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Fri, 01 Aug 2025 10:13:55 -0700 (PDT) X-Google-Smtp-Source: AGHT+IG8+K6i2QRol52KGBLA9S6CAT2dneVBlknXiG0f/LH2d8sJO8oI3hxjPNdTh/rQMWdVl3tj8g== X-Received: by 2002:a05:622a:a953:20b0:4af:36a:a60f with SMTP id d75a77b69052e-4af036aa957mr25037891cf.21.1754068434728; Fri, 01 Aug 2025 10:13:54 -0700 (PDT) Received: from x1.local ([174.89.135.171]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4aeeebde17fsm22954801cf.1.2025.08.01.10.13.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Aug 2025 10:13:54 -0700 (PDT) Date: Fri, 1 Aug 2025 13:13:42 -0400 From: Peter Xu To: Suren Baghdasaryan Cc: David Hildenbrand , akpm@linux-foundation.org, aarcange@redhat.com, lokeshgidra@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com, stable@vger.kernel.org Subject: Re: [PATCH v2 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole Message-ID: References: <20250731154442.319568-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Aug 01, 2025 at 09:41:31AM -0700, Suren Baghdasaryan wrote: > On Fri, Aug 1, 2025 at 9:23 AM Peter Xu wrote: > > > > On Fri, Aug 01, 2025 at 08:28:38AM -0700, Suren Baghdasaryan wrote: > > > On Fri, Aug 1, 2025 at 7:16 AM Peter Xu wrote: > > > > > > > > On Fri, Aug 01, 2025 at 09:21:30AM +0200, David Hildenbrand wrote: > > > > > On 31.07.25 17:44, Suren Baghdasaryan wrote: > > > > > > > > > > Hi! > > > > > > > > > > Did you mean in you patch description: > > > > > > > > > > "userfaultfd: fix a crash in UFFDIO_MOVE with some non-present PMDs" > > > > > > > > > > Talking about THP holes is very very confusing. > > > > > > > > > > > When UFFDIO_MOVE is used with UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES and it > > > > > > encounters a non-present THP, it fails to properly recognize an unmapped > > > > > > > > > > You mean a "non-present PMD that is not a migration entry". > > > > > > > > > > > hole and tries to access a non-existent folio, resulting in > > > > > > a crash. Add a check to skip non-present THPs. > > > > > > > > > > That makes sense. The code we have after this patch is rather complicated > > > > > and hard to read. > > > > > > > > > > > > > > > > > Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") > > > > > > Reported-by: syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com > > > > > > Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/ > > > > > > Signed-off-by: Suren Baghdasaryan > > > > > > Cc: stable@vger.kernel.org > > > > > > --- > > > > > > Changes since v1 [1] > > > > > > - Fixed step size calculation, per Lokesh Gidra > > > > > > - Added missing check for UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES, per Lokesh Gidra > > > > > > > > > > > > [1] https://lore.kernel.org/all/20250730170733.3829267-1-surenb@google.com/ > > > > > > > > > > > > mm/userfaultfd.c | 45 +++++++++++++++++++++++++++++---------------- > > > > > > 1 file changed, 29 insertions(+), 16 deletions(-) > > > > > > > > > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > > > > > index cbed91b09640..b5af31c22731 100644 > > > > > > --- a/mm/userfaultfd.c > > > > > > +++ b/mm/userfaultfd.c > > > > > > @@ -1818,28 +1818,41 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, > > > > > > ptl = pmd_trans_huge_lock(src_pmd, src_vma); > > > > > > if (ptl) { > > > > > > - /* Check if we can move the pmd without splitting it. */ > > > > > > - if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) || > > > > > > - !pmd_none(dst_pmdval)) { > > > > > > - struct folio *folio = pmd_folio(*src_pmd); > > > > > > + if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) { > > > > > > > > [1] > > > > > > > > > > + /* Check if we can move the pmd without splitting it. */ > > > > > > + if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) || > > > > > > + !pmd_none(dst_pmdval)) { > > > > > > + if (pmd_present(*src_pmd)) { > > > > [2] > > > > > > > > + struct folio *folio = pmd_folio(*src_pmd); > > > > [3] > > > > > > > > + > > > > > > + if (!folio || (!is_huge_zero_folio(folio) && > > > > > > + !PageAnonExclusive(&folio->page))) { > > > > > > + spin_unlock(ptl); > > > > > > + err = -EBUSY; > > > > > > + break; > > > > > > + } > > > > > > + } > > > > > > > > > > ... in particular that. Is there some way to make this code simpler / easier > > > > > to read? Like moving that whole last folio-check thingy into a helper? > > > > > > > > One question might be relevant is, whether the check above [1] can be > > > > dropped. > > > > > > > > The thing is __pmd_trans_huge_lock() does double check the pmd to be !none > > > > before returning the ptl. I didn't follow closely on the recent changes on > > > > mm side on possible new pmd swap entries, if migration is the only possible > > > > one then it looks like [1] can be avoided. > > > > > > Hi Peter, > > > is_swap_pmd() check in __pmd_trans_huge_lock() allows for (!pmd_none() > > > && !pmd_present()) PMD to pass and that's when this crash is hit. > > > > First for all, thanks for looking into the issue with Lokesh; I am still > > catching up with emails after taking weeks off. > > > > I didn't yet read into the syzbot report, but I thought the bug was about > > referencing the folio on top of a swap entry after reading your current > > patch, which has: > > > > if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) || > > !pmd_none(dst_pmdval)) { > > struct folio *folio = pmd_folio(*src_pmd); <---- > > > > Here looks like *src_pmd can be a migration entry. Is my understanding > > correct? > > Correct. > > > > > > If we drop the check at [1] then the path that takes us to > > > > If my above understanding is correct, IMHO it should be [2] above that > > makes sure the reference won't happen on a swap entry, not necessarily [1]? > > Yes, in case of migration entry this is what protects us. > > > > > > split_huge_pmd() will bail out inside split_huge_pmd_locked() with no > > > indication that split did not happen. Afterwards we will retry > > > > So we're talking about the case where it's a swap pmd entry, right? > > Hmm, my understanding is that it's being treated as a swap entry but > in reality is not. I thought THPs are always split before they get > swapped, no? Yes they should be split, afaiu. > > > Could you elaborate why the split would fail? > > Just looking at the code, split_huge_pmd_locked() checks for > (pmd_trans_huge(*pmd) || is_pmd_migration_entry(*pmd)). > pmd_trans_huge() is false if !pmd_present() and it's not a migration > entry, so __split_huge_pmd_locked() will be skipped. Here might be the major part of where confusion came from: I thought it must be a migration pmd entry to hit the issue, so it's not? I checked the code just now: __handle_mm_fault: if (unlikely(is_swap_pmd(vmf.orig_pmd))) { VM_BUG_ON(thp_migration_supported() && !is_pmd_migration_entry(vmf.orig_pmd)); So IIUC pmd migration entry is still the only possible way to have a swap entry. It doesn't look like we have "real" swap entries for PMD (which can further points to some swapfiles)? -- Peter Xu