[Adding missing mailing lists to Cc that I omitted by mistake] On Mon, Mar 16, 2026 at 04:45:24PM +0900, Harry Yoo wrote: > On Fri, Mar 06, 2026 at 07:18:04PM +0200, Mike Rapoport (Microsoft) wrote: > > Split the code that finds, locks and verifies VMA from mfill_atomic() > > into a helper function. > > > > This function will be used later during refactoring of > > mfill_atomic_pte_copy(). > > > > Add a counterpart mfill_put_vma() helper that unlocks the VMA and > > releases map_changing_lock. > > > > Signed-off-by: Mike Rapoport (Microsoft) <[email protected]> > > Signed-off-by: Peter Xu <[email protected]> > > --- > > Apparently this patch seems to have two issues... > Fortunately, it did not land mainline yet and can be addressed. > > Deepanshu and Edward sent fixes, but it should be fixed > as part of v3 (if Mike plans to do so) or as a fix-up on patch 4. > > Please keep in mind that my understanding of this patchset is limited. > I'm just doing some mechanical analysis. > > > mm/userfaultfd.c | 124 ++++++++++++++++++++++++++++------------------- > > 1 file changed, 73 insertions(+), 51 deletions(-) > > > > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c > > index 224b55804f99..baff11e83101 100644 > > --- a/mm/userfaultfd.c > > +++ b/mm/userfaultfd.c > > @@ -157,6 +157,73 @@ static void uffd_mfill_unlock(struct vm_area_struct > > *vma) > > } > > #endif > > > > +static void mfill_put_vma(struct mfill_state *state) > > +{ > > + up_read(&state->ctx->map_changing_lock); > > + uffd_mfill_unlock(state->vma); > > + state->vma = NULL; > > +} > > + > > +static int mfill_get_vma(struct mfill_state *state) > > +{ > > + struct userfaultfd_ctx *ctx = state->ctx; > > + uffd_flags_t flags = state->flags; > > + struct vm_area_struct *dst_vma; > > + int err; > > + > > + /* > > + * Make sure the vma is not shared, that the dst range is > > + * both valid and fully within a single existing vma. > > + */ > > + dst_vma = uffd_mfill_lock(ctx->mm, state->dst_start, state->len); > > + if (IS_ERR(dst_vma)) > > + return PTR_ERR(dst_vma); > > state->len is always initialized to zero in patch 2. > syzbot triggered a warning in folio_add_new_anon_rmap(), > which appears to be due to failing to verify the range > in uffd_mfill_lock() (at least syzbot says it's fixed [1]). > > [1] > https://lore.kernel.org/linux-mm/[email protected] > > It seems there's another attempt to fix the syzbot report from > Deepanshu Kartikey [2], which I didn't take a deeper look. > > At first look [2] looks a bit wrong way to fix to me though, > because it allows operating only on a single VMA nothing should really split > or shrink the VMA if somebody is holding the VMA lock in read mode > (and the validation of the range is done while holding the lock). > > [2] > https://lore.kernel.org/linux-mm/[email protected] > > > + /* > > + * If memory mappings are changing because of non-cooperative > > + * operation (e.g. mremap) running in parallel, bail out and > > + * request the user to retry later > > + */ > > + down_read(&ctx->map_changing_lock); > > + err = -EAGAIN; > > + if (atomic_read(&ctx->mmap_changing)) > > + goto out_unlock; > > + > > + err = -EINVAL; > > + > > + /* > > + * shmem_zero_setup is invoked in mmap for MAP_ANONYMOUS|MAP_SHARED but > > + * it will overwrite vm_ops, so vma_is_anonymous must return false. > > + */ > > + if (WARN_ON_ONCE(vma_is_anonymous(dst_vma) && > > + dst_vma->vm_flags & VM_SHARED)) > > + goto out_unlock; > > + > > + /* > > + * validate 'mode' now that we know the dst_vma: don't allow > > + * a wrprotect copy if the userfaultfd didn't register as WP. > > + */ > > + if ((flags & MFILL_ATOMIC_WP) && !(dst_vma->vm_flags & VM_UFFD_WP)) > > + goto out_unlock; > > + > > + if (is_vm_hugetlb_page(dst_vma)) > > + goto out; > > + > > + if (!vma_is_anonymous(dst_vma) && !vma_is_shmem(dst_vma)) > > + goto out_unlock; > > + if (!vma_is_shmem(dst_vma) && > > + uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE)) > > + goto out_unlock; > > In case of error, mfill_put_vma() is supposed to unlock appropriate > locks but state->vma is not set, so it does nothing. > > Pointed out by Edward Adam Davis [3] > [3] > https://lore.kernel.org/linux-mm/[email protected] > > I think, this should probably be fine in patch 4, but patch 5 adds > `if (!state->vma) return;`, leading to unreleased locks on error paths. > > > + > > +out: > > + state->vma = dst_vma; > > + return 0; > > + > > +out_unlock: > > + mfill_put_vma(state); > > + return err; > > +} > > + > > static pmd_t *mm_alloc_pmd(struct mm_struct *mm, unsigned long address) > > { > > pgd_t *pgd;
-- Cheers, Harry / Hyeonggon
