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Fri, 01 Aug 2025 09:23:19 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEpZI71GEayMeGwYSIqUSspyQlyLLtHuQaSsWRKAdHTVneK9AyXLakZUxoMF71iimA1I9hOJg== X-Received: by 2002:a05:6214:623:b0:707:1eae:17e9 with SMTP id 6a1803df08f44-70935f6c42emr3327936d6.21.1754065399325; Fri, 01 Aug 2025 09:23:19 -0700 (PDT) Received: from x1.local ([174.89.135.171]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-7077c9dc383sm22996276d6.16.2025.08.01.09.23.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 01 Aug 2025 09:23:18 -0700 (PDT) Date: Fri, 1 Aug 2025 12:23:05 -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 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? > 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]? > 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? Could you elaborate why the split would fail? AFAIU, split_huge_pmd_locked() should still work for a migration pmd entry. Thanks, > thinking that PMD got split and leaving further remapping to > move_pages_pte() (see the comment before "continue"). I think in this > case it will end up in the same path again instead (infinite loop). I > didn't test this but from the code I think that's what would happen. > Does that make sense? > > > > > And it also looks applicable to also drop the "else" later, because in "if > > (ptl)" it cannot hit pmd_none(). > > > > Thanks, > > > > -- > > Peter Xu > > > -- Peter Xu