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 34A6D23395D; Thu, 10 Sep 2026 08:43:51 +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=1789029832; cv=none; b=i+W/PPiupeQ5GxQ1Q2zySiFYsOX/R1doyEG2cPiXXdn8z7nkQkbkE/c1G7yBzDltO1Kk9VZEdWWpRyOH0HMZBWjB92J1TsMGcgu29BFjgBv/uHFqH0YEZfolv5VjLsqrbPOPLYZRr3/bbri+kRiWhpCw73YAxhxLt7DUYr6LEio= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789029832; c=relaxed/simple; bh=GwscfBrlwlJ4OfLnuhq6+VRNnnQ2CC3GGxRnzmZy45c=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=N8bOaf3GUMtSKi5xvsPQAQHdGRIbQOsD7gjMDAAudSjlNzpfHNSayfNd83CkXsQ48CTsPeGk/EXss8F2nGkJ9dmrQ5IRj/nM6WxWaUs/IUssea4w5iIgrvrfHy0Ok7KQJSH/K3Fb3yIsOnm7UUMrlE1MUj7UZX6eEYp0npohh5Q= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=P2HP7y+v; 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="P2HP7y+v" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4006D1F000FF; Thu, 10 Sep 2026 08:43:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1789029831; bh=ErRlIXkdJ4/rRpN+QKa7XDjPMLHUV1wufabVSUtAsQI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=P2HP7y+vQFK2rr4JQhfnWLQMcAoRUjYl4ShV0NJRFVhOU01TwpojXqi5Jbf1VLxtt 5H9xKB2rEUeMkwUkTm5zk539OYXH0eiWTbsiy9jycZVsGQzCIayr6NiL0YWH1JR3l1 buZNmfJ+GpiTDrFMdLrNywunrZUioJ75icRB1IFBoL2ji1IPNU3+4jgJinv8TZG+TW L439PYO2ICtBackUaHH4CaeZWo0b/I8QDxYt6BXfeZdBBeVop2JbHKCVChZvjoYU5Q Xb1w6rKuHpgTSCZnAML1njltCXVnefNYTP1iYvbYePrxP4gp1ZbjAGRaM5XdClNwnT yVPzcZRhjUumw== Message-ID: Date: Thu, 10 Sep 2026 10:43:46 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] KVM: guest_memfd: take the invalidate lock when unbinding a dying file To: Shivank Garg , Sean Christopherson , Paolo Bonzini Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org, Sashiko References: <20260901-gmem-unbind-fix-v2-1-12febe1d2216@amd.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 9/1/26 09:20, Shivank Garg wrote: > kvm_gmem_unbind() takes mapping->invalidate_lock only if the > get_file_active() succeeds. When the guest_memfd file is dying, its > reference count is already zero so get_file_active fails and the binding > is removed without holding the invalidate lock. > > kvm_gmem_invalidate_{start,end}() checks f->bindings independently to > decide whether to begin or end KVM MMU invalidation. So, the bindings > must therefore remain stable between the two calls. Otherwise, unbind > can remove a binding after start increments mmu_invalidate_in_progress, > but before end finds the binding and decrements it. > Example, unbind race with memory failure: > > CPU 0: memory failure CPU 1: memslot delete > ---------------------------------- --------------------------- > (guest_memfd file is dying) > kvm_gmem_error_folio() > kvm_gmem_invalidate_start() > finds binding > mmu_invalidate_in_progress++ > kvm_gmem_unbind() > get_file_active() fails > removes binding > kvm_gmem_invalidate_end() > no binding found > counter stays elevated > > mmu_invalidate_retry() then returns 1 forever, so guest page faults > retry without ever installing a mapping and the guest hangs. > > Remove the distinction between live and dying files in the unbind path. > Always use slot->gmem.file and take the invalidate lock when it's non-NULL. > Normal unbind callers hold slots_lock, which prevents release from clearing > the pointer or freeing the file state until unbind completes. Final VM > teardown can only see a NULL pointer because guest_memfd pins the KVM until > release has cleared all bindings. > > Fixes: ae431059e75d ("KVM: guest_memfd: Remove bindings on memslot deletion when gmem is dying") > Cc: stable@vger.kernel.org > Reported-by: Sashiko > Closes: https://lore.kernel.org/all/20260728092027.225CF1F000E9@smtp.kernel.org > Suggested-by: Sean Christopherson > Signed-off-by: Shivank Garg > --- > Tested on a 7.3-rc1, AMD EPYC 7713. > > Changes in V2: > - Treat a dying file as the normal case: always use slot->gmem.file and > take invalidate lock, drop use of gmem_get_file for unbind and fold > __kvm_gmem_unbind() into caller. (Sean) > - Drop NUMA selftests changes from this series, and will be sent separately. (Sean) > - Link to V1: https://lore.kernel.org/kvm/20260823-shivank-gmem-fix-split-v1-0-512a29fb8e86@amd.com > --- > virt/kvm/guest_memfd.c | 44 ++++++++++++++++++-------------------------- > 1 file changed, 18 insertions(+), 26 deletions(-) > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 625e62e1a031..21bb3710edfe 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -668,48 +668,40 @@ int kvm_gmem_bind(struct kvm *kvm, struct kvm_memory_slot *slot, > return r; > } > > -static void __kvm_gmem_unbind(struct kvm_memory_slot *slot, struct gmem_file *f) > +void kvm_gmem_unbind(struct kvm_memory_slot *slot) > { > + struct file *file = slot->gmem.file; > unsigned long start = slot->gmem.pgoff; > unsigned long end = start + slot->npages; > + struct gmem_file *f; > > - xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL); > - > - /* > - * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn() > - * cannot see this memslot. > - */ > - WRITE_ONCE(slot->gmem.file, NULL); > -} > - > -void kvm_gmem_unbind(struct kvm_memory_slot *slot) > -{ > /* > * Nothing to do if the underlying file was _already_ closed, as > * kvm_gmem_release() invalidates and nullifies all bindings. > */ > - if (!slot->gmem.file) > + if (!file) > return; > > - CLASS(gmem_get_file, file)(slot); > - > /* > * However, if the file is _being_ closed, then the bindings need to be > * removed as kvm_gmem_release() might not run until after the memslot > - * is freed. Note, modifying the bindings is safe even though the file > - * is dying as kvm_gmem_release() nullifies slot->gmem.file under > - * slots_lock, and only puts its reference to KVM after destroying all > - * bindings. I.e. reaching this point means kvm_gmem_release() hasn't > - * yet destroyed the bindings or freed the gmem_file, and can't do so > - * until the caller drops slots_lock. > + * is freed. Modifying the bindings is safe even if the file is dying > + * as kvm_gmem_release() nullifies slot->gmem.file under slots_lock, > + * and only puts its reference to KVM after destroying all bindings. > + * I.e. reaching this point means kvm_gmem_release() hasn't destroyed > + * the bindings or freed the gmem_file and can't do so until the caller > + * drops slots_lock, so there's no need to verify the file is live. > */ > - if (!file) { > - __kvm_gmem_unbind(slot, slot->gmem.file->private_data); > - return; > - } > + f = file->private_data; > > filemap_invalidate_lock(file->f_mapping); > - __kvm_gmem_unbind(slot, file->private_data); > + xa_store_range(&f->bindings, start, end - 1, NULL, GFP_KERNEL); > + > + /* > + * synchronize_srcu(&kvm->srcu) ensured that kvm_gmem_get_pfn() > + * cannot see this memslot. > + */ > + WRITE_ONCE(slot->gmem.file, NULL); > filemap_invalidate_unlock(file->f_mapping); > } Besides fixing something, this does make the code simpler :) Reviewed-by: David Hildenbrand (Arm) -- Cheers, David