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Tue, 05 Aug 2025 07:39:30 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEbzW4UiXSJV9RVR64B6UcBQaLqjWbfSx5kR9/jhgtO/J/1mSovc/gLubrthIt8BIT18R/6Yw== X-Received: by 2002:a05:622a:341:b0:4b0:7372:1bcb with SMTP id d75a77b69052e-4b073722829mr97740261cf.15.1754404769737; Tue, 05 Aug 2025 07:39:29 -0700 (PDT) Received: from x1.local ([174.89.135.171]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7e67f5c55a2sm685723885a.36.2025.08.05.07.39.28 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 05 Aug 2025 07:39:28 -0700 (PDT) Date: Tue, 5 Aug 2025 10:39:16 -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: 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 Mon, Aug 04, 2025 at 07:55:42AM -0700, Suren Baghdasaryan wrote: > On Fri, Aug 1, 2025 at 5:32 PM Peter Xu wrote: > > > > 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? > > In this case it's because earlier we allocated that PMD here: > https://elixir.bootlin.com/linux/v6.16/source/mm/userfaultfd.c#L1797 AFAIU, this line is not about allocation of any pmd entry, but the pmd pgtable page that _holds_ the PMDs: static inline pmd_t *pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address) { return (unlikely(pud_none(*pud)) && __pmd_alloc(mm, pud, address))? NULL: pmd_offset(pud, address); } It makes sure the PUD entry, not the PMD entry, be populated. > but wouldn't that be the same if the PMD was mapped and then got > unmapped later? My understanding is that we allocate the PMD at the > line I pointed to make UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES case the same > as this unmapped PMD case. If my assumption is incorrect then we could > skip the hole earlier instead of allocating the PMD for it. > > > > > 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. > > Yes, for migration entries adding the "if (pmd_present(*src_pmd))" > before getting the folio is enough. The problematic case is > (!pmd_none(*src_pmd) && !pmd_present(*src_pmd)) and not a migration > entry. I thought we could have any of below here on the pmd entry: (0) pmd_none, which should constantly have pmd_trans_huge_lock -> NULL (1) pmd pgtable entry, which must have PRESENT && !TRANS, so pmd_trans_huge_lock -> NULL, (2) pmd migration, pmd_trans_huge_lock -> valid (3) pmd thp, pmd_trans_huge_lock -> valid I thought (2) was broken, which we seem to agree upon.. however if so the smaller patch should fix it, per explanation in my previous reply. OTOH I can't think of (4). Said that, I just noticed (3) can be broken as well - could it be a prot_none entry? The very confusing part of this patch is it seems to think it's pmd_none() here as holes: if (pmd_present(*src_pmd) || is_pmd_migration_entry(*src_pmd)) { ... } else { spin_unlock(ptl); if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) { err = -ENOENT; break; } /* nothing to do to move a hole */ err = 0; step_size = min(HPAGE_PMD_SIZE, src_start + len - src_addr); } But is it really? Again, I don't think pmd_none() could happen with pmd_trans_huge_lock() returning the ptl. Could you double check this? E.g. with this line if that makes sense to you: diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 8bf8ff0be990f..d2d4f2a0ae69f 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1903,6 +1903,7 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, dst_addr, src_addr); step_size = HPAGE_PMD_SIZE; } else { + BUG_ON(!pmd_none(*src_pmd)); spin_unlock(ptl); if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) { err = -ENOENT; I would expect it constantly BUG() here, if that explains my thoughts. Now I doubt it's a prot_none THP.. aka, a THP that got numa hint to be moved. If so, we may need to process it / move it / .. but we likely should never skip it. We can double check the buggy pmd entry you hit (besides migration entry) first. Thanks, -- Peter Xu