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AKwUvBy2kqjinEWPQGqrTK7R9MqdJk4M1Cs1Rqr1kkXmwi8ESAKFK8ksfeC9FEJuv1KX21GeOWJSQ5ioWaxvOdU=@vger.kernel.org X-Gm-Message-State: AFuF++mjhhMbtdR/gcdiJnItgbxy0cnCOZYQUlMZ70h0DRAbf7C21idY 2anH40jf4O6LDxI5Wj4BoMkCa8lBqSRn2W5RPt2+yNJOU/cbyFNJ0y9ChRSm/pQX+9iAR1s13EM Doa2ZHw== X-Received: from pfbmt7.prod.google.com ([2002:a05:6a00:6e67:b0:869:6fdc:2a86]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:9e4e:b0:3d3:ae40:51e0 with SMTP id adf61e73a8af0-3da3a045196mr58708633637.20.1788982063138; Wed, 09 Sep 2026 12:27:43 -0700 (PDT) Date: Wed, 9 Sep 2026 12:27:42 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260904004342.3162959-1-seanjc@google.com> <20260904004342.3162959-2-seanjc@google.com> Message-ID: Subject: Re: [PATCH v3 1/4] KVM: guest_memfd: Gracefully handle xarray errors when binding a memslot From: Sean Christopherson To: "David Hildenbrand (Arm)" Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Stefan Teodorescu , Dennis Tighe , Sashiko Bot , Yan Zhao Content-Type: text/plain; charset="us-ascii" On Mon, Sep 07, 2026, David Hildenbrand (Arm) wrote: > On 9/4/26 02:43, Sean Christopherson wrote: > > If inserting a memslot into a guest_memfd's bindings xarray fails, > > propagate the error back to the caller, i.e. fail memslot creation as well. > > Signalling success and continuing on with memslot creation results in > > use-after-free, as the guest_memfd instance will remain reachable via the > > memslot after the file is freed (kvm_gmem_release() won't nullify the file > > pointer due to lack of a valid binding). > > > > Opportunistically WARN and reject binding if KVM_MEMSLOT_GMEM_ONLY is > > already set, partly to guard against goofs elsewhere, but mostly so that > > KVM doesn't need to worry about clobbering flags when unwinding on failure. > > > > Fixes: a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory") > > Cc: stable@vger.kernel.org > > Reported-by: Stefan Teodorescu > > Reported-by: Dennis Tighe > > Reported-by: Sashiko Bot > > Closes: https://lore.kernel.org/all/20260823135031.4F6DC1F000E9%40smtp.kernel.org > > Signed-off-by: Sean Christopherson > > --- > > virt/kvm/guest_memfd.c | 15 +++++++++++++-- > > 1 file changed, 13 insertions(+), 2 deletions(-) > > > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > > index b596486d184c..0b48e9a775aa 100644 > > --- a/virt/kvm/guest_memfd.c > > +++ b/virt/kvm/guest_memfd.c > > @@ -612,10 +612,14 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > > struct inode *inode; > > struct file *file; > > int r = -EINVAL; > > + void *xar; > > > > BUILD_BUG_ON(sizeof(gpa_t) != sizeof(offset)); > > BUILD_BUG_ON(sizeof(gfn_t) != sizeof(slot->gmem.pgoff)); > > > > + if (WARN_ON_ONCE(slot->flags & KVM_MEMSLOT_GMEM_ONLY)) > > + return -EINVAL; > > + > > file = fget(fd); > > if (!file) > > return -EBADF; > > @@ -654,7 +658,15 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > > if (kvm_gmem_supports_mmap(inode)) > > slot->flags |= KVM_MEMSLOT_GMEM_ONLY; > > > > - xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL); > > + xar = xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL); > > + > > + r = xa_is_err(xar) ? xa_err(xar) : 0; > > > r = xa_err(xar); > > Should be sufficient, right? Yes. I didn't like relying on what I thought were internal xarray details, but I missed that xa_err() itself checks xa_is_err(). > mm/memremap.c:pagemap_range() uses that and just avoids the intermediate xar > value completely. > > r = xa_err(xa_store_range(...); Ya, it's ugly, but I do think it's less ugly than the intermediate xar. r = xa_err(xa_store_range(&f->bindings, start, end - 1, slot, GFP_KERNEL));