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Wed, 18 Dec 2019 08:17:57 -0800 (PST) Received: from xz-x1 ([104.156.64.74]) by smtp.gmail.com with ESMTPSA id o16sm769610qkj.91.2019.12.18.08.17.55 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 18 Dec 2019 08:17:56 -0800 (PST) Date: Wed, 18 Dec 2019 11:17:55 -0500 From: Peter Xu To: Sean Christopherson Cc: James Hogan , Paul Mackerras , Christian Borntraeger , Janosch Frank , Paolo Bonzini , Marc Zyngier , linux-arm-kernel@lists.infradead.org, Wanpeng Li , Philippe =?utf-8?Q?Mathieu-Daud=C3=A9?= , kvm@vger.kernel.org, David Hildenbrand , Joerg Roedel , Cornelia Huck , linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org, kvm-ppc@vger.kernel.org, Vitaly Kuznetsov , kvmarm@lists.cs.columbia.edu, Jim Mattson Subject: Re: [PATCH v4 07/19] KVM: Explicitly free allocated-but-unused dirty bitmap Message-ID: <20191218161755.GB26669@xz-x1> References: <20191217204041.10815-1-sean.j.christopherson@intel.com> <20191217204041.10815-8-sean.j.christopherson@intel.com> <20191217222446.GK7258@xz-x1> <20191217225118.GF11771@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20191217225118.GF11771@linux.intel.com> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Tue, Dec 17, 2019 at 02:51:18PM -0800, Sean Christopherson wrote: > On Tue, Dec 17, 2019 at 05:24:46PM -0500, Peter Xu wrote: > > On Tue, Dec 17, 2019 at 12:40:29PM -0800, Sean Christopherson wrote: > > > Explicitly free an allocated-but-unused dirty bitmap instead of relying > > > on kvm_free_memslot() if an error occurs in __kvm_set_memory_region(). > > > There is no longer a need to abuse kvm_free_memslot() to free arch > > > specific resources as arch specific code is now called only after the > > > common flow is guaranteed to succeed. Arch code can still fail, but > > > it's responsible for its own cleanup in that case. > > > > > > Eliminating the error path's abuse of kvm_free_memslot() paves the way > > > for simplifying kvm_free_memslot(), i.e. dropping its @dont param. > > > > > > Signed-off-by: Sean Christopherson > > > --- > > > virt/kvm/kvm_main.c | 7 ++++--- > > > 1 file changed, 4 insertions(+), 3 deletions(-) > > > > > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > > > index d403e93e3028..6b2261a9e139 100644 > > > --- a/virt/kvm/kvm_main.c > > > +++ b/virt/kvm/kvm_main.c > > > @@ -1096,7 +1096,7 @@ int __kvm_set_memory_region(struct kvm *kvm, > > > > > > slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT); > > > if (!slots) > > > - goto out_free; > > > + goto out_bitmap; > > > memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots)); > > > > > > if ((change == KVM_MR_DELETE) || (change == KVM_MR_MOVE)) { > > > @@ -1144,8 +1144,9 @@ int __kvm_set_memory_region(struct kvm *kvm, > > > if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) > > > slots = install_new_memslots(kvm, as_id, slots); > > > kvfree(slots); > > > -out_free: > > > - kvm_free_memslot(kvm, &new, &old); > > > +out_bitmap: > > > + if (new.dirty_bitmap && !old.dirty_bitmap) > > > + kvm_destroy_dirty_bitmap(&new); > > > > What if both the old and new have KVM_MEM_LOG_DIRTY_PAGES set? > > kvm_free_memslot() did cover that but I see that you explicitly > > dropped it. Could I ask why? Thanks, > > In that case, old.dirty_bitmap == new.dirty_bitmap, i.e. shouldn't be freed > by this error path since doing so would result in a use-after-free via the > old memslot. > > The kvm_free_memslot() logic is the same, albeit in a very twisted way. Yes it is. :) > > In __kvm_set_memory_region(), @old and @new start with the same dirty_bitmap. > > new = old = *slot; > > And @new is modified based on KVM_MEM_LOG_DIRTY_PAGES. If LOG_DIRTY_PAGES > is set in both @new and @old, then both the "if" and "else if" evaluate > false, i.e. new.dirty_bitmap == old.dirty_bitmap. > > /* Allocate/free page dirty bitmap as needed */ > if (!(new.flags & KVM_MEM_LOG_DIRTY_PAGES)) > new.dirty_bitmap = NULL; > else if (!new.dirty_bitmap) { > r = kvm_create_dirty_bitmap(&new); > if (r) > return r; > } > > Subbing "@free <= @new" and "@dont <= @old" in kvm_free_memslot() > > static void kvm_free_memslot(struct kvm *kvm, struct kvm_memory_slot *free, > struct kvm_memory_slot *dont) > { > if (!dont || free->dirty_bitmap != dont->dirty_bitmap) > kvm_destroy_dirty_bitmap(free); > > > yeids this, since @old is obviously non-NULL > > if (new.dirty_bitmap != old.dirty_bitmap) > kvm_destroy_dirty_bitmap(&new); > > The dirty_bitmap allocation logic guarantees that new.dirty_bitmap is > a) NULL (the "if" case") > b) != old.dirty_bitmap iff old.dirty_bitmap == NULL (the "else if" case) > c) == old.dirty_bitmap (the implicit "else" case). > > kvm_free_memslot() frees @new.dirty_bitmap iff its != @old.dirty_bitmap, > thus the explicit destroy only needs to check for (b). Thanks for explaining with such a detail. Reviewed-by: Peter Xu -- Peter Xu