From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 EA9183F23C3; Mon, 9 Feb 2026 19:38:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770665893; cv=none; b=nHg32Gibvsnmt+cB02Pa7El92Qksg+KXi4/hIpq8z96ABoBT67YWb7wLFpocUy1pXHoF9TuTzAOfGglwdq9MHI0ITPzd3IqazhUzjwXbW6Ai+ajCUdVjiOESCydJGp+OQDgEHwIFiM5fDkwpzGalPnAMti3ePHe5WYxEiQcz5qc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1770665893; c=relaxed/simple; bh=WUrsjMXh3pCtMDG6ztjXdnirrLv7ZfvESTsITHNcuIk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=bp0YXh0YjDCWYGoqNN/Ptgl+diYjP3PSGk8IGfx/B/XOh3Q6JEoAfVCybnRe5RzZrUQrJlMynnLjTsP2rLQkWB/uGsgHzZlYImaFMJfu+F7ssmMaXUrexL6I6Ui1gdozfOAiGC1Oe3myS3Y4IRphTPROLvqCRIyy+smh8nPX9z8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=NROPUwIV; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="NROPUwIV" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8445FC116C6; Mon, 9 Feb 2026 19:38:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1770665892; bh=WUrsjMXh3pCtMDG6ztjXdnirrLv7ZfvESTsITHNcuIk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=NROPUwIVpEs+0lkbQgRVAMYM0KWltV1UEvnfc5KeyE50drflHAoFbtUqFjiqTqFs6 tohg1prbhlL0gLz9/ViTyhkgUikNRuISrKQ5Xw8R0JOpk8qWWY2xWfb52km+lUDIoR v3oe8Ntt9fqCwZfOJIcKlqkdUeGnFsZXeyr6C80qbEwr8+ykoUgpicU/44Pc6rYWCu 5mLHMoR13r9ctMMhCsgJYt0+cblqjqcBcTd4UXjMzaQg0q1TZvibzmToKWZBSGMNsW Kro4PB4mYCMZ6GXGq8fn7huOjXf2YEggPuoHZEHFEo5IpExc1teHwWUfmzMIP5LTRW d9aVaLJ+MchTw== Message-ID: Date: Mon, 9 Feb 2026 20:38:06 +0100 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] KVM: guest_memfd: Disable VMA merging with VM_DONTEXPAND To: Ackerley Tng , Sean Christopherson Cc: syzbot+33a04338019ac7e43a44@syzkaller.appspotmail.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, pbonzini@redhat.com, syzkaller-bugs@googlegroups.com, michael.roth@amd.com, vannapurve@google.com, kartikey406@gmail.com References: <697d115a.050a0220.1d61ec.0004.GAE@google.com> <20260204170144.2904483-1-ackerleytng@google.com> <16e5a36e-fff0-4a54-9c5c-a8e411659108@kernel.org> <442d7ce8-0de3-4f5a-95ed-3be9bdaa7e47@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/9/26 19:24, Ackerley Tng wrote: > "David Hildenbrand (Arm)" writes: > >> On 2/8/26 18:34, Ackerley Tng wrote: >>> Ackerley Tng writes: >>> >>> >>> I trimmed the repro to this: >>> >>> static void test_guest_memfd_repro(void) >>> { >>> struct kvm_vcpu *vcpu; >>> uint8_t *unaligned_mem; >>> struct kvm_vm *vm; >>> uint8_t *mem; >>> int fd; >>> >>> vm = __vm_create_shape_with_one_vcpu(VM_SHAPE_DEFAULT, &vcpu, 1, guest_code); >>> >>> fd = vm_create_guest_memfd(vm, SZ_2M * 2, GUEST_MEMFD_FLAG_MMAP | >>> GUEST_MEMFD_FLAG_INIT_SHARED); >>> >>> unaligned_mem = mmap(NULL, SZ_2M + SZ_2M, PROT_READ | PROT_WRITE, >>> MAP_FIXED | MAP_SHARED, fd, 0); >>> mem = align_ptr_up(unaligned_mem, SZ_2M); >>> TEST_ASSERT(((unsigned long)mem & (SZ_2M - 1)) == 0, "returned >>> address must be aligned to SZ_2M"); >>> >>> TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_HUGEPAGE), 0); >>> >>> for (int i = 0; i < SZ_2M; i += SZ_4K) >>> READ_ONCE(mem[i]); >>> >>> TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_COLLAPSE), 0); >>> >>> TEST_ASSERT_EQ(madvise(mem, SZ_2M, MADV_DONTNEED), 0); >>> >>> /* This triggers the WARNing. */ >>> READ_ONCE(mem[0]); >>> >>> munmap(unaligned_mem, SZ_2M * 2); >>> >>> close(fd); >>> kvm_vm_free(vm); >>> } >>> >>> And tried to replace the fd creation the secretmem equivalent >>> >>> fd = syscall(__NR_memfd_secret, 0); >>> TEST_ASSERT(fd >= 0, "Couldn't create secretmem fd."); >>> TEST_ASSERT_EQ(ftruncate(fd, SZ_2M * 2), 0); >>> >>> Should a guest_memfd selftest be added to cover this? >>> >>> MADV_COLLAPSE fails with EINVAL, but it does go through to >>> hpage_collapse_scan_file() -> collapse_file(), before failing because >>> when collapsing the page, copy_mc_highpage() returns > 0. >> >> Just what I suspected. :) >> >> Thanks for digging into the details! >> > > Happy to help :) > > In general, do we want the reproducers added as selftests? Should this > be added as part of tools/testing/selftests/kvm/guest_memfd_test.c I guess adding it to guest_memfd_test.c and asserting that MADV_COLLAPSE fails as expected could be a reasonable test case. It's not a lot of code and easy to verify. -- Cheers, David