From patchwork Wed Aug 28 18:14:46 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sean Christopherson X-Patchwork-Id: 13781716 Received: from mail-yb1-f201.google.com (mail-yb1-f201.google.com [209.85.219.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DB3781AB50D for ; Wed, 28 Aug 2024 18:14:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.219.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724868896; cv=none; b=BbeR4GKhq/qLcIwSii6E22LOm4W56KEW6OYiiySojbHkzu05DJnUw9/0Ajast+9aMc9IYsnZrR9GncNbZy9ThGfpI7do4/0TRFfHRgVvpagWVL1RIP99uQIBI5ATwrFJEeToYs8PMeDjvVa7uqy626urprwYE+z80LDHjtiUiAM= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1724868896; c=relaxed/simple; bh=LnPdXA7Jg8T29YlpMD9bHqpviTaW8w6gYDIQ+3cm8rw=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=UgErZaQ7fjHKeRDv75fzlZ4ODD5AdXkaVZw7eNrG9+78DW5AgwuqgaA98zJITKqDFcWk19xu3qurw4M7xyxq3fzXruHEvzIEoQvd5JySbHlcuzF8lfJeNcgvp1V/aanMNYcjDUKs/jq5Tc/4bSCK4jcDAyTvBQoTK3R09hlw+po= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=v1bOuD6n; arc=none smtp.client-ip=209.85.219.201 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="v1bOuD6n" Received: by mail-yb1-f201.google.com with SMTP id 3f1490d57ef6-e17bb508bb9so9342389276.2 for ; Wed, 28 Aug 2024 11:14:54 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1724868894; x=1725473694; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:from:to:cc:subject:date:message-id:reply-to; bh=g8CEH7SnnSsHaiw39Cx2W/imrlcDKaVqaPZKBSnVoqE=; b=v1bOuD6nVgxW03i0FbFjLL49r3B17a9fURUdaJXVd4md5Bw+9swKXZsdU2stECb2L7 2ztjqihgJk7LYGHFat04MenFBnC6qSW0BeTSlQOX9NfpnQPcnyEilVXtX3JHCpsCfg9U 5takWcb12GGwsigyQfIMjyMm9tuKY+alReASYkueasAvnc9N7tOdLbj5ukExlKe+Vksz O/J+me4tkjR0ovYSbGLColJs3ZCKRCx7aGploTOTWxJpaX9VLHgm1Fyf9Ue+YAHaKtUV WnOe8zmQgtHE/oA+7lLv3Z8DmB6DZVFut/AVZqKk6fTCyp6YSzu4GFljkkPmsqUuwr72 tdGg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1724868894; x=1725473694; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:reply-to:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=g8CEH7SnnSsHaiw39Cx2W/imrlcDKaVqaPZKBSnVoqE=; b=H8YC3de4nWraicoVqx6xlpWQnqC9pIoHf+5RE+5qNGs8Td/5XVeYZncA8eDsAI4haK 7pxWFXoehQq5jOOpAXQvL/Wav2hzt8PzLp5SOdc/N7VDwvj6RrBkoESTElNFIev3lZXj g/Xy6MxnpvAcdR6e62rrmbUJIerqszRb3F4gFIAD9dvQFpvLw8EYz8zavEOPx3Rvfggg gjaHpV/5/XdgjU9w1pduOfemi+Uo4uwZPECvsxMysSHlou+wq9Z01PY9Urp6L/mcE9RU LrIOD8gL2AyXcDfNHoN06vn72r5isktl+BtyK6YcNvM1UQj0wfKnSKz5hS9/kRseDC6g GGmw== X-Gm-Message-State: AOJu0YzEvkpKuj+tsHFKSYXRu+g5CO9tNkAi47ZfZUtqN8w8A9WAiuKF YKIE+6TvIJ9PmTGE5wr3v0BvOjHbiFGPB2/k8z4GNY8Mt3D5ZDy5AQcCDuvyTn8otz4BBRgitxq CGQ== X-Google-Smtp-Source: AGHT+IHNeuQKAVCun7jGA+4F4NSXhmCTAL6hk/rIp4B1oyD7UZlqqmU2e4hcnFIXLWY3/GKLuflJ7LotGs0= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a25:e304:0:b0:e11:7112:6b9b with SMTP id 3f1490d57ef6-e1a5ab3d138mr6965276.3.1724868893779; Wed, 28 Aug 2024 11:14:53 -0700 (PDT) Reply-To: Sean Christopherson Date: Wed, 28 Aug 2024 11:14:46 -0700 In-Reply-To: <20240828181446.652474-1-seanjc@google.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240828181446.652474-1-seanjc@google.com> X-Mailer: git-send-email 2.46.0.295.g3b9ea8a38a-goog Message-ID: <20240828181446.652474-3-seanjc@google.com> Subject: [PATCH v2 2/2] KVM: Clean up coalesced MMIO ring full check From: Sean Christopherson To: Paolo Bonzini , Paul Walmsley , Palmer Dabbelt , Albert Ou Cc: kvm@vger.kernel.org, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Ilias Stamatis , Marc Zyngier , Oliver Upton , Anup Patel , Sean Christopherson , Paul Durrant Fold coalesced_mmio_has_room() into its sole caller, coalesced_mmio_write(), as it's really just a single line of code, has a goofy return value, and is unnecessarily brittle. E.g. if coalesced_mmio_has_room() were to check ring->last directly, or the caller failed to use READ_ONCE(), KVM would be susceptible to TOCTOU attacks from userspace. Opportunistically add a comment explaining why on earth KVM leaves one entry free, which may not be obvious to readers that aren't familiar with ring buffers. No functional change intended. Reviewed-by: Ilias Stamatis Cc: Paul Durrant Signed-off-by: Sean Christopherson --- virt/kvm/coalesced_mmio.c | 29 ++++++++--------------------- 1 file changed, 8 insertions(+), 21 deletions(-) diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c index 184c5c40c9c1..375d6285475e 100644 --- a/virt/kvm/coalesced_mmio.c +++ b/virt/kvm/coalesced_mmio.c @@ -40,25 +40,6 @@ static int coalesced_mmio_in_range(struct kvm_coalesced_mmio_dev *dev, return 1; } -static int coalesced_mmio_has_room(struct kvm_coalesced_mmio_dev *dev, u32 last) -{ - struct kvm_coalesced_mmio_ring *ring; - - /* Are we able to batch it ? */ - - /* last is the first free entry - * check if we don't meet the first used entry - * there is always one unused entry in the buffer - */ - ring = dev->kvm->coalesced_mmio_ring; - if ((last + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) { - /* full */ - return 0; - } - - return 1; -} - static int coalesced_mmio_write(struct kvm_vcpu *vcpu, struct kvm_io_device *this, gpa_t addr, int len, const void *val) @@ -72,9 +53,15 @@ static int coalesced_mmio_write(struct kvm_vcpu *vcpu, spin_lock(&dev->kvm->ring_lock); + /* + * last is the index of the entry to fill. Verify userspace hasn't + * set last to be out of range, and that there is room in the ring. + * Leave one entry free in the ring so that userspace can differentiate + * between an empty ring and a full ring. + */ insert = READ_ONCE(ring->last); - if (!coalesced_mmio_has_room(dev, insert) || - insert >= KVM_COALESCED_MMIO_MAX) { + if (insert >= KVM_COALESCED_MMIO_MAX || + (insert + 1) % KVM_COALESCED_MMIO_MAX == READ_ONCE(ring->first)) { spin_unlock(&dev->kvm->ring_lock); return -EOPNOTSUPP; }