Sean Christopherson 771dfb3c53 KVM: x86: Protect userspace MSR filter with SRCU, and set atomically-ish ... [ Upstream commit b318e8decf6b9ef1bcf4ca06fae6d6a2cb5d5c5c ] Fix a plethora of issues with MSR filtering by installing the resulting filter as an atomic bundle instead of updating the live filter one range at a time. The KVM_X86_SET_MSR_FILTER ioctl() isn't truly atomic, as the hardware MSR bitmaps won't be updated until the next VM-Enter, but the relevant software struct is atomically updated, which is what KVM really needs. Similar to the approach used for modifying memslots, make arch.msr_filter a SRCU-protected pointer, do all the work configuring the new filter outside of kvm->lock, and then acquire kvm->lock only when the new filter has been vetted and created. That way vCPU readers either see the old filter or the new filter in their entirety, not some half-baked state. Yuan Yao pointed out a use-after-free in ksm_msr_allowed() due to a TOCTOU bug, but that's just the tip of the iceberg... - Nothing is __rcu annotated, making it nigh impossible to audit the code for correctness. - kvm_add_msr_filter() has an unpaired smp_wmb(). Violation of kernel coding style aside, the lack of a smb_rmb() anywhere casts all code into doubt. - kvm_clear_msr_filter() has a double free TOCTOU bug, as it grabs count before taking the lock. - kvm_clear_msr_filter() also has memory leak due to the same TOCTOU bug. The entire approach of updating the live filter is also flawed. While installing a new filter is inherently racy if vCPUs are running, fixing the above issues also makes it trivial to ensure certain behavior is deterministic, e.g. KVM can provide deterministic behavior for MSRs with identical settings in the old and new filters. An atomic update of the filter also prevents KVM from getting into a half-baked state, e.g. if installing a filter fails, the existing approach would leave the filter in a half-baked state, having already committed whatever bits of the filter were already processed. [*] https://lkml.kernel.org/r/20210312083157.25403-1-yaoyuan0329os@gmail.com Fixes: 1a155254ff ("KVM: x86: Introduce MSR filtering") Cc: stable@vger.kernel.org Cc: Alexander Graf <graf@amazon.com> Reported-by: Yuan Yao <yaoyuan0329os@gmail.com> Signed-off-by: Sean Christopherson <seanjc@google.com> Message-Id: <20210316184436.2544875-2-seanjc@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Sasha Levin <sashal@kernel.org>		2021-03-30 14:31:53 +02:00
..
kvm	KVM: x86: Protect userspace MSR filter with SRCU, and set atomically-ish	2021-03-30 14:31:53 +02:00
uml	docs: virt: user_mode_linux_howto_v2.rst: fix a literal block markup	2020-10-16 07:28:20 +02:00
guest-halt-polling.rst	docs: virt: guest-halt-polling.txt convert to ReST	2020-02-12 20:10:08 +01:00
index.rst	Char/Misc driver patches for 5.10-rc1	2020-10-15 10:01:51 -07:00
ne_overview.rst	nitro_enclaves: Add overview documentation	2020-09-22 13:58:41 +02:00
paravirt_ops.rst