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()?
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().
IOW, I still don't understand why below won't already work:
===8<=== diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 52d7d5f144b8e..33e78f52ee9f5 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1880,13 +1880,15 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start, /* 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; + if (pmd_present(*src_pmd)) { + 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);
===8<===
Likely I missed something important.. I'll be afk for a while soon, I'll also double check (maybe early next week) on the reproducer.
Thanks,