another kmap address with flush. After returning to user= space, >>>> the user application works on the page but when the cache line used by >>>> init_on_alloc is written back (with 0s) at eviction, user data is corr= upted. >>>> Am I missing anything? Or all arch with cache aliasing never enables >>>> init_on_alloc? >>> >>> Hi Geert, >>> >>> Regarding the above concern, have you ever had CONFIG_INIT_ON_ALLOC_DEF= AULT_ON >>> for your MIPS machine and encountered any issue? Or let me know if my r= easoning >>> above is flawed. >>> >>> To test it, I wonder if you can 1) revert my patch and 2) turn on >>> CONFIG_INIT_ON_ALLOC_DEFAULT_ON for your MIPS machine and run some appl= ications >>> to see if any error happens. >> >> That seems to work fine... >> >> 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 does= n'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 userspac= e. 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 powe= rpc 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. Best Regards, Yan, Zi