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Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Andrew Morton , Rik van Riel , Yuan-Hao Hsu , stable@vger.kernel.org Subject: Re: [PATCH v3 1/6] mm: khugepaged: ignore MADV_HUGEPAGE for shadow stacks Message-ID: References: <20261109-refactor_gup_fast-v3-0-2aaef02df0e1@arm.com> <20261109-refactor_gup_fast-v3-1-2aaef02df0e1@arm.com> <3a2d4135-f531-4797-8dd2-a2732a846e45@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <3a2d4135-f531-4797-8dd2-a2732a846e45@kernel.org> X-Rspamd-Server: rspam06 X-Stat-Signature: 444ayzghpu871d3p416nscei7w796du1 X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Queue-Id: 9CA151A0002 X-Rspam-User: X-HE-Tag: 1794237875-695957 X-HE-Meta: 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 ATZ1ELpu SPRCbUStSAgFa5OvBaOdApMysidPLNkkjIumn22whYT4xbv6Pv6p9BpwoWDFeqJxMcLsdrPmGH9rHXtmq4zgPhzFA3rdlZng6bin7ArC7rAma/164EP3YzDrf2aQpXmXyVegHe4XeiDJ050UeHQWafAnny7yEufPT+IOXvW7Y9IN7GjuhYHAJ5YuYhiH8Nh3Bw+fmkXobSjtrA+TZTb86Nlq31aLq58uIb9jnThfhSFOxhJw+Ma8IpZlOxNO0I9UTOn1J7sefOhTI/li+HWZa+MplQHIpPERqlnhOnGIcC78DwQNo+zCO/tdXqeD7V+IjeduHlrwJCZdOKsFfgEf3LqddjNYCzGCLjSqnLlzRESuJ76Y= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 > On 11/9/26 13:32, Yeoreum Yun wrote: > > commit 3efb9800557b ("mm: do not map the shadow stack as THP") makes > > shadow stack mappings default to VM_NOHUGEPAGE. However, userspace can > > still clear this flag and set VM_HUGEPAGE through madvise(). > > > > Allowing large folios in x86 shadow stacks exposes a limitation in > > folio_pte_batch_flags(). For example: > > > > 1. create a shadow stack mapping. > > 2. apply MADV_HUGEPAGE and populate the mapping with a PTE-mapped > > large folio. > > 3. apply MADV_DONTFORK to the first base page of that folio. > > 4. call fork(). > > > > In the parent, the first page remains writable and anonymous-exclusive, > > while the remaining pages shared with the child become write-protected > > and lose PageAnonExclusive(). These pages can still belong to the same > > large folio. > > > > On x86, writable shadow-stack PTEs have _PAGE_RW clear and _PAGE_DIRTY > > set. Write-protected PTEs have both bits clear, with the dirty state > > preserved in _PAGE_SAVED_DIRTY. > > > > When FPB_RESPECT_WRITE is set without FPB_RESPECT_DIRTY, > > folio_pte_batch_flags() normalizes PTEs using pte_mkclean(), which clears > > both dirty bits. This loses the distinction between writable > > shadow-stack PTEs and write-protected PTEs, allowing them to be grouped > > into the same batch. GUP-fast can encounter such a batch when walking > > across the VMA boundary created by MADV_DONTFORK. > > > > Since there is no practical use-case shadow stack with large folio, > > ignore MADV_HUGEPAGE for shadow stack mappings, preserving their > > VM_NOHUGEPAGE setting and preventing large-folio allocation. > > > > Fixes: 3efb9800557b ("mm: do not map the shadow stack as THP") > > Suggested-by: David Hildenbrand (Arm) > > Signed-off-by: Yeoreum Yun > > Cc: > > --- > > mm/khugepaged.c | 3 +++ > > 1 file changed, 3 insertions(+) > > > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > > index 40fad28c52f6f..51ec043fa26c3 100644 > > --- a/mm/khugepaged.c > > +++ b/mm/khugepaged.c > > @@ -367,6 +367,9 @@ int hugepage_madvise(struct vm_area_struct *vma, > > { > > switch (advice) { > > case MADV_HUGEPAGE: > > + /* PTE batching doesn't work reliably. */ > > + if (vma_test_single_mask(vma, VMA_SHADOW_STACK)) > > + break; > > *vm_flags &= ~VM_NOHUGEPAGE; > > *vm_flags |= VM_HUGEPAGE; > > break; > > > > Ack, I prefer this solution for now until we know how to make pte_mkclean() on > x86 shadow stack PTEs not result in an implicit pte_wrprotect(). Yeap. As we discussed in offline, just skip clearing dirty bit for shstk in pte_mkclear() wouldn't work since it would make a another regression -- wouldn't be page-outed. Might it would be good to use another soft-bit for shstk in x86. But, for now, it seems enough as you said. > > Acked-by: David Hildenbrand (Arm) Thanks! -- Sincerely, Yeoreum Yun