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AJvYcCUUfmrIItf4pPDB7ICnUZn19MMNn+s681tC4+icandGDASfhnq1B3zSPpzAjQQe2ztsqaeDnTYJlg==@kvack.org X-Gm-Message-State: AOJu0YwfIx/Tog18wqPlr5IqI1ZNZNssyERT/8McZDcvc7YzC/xk3LYK kCfaSovjMlrlwrDP5V8pvVE0SW0F3S9YrHh2nyG6LX8nB2dh3M6QlEJdaHPRl5oXYBInJwBjtGG enBFZA7Ex0Faa2AJXmvplOuu6PLAJkskWCJ6Kctz4 X-Gm-Gg: ASbGncvEVwC4+SXjc2QXNZ2GdnwMbluuS1UHCAfnDJN6e4kTr1pZiZ3a/WGTwtfURQ6 i3qAuv3SacMkWaS5fAO3TuqHDZ0dr32qdqRrkSyK62XxaCuifLqb/5h7L6EnkjaL/YFdxoS9HJD RwsYUawg31QTXZh7g4HPiZNdzucvFkGcY55Hoya9Uoxdu3DFk1K1F3y1aZ9tMpSH+kR/NiK2tT8 ZOxvqpc3Ur2suRepJIG0Y7dCeuHKhyH0P864oCzZg== X-Google-Smtp-Source: AGHT+IHpZroMnR8gNbnJ6Hl9Azyb+Hb8r/YpRDMj/t8qAd1gcDSdLIfF7iA0Z5SXN0Jk5/1OoME3F4GcnV0AJtVw4OA= X-Received: by 2002:a50:d75c:0:b0:607:d206:7657 with SMTP id 4fb4d7f45d1cf-615ab47613amr78190a12.2.1753974522596; Thu, 31 Jul 2025 08:08:42 -0700 (PDT) MIME-Version: 1.0 References: <20250730170733.3829267-1-surenb@google.com> <20250731015241.3576-1-hdanton@sina.com> In-Reply-To: From: Lokesh Gidra Date: Thu, 31 Jul 2025 08:08:30 -0700 X-Gm-Features: Ac12FXxDR_jDXV2_itlIrHvfQvtXRY2GLBRhyzMxKRUo0_JmXQBqs2eQDyu-PXM Message-ID: Subject: Re: [PATCH 1/1] userfaultfd: fix a crash when UFFDIO_MOVE handles a THP hole To: Suren Baghdasaryan Cc: Hillf Danton , akpm@linux-foundation.org, peterx@redhat.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, syzkaller-bugs@googlegroups.com, syzbot , stable@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Queue-Id: 802EB120013 X-Stat-Signature: 7u9hmsxeynrx9zk68errgkiis1x48igu X-Rspam-User: X-Rspamd-Server: rspam08 X-HE-Tag: 1753974524-741943 X-HE-Meta: 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 NwRPntRs 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 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 Thu, Jul 31, 2025 at 7:24=E2=80=AFAM Suren Baghdasaryan wrote: > > On Thu, Jul 31, 2025 at 12:35=E2=80=AFAM Lokesh Gidra wrote: > > > > On Wed, Jul 30, 2025 at 6:58=E2=80=AFPM Hillf Danton = wrote: > > > > > > #syz test > > > > > > 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 unmap= ped > > > hole and tries to access a non-existent folio, resulting in > > > a crash. Add a check to skip non-present THPs. > > > > > Thanks Suren for promptly addressing this issue. > > > > > 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 > > > --- > > > mm/userfaultfd.c | 38 +++++++++++++++++++++++--------------- > > > 1 file changed, 23 insertions(+), 15 deletions(-) > > > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > > index cbed91b09640..60be8080ddd0 100644 > > > --- a/mm/userfaultfd.c > > > +++ b/mm/userfaultfd.c > > > @@ -1818,27 +1818,35 @@ ssize_t move_pages(struct userfaultfd_ctx *ct= x, unsigned long dst_start, > > > > > > ptl =3D pmd_trans_huge_lock(src_pmd, src_vma); > > > if (ptl) { > > > - /* Check if we can move the pmd without split= ting it. */ > > > - if (move_splits_huge_pmd(dst_addr, src_addr, = src_start + len) || > > > - !pmd_none(dst_pmdval)) { > > > - struct folio *folio =3D pmd_folio(*sr= c_pmd); > > > + if (pmd_present(*src_pmd) || is_pmd_migration= _entry(*src_pmd)) { > > > + /* Check if we can move the pmd witho= ut splitting it. */ > > > + if (move_splits_huge_pmd(dst_addr, sr= c_addr, src_start + len) || > > > + !pmd_none(dst_pmdval)) { > > > + if (pmd_present(*src_pmd)) { > > > + struct folio *folio = =3D pmd_folio(*src_pmd); > > > + > > > + if (!folio || (!is_hu= ge_zero_folio(folio) && > > > + !PageA= nonExclusive(&folio->page))) { > > > + spin_unlock(p= tl); > > > + err =3D -EBUS= Y; > > > + break; > > > + } > > > + } > > > > > > - if (!folio || (!is_huge_zero_folio(fo= lio) && > > > - !PageAnonExclusive(&fo= lio->page))) { > > > spin_unlock(ptl); > > > - err =3D -EBUSY; > > > - break; > > > + split_huge_pmd(src_vma, src_p= md, src_addr); > > > + /* The folio will be split by= move_pages_pte() */ > > > + continue; > > > } > > > > > > + err =3D move_pages_huge_pmd(mm, dst_p= md, src_pmd, > > > + dst_pmdval,= dst_vma, src_vma, > > > + dst_addr, s= rc_addr); > > > + } else { > > > + /* nothing to do to move a hole */ > > > spin_unlock(ptl); > > > - split_huge_pmd(src_vma, src_pmd, src_= addr); > > > - /* The folio will be split by move_pa= ges_pte() */ > > > - continue; > > > + err =3D 0; > > I think we need to act here depending on whether > > UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not. > > Hmm, yes, I think you are right. I thought we would bail out earlier > if !UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES but I think it's possible to get > here if the PMD was established earlier but then unmapped. That makes sense too. My thinking was that the !UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES check above is only when !src_pmd, which means the src pmd-page itself isn't present. However, the case where pmd-page is present, but the pmd entry is not; continuing skipping the hole even when user cannot tolerate src-holes would break the userspace logic. > > > > > err =3D (mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES) ? 0 : -ENO= ENT; > > > > Also, IMO, the step_size in this case should be the minimum of > > remaining length and HPAGE_PMD_SIZE. > > Ah, ok. I think it matters only for incrementing "moved" correctly > because otherwise the functionality is the same. > Right. Returning an incorrect "moved" value to userspace can break the logic, possibly causing data corruption. > > > } > > > - > > > - err =3D move_pages_huge_pmd(mm, dst_pmd, src_= pmd, > > > - dst_pmdval, dst_vma= , src_vma, > > > - dst_addr, src_addr)= ; > > > step_size =3D HPAGE_PMD_SIZE; > > > } else { > > > if (pmd_none(*src_pmd)) { > > I have a related question/doubt: why do we populate the page-table > > hierarchy on the src side [1] (and then also at line 1857) when a hole > > is found? IMHO, it shouldn't be needed. Depending on whether > > UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES is set or not, it should either > > return -ENOENT, or continue past the hole. Please correct me if I'm > > wrong. > > I thought about that too. I think it's done to simplify the logic. > This way we can treat the cases when PMD was never allocated and when > PMD was allocated, mapped and then unmapped the same way. > Makes sense. Thanks for clarifying. > > > > [1] https://elixir.bootlin.com/linux/v6.16/source/mm/userfaultfd.c#L179= 7 > > > > > > > > base-commit: 01da54f10fddf3b01c5a3b80f6b16bbad390c302 > > > -- > > > 2.50.1.552.g942d659e1b-goog