From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2B42D3537E5; Mon, 7 Sep 2026 17:47:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788803234; cv=none; b=m9S0DdyUbcjp/pd2w2+TBXoJJhRxwMAaynEOtgwljpZM0Trjrorj+V1iY75mIA47YMg5cFzkXtgtZgyMpovxmdMd783gO/8kq1oBB9klkzU4L03LcrB7qK3tnJihdCQCit3DCJsgThdV1eBeTor7crJ1mWOkh4IiYI4S1sSIyP8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788803234; c=relaxed/simple; bh=m9FMQDOcT4FQPnknbDFlfjy6QjpVnNqyIJ8B8YERdZI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=tT/JzyPDy7WYYvApsDVRteZBaFWUPpM9Jbls3TWaiNkL03Rfa7GEuhB966intgcPj/bMSMpQK3B3tQrUAWL7x47xtZ+hvZ+jjuPLasJpnQQlfWYR+Lc+6Vn2sx5WtS0fNhht87PdQxQT1OwL4vu/Vyzkjq1ponMG2mmjfzYBklI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=K5CKxkTz; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="K5CKxkTz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4FC751F00A3A; Mon, 7 Sep 2026 17:47:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788803232; bh=idlbhkKiTt/twN0uI3E3GTXch8QGLkjoueh0C9NXOMI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=K5CKxkTzIc2iHDlS5Ls7aScP2LHvSXuI6yB7Z2S2jJdEgKw9XanNUxg/+6EG+gk20 5V89+AZPWGeZYRS3iFTp4yOHkS7tiSgJ0JeTDl0fQsKFpUr5l1lNMSAKbf+RT7NmLV MYPav4GMBELPcLE+0R/F9VWpNSGEpZmf0JrJlrxdSsB1JbBC/oFVCriNIxAQcnYXGA EIFzAU4ZOGITcKP6WAwj3+vgi7vb0zxfVH4PBMHskCkGz6jJ5raUnjRpr51TAJhqrX CDxWO1wAbaPA7grjy/CG9uLiLyrX+8jxsEGvgTEdrzZPPNFd58xDVlE2X/PJPwpavk 2MdN3pTLIt3BQ== Message-ID: Date: Mon, 7 Sep 2026 19:47:08 +0200 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 3/4] KVM: guest_memfd: Establish memslot<=>guest_memfd bindings *after* memslot is ready To: Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Stefan Teodorescu , Dennis Tighe , Sashiko Bot , Yan Zhao References: <20260904004342.3162959-1-seanjc@google.com> <20260904004342.3162959-4-seanjc@google.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/4/26 02:43, Sean Christopherson wrote: > Wait to bind a memslot to a guest_memfd instance until *after* the memslot > is fully prepared, as creating the binding in guest_memfd will effectively > expose the memslot to readers. As pointed out by Sashiko, binding the > memslot before it's ready to be exposed to the rest of the world can break > various memslot assumption and rules. E.g. x86 could observe a NULL rmap > pointer if a PUNCH_HOLE hit the guest_memfd after the binding was created, > but before KVM made it through kvm_prepare_memory_region(). > > Begrudgingly resort to passing in the guest_memfd fd+offset pair to > kvm_set_memslot(), as creating the binding really does need to happen in > the middle of setting the new memslot. Alternatively, to preserve the > aesthetically pleasing function prototype, "struct kvm_memory_slot" could > be expanded to track the fd and the file, but that would create the > possibility for TOCTOU bugs on the fd vs. file, and would add zero value > beyond making kvm_set_memslot() look pretty. > > Fixes:a7800aa80ea4 ("KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for guest-specific backing memory") > Cc: stable@vger.kernel.org > Reported-by: Sashiko Bot > Closes: https://lore.kernel.org/all/20260826170551.BEF801F000E9@smtp.kernel.org > Signed-off-by: Sean Christopherson > --- > virt/kvm/kvm_main.c | 34 ++++++++++++++++++++++------------ > 1 file changed, 22 insertions(+), 12 deletions(-) > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 3c0dbe60a5b4..21c10cbbac66 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -1887,7 +1887,8 @@ static void kvm_update_flags_memslot(struct kvm *kvm, > static int kvm_set_memslot(struct kvm *kvm, > struct kvm_memory_slot *old, > struct kvm_memory_slot *new, > - enum kvm_mr_change change) > + enum kvm_mr_change change, > + unsigned int gmem_fd, uoff_t gmem_offset) > { > struct kvm_memory_slot *invalid_slot; > int r; > @@ -1934,6 +1935,15 @@ static int kvm_set_memslot(struct kvm *kvm, > if (r) > goto err; > > + if (new && new->flags & KVM_MEM_GUEST_MEMFD) { For readability I'd throw in an extra pair of (). But KVM seems to use both styles, so there is no clear preference when staring at the existing code :) > + if (WARN_ON_ONCE(change != KVM_MR_CREATE)) > + goto err_bind; > + > + r = kvm_gmem_bind(kvm, new, gmem_fd, gmem_offset); > + if (r) > + goto err_bind; > + } > + > /* > * For DELETE and MOVE, the working slot is now active as the INVALID > * version of the old slot. MOVE is particularly special as it reuses > @@ -1965,6 +1975,13 @@ static int kvm_set_memslot(struct kvm *kvm, > > return 0; > > +err_bind: > + if (new) { We'd never end up here with !new, right? > + kvm_arch_free_memslot(kvm, new); > + > + if (new->dirty_bitmap && (!old || !old->dirty_bitmap)) > + kvm_destroy_dirty_bitmap(new); That's essentially the cleanup path in kvm_prepare_memory_region(). I guess with some more reshuffling we could have a single dirty bitmap cleanup path in this code. > + } > err: > /* > * For DELETE/MOVE, revert the above INVALID change. No modifications > @@ -2059,7 +2076,7 @@ static int kvm_set_memory_region(struct kvm *kvm, > if (WARN_ON_ONCE(kvm->nr_memslot_pages < old->npages)) > return -EIO; > > - return kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE); > + return kvm_set_memslot(kvm, old, NULL, KVM_MR_DELETE, -1, 0); > } > > base_gfn = (mem->guest_phys_addr >> PAGE_SHIFT); > @@ -2106,21 +2123,14 @@ static int kvm_set_memory_region(struct kvm *kvm, > new->npages = npages; > new->flags = mem->flags; > new->userspace_addr = mem->userspace_addr; > - if (mem->flags & KVM_MEM_GUEST_MEMFD) { > - r = kvm_gmem_bind(kvm, new, mem->guest_memfd, mem->guest_memfd_offset); > - if (r) > - goto out; > - } > > - r = kvm_set_memslot(kvm, old, new, change); > + r = kvm_set_memslot(kvm, old, new, change, > + mem->guest_memfd, mem->guest_memfd_offset); > if (r) > - goto out_unbind; > + goto out; > > return 0; > > -out_unbind: > - if (mem->flags & KVM_MEM_GUEST_MEMFD) > - kvm_gmem_unbind(new); > out: > kfree(new); > return r; In general LGTM. -- Cheers, David