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Fri, 01 Aug 2025 17:32:30 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGcyIfCLQARG8ajrAkxw6gecilHP9OwpIaR2rAKxNNqso1ts3GOWxdhyD6yzWs4HO8O8IpDiQ== X-Received: by 2002:a05:622a:424e:b0:4af:23a:4d8f with SMTP id d75a77b69052e-4af1094bb33mr29893491cf.1.1754094749728; Fri, 01 Aug 2025 17:32:29 -0700 (PDT) Received: from x1.local ([174.89.135.171]) by smtp.gmail.com with ESMTPSA id d75a77b69052e-4aeeec005c2sm25861691cf.16.2025.08.01.17.32.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Aug 2025 17:32:28 -0700 (PDT) Date: Fri, 1 Aug 2025 20:32:16 -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 07:30:02PM +0000, Suren Baghdasaryan wrote: > On Fri, Aug 1, 2025 at 6:21 PM Peter Xu wrote: > > > > On Fri, Aug 01, 2025 at 05:45:10PM +0000, Suren Baghdasaryan wrote: > > > On Fri, Aug 1, 2025 at 5:13 PM Peter Xu wrote: > > > > > > > > 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)? > > > > > > Correct. AFAIU here we stumble on a pmd entry which was allocated but > > > never populated. > > > > Do you mean a pmd_none()? > > Yes. > > > > > If so, that goes back to my original question, on why > > __pmd_trans_huge_lock() returns non-NULL if it's a pmd_none()? IMHO it > > really should have returned NULL for pmd_none(). > > That was exactly the answer I gave Lokesh when he theorized about the > cause of this crash but after reproducing it I saw that > pmd_trans_huge_lock() happily returns the PTL as long as PMD is not > pmd_none(). And that's because it passes as is_swap_pmd(). But even if > we change that we still need to implement the code to skip the entire > PMD. The thing is I thought if pmd_trans_huge_lock() can return non-NULL, it must be either a migration entry or a present THP. So are you describing a THP but with present bit cleared? Do you know what is that entry, and why it has present bit cleared? I think my attention got attracted to pmd migration entry too much, so I didn't really notice such possibility, as I believe migration pmd is broken already in this path. The original code: 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 (!folio || (!is_huge_zero_folio(folio) && !PageAnonExclusive(&folio->page))) { spin_unlock(ptl); err = -EBUSY; break; } spin_unlock(ptl); split_huge_pmd(src_vma, src_pmd, src_addr); /* The folio will be split by move_pages_pte() */ continue; } err = move_pages_huge_pmd(mm, dst_pmd, src_pmd, dst_pmdval, dst_vma, src_vma, dst_addr, src_addr); step_size = HPAGE_PMD_SIZE; } else { It'll get ptl for a migration pmd, then pmd_folio is risky without checking present bit. That's what my previous smaller patch wanted to fix. But besides that, IIUC it's all fine at least for a pmd migration entry, because when with the smaller patch applied, either we'll try to split the pmd migration entry, or we'll do move_pages_huge_pmd(), which internally handles the pmd migration entry too by waiting on it: if (!pmd_trans_huge(src_pmdval)) { spin_unlock(src_ptl); if (is_pmd_migration_entry(src_pmdval)) { pmd_migration_entry_wait(mm, &src_pmdval); return -EAGAIN; } return -ENOENT; } Then logically after the migration entry got recovered, we'll either see a real THP or pmd none next time. Some explanation on the problematic non-present THP entry would be helpful. Thanks, -- Peter Xu