On Fri, Aug 01, 2025 at 07:30:02PM +0000, Suren Baghdasaryan wrote:
On Fri, Aug 1, 2025 at 6:21 PM Peter Xu peterx@redhat.com 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 peterx@redhat.com 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 peterx@redhat.com 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 peterx@redhat.com 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 surenb@google.com > > > > 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,