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Shutemov" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20231030_171823_524687_BBD26888 X-CRM114-Status: GOOD ( 33.53 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Oct 31, 2023, Paolo Bonzini wrote: > On 10/30/23 21:25, Sean Christopherson wrote: > > > Probably worth adding a check on valid flags here. > > > > Definitely needed. There's a very real bug here. But rather than duplicate flags > > checking or plumb @ioctl all the way to __kvm_set_memory_region(), now that we > > have the fancy guard(mutex) and there are no internal calls to kvm_set_memory_region(), > > what if we: > > > > 1. Acquire/release slots_lock in __kvm_set_memory_region() > > 2. Call kvm_set_memory_region() from x86 code for the internal memslots > > 3. Disallow *any* flags for internal memslots > > 4. Open code check_memory_region_flags in kvm_vm_ioctl_set_memory_region() > > I dislike this step, there is a clear point where all paths meet > (ioctl/internal, locked/unlocked) and that's __kvm_set_memory_region(). > I think that's the place where flags should be checked. (I don't mind > the restriction on internal memslots; it's just that to me it's not a > particularly natural way to structure the checks). Yeah, I just don't like the discrepancy it causes where some flags are explicitly checked and allowed, allowed and then later disallowed. > On the other hand, the place where to protect from out-of-bounds > accesses, is the place where you stop caring about struct > kvm_userspace_memory_region vs kvm_userspace_memory_region2 (and > your code gets it right, by dropping "ioctl" as soon as possible). > > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c > index 87f45aa91ced..fe5a2af14fff 100644 > --- a/virt/kvm/kvm_main.c > +++ b/virt/kvm/kvm_main.c > @@ -1635,6 +1635,14 @@ bool __weak kvm_arch_dirty_log_supported(struct kvm *kvm) > return true; > } > +/* > + * Flags that do not access any of the extra space of struct > + * kvm_userspace_memory_region2. KVM_SET_USER_MEMORY_REGION_FLAGS > + * only allows these. > + */ > +#define KVM_SET_USER_MEMORY_REGION_FLAGS \ Can we name this KVM_SET_USER_MEMORY_REGION_LEGACY_FLAGS, or something equally horrific? As is, this sounds way too much like a generic "allowed flags for any memory region". Or maybe invert the macro? I.e. something to make it more obvious that it's effectively a versioning check, not a generic "what's supported?" check. #define KVM_SET_USER_MEMORY_FLAGS_V2_ONLY \ (~(KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY)) > + (KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY) > + > static int check_memory_region_flags(struct kvm *kvm, > const struct kvm_userspace_memory_region2 *mem) > { > @@ -5149,10 +5149,16 @@ static long kvm_vm_ioctl(struct file *filp, > struct kvm_userspace_memory_region2 mem; > unsigned long size; > - if (ioctl == KVM_SET_USER_MEMORY_REGION) > + if (ioctl == KVM_SET_USER_MEMORY_REGION) { > + /* > + * Fields beyond struct kvm_userspace_memory_region shouldn't be > + * accessed, but avoid leaking kernel memory in case of a bug. > + */ > + memset(&mem, 0, sizeof(mem)); > size = sizeof(struct kvm_userspace_memory_region); > - else > + } else { > size = sizeof(struct kvm_userspace_memory_region2); > + } > /* Ensure the common parts of the two structs are identical. */ > SANITY_CHECK_MEM_REGION_FIELD(slot); > @@ -5165,6 +5167,11 @@ static long kvm_vm_ioctl(struct file *filp, > if (copy_from_user(&mem, argp, size)) > goto out; > + r = -EINVAL; > + if (ioctl == KVM_SET_USER_MEMORY_REGION && > + (mem->flags & ~KVM_SET_USER_MEMORY_REGION_FLAGS)) > + goto out; > + > r = kvm_vm_ioctl_set_memory_region(kvm, &mem); > break; > } > > > That's a kind of patch that you can't really get wrong (though I have > the brown paper bag ready). > > Maintainance-wise it's fine, since flags are being added at a pace of > roughly one every five years, Heh, true. > and anyway it's also future proof: I placed the #define near > check_memory_region_flags so that in five years we remember to keep it up to > date. But worst case, the new flags will only be allowed by > KVM_SET_USER_MEMORY_REGION2 unnecessarily; there are no security issues > waiting to bite us. > > In sum, this is exactly the only kind of fix that should be in the v13->v14 > delta. Boiling the ocean can be fun too ;-) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel