1/5/2024 0 Comments Kodi for mac 10.12.5![]() ![]() ![]() Unfortunately, making recursive pagetable promotion and demotion operations restartable is incredibly complicated, and the code contains several races which, if triggered, can cause Xen to drop or retain extra type counts, potentially allowing guests to get write access to in-use pagetables. These operations may take an arbitrarily large amount of time, and so must be re-startable. Xen also allows for "recursive" promotions: i.e., an operating system promoting a page to an L4 pagetable may end up causing pages to be promoted to 元s, which may in turn cause pages to be promoted to L2s, and so on. In order to prevent the guest from modifying these page tables directly, Xen keeps track of how pages are used using a type system pages must be "promoted" before being used as a pagetable, and "demoted" before being used for any other type. To avoid using shadow pagetables for PV guests, Xen exposes the actual hardware pagetables to the guest. There are issues with restartable PV type change operations. This was fixed without realizing the impact in 4.18 with the following three commits, though it's believed the first is the only strictly necessary commit: 6f597c6b63b6 ("KVM: PPC: Book3S PR: Add guest MSR parameter for kvmppc_save_tm()/kvmppc_restore_tm()") 7b0e827c6970 ("KVM: PPC: Book3S HV: Factor fake-suspend handling out of kvmppc_save/restore_tm") 009c872a8bc4 ("KVM: PPC: Book3S PR: Move kvmppc_save_tm/kvmppc_restore_tm to separate file")Īn issue was discovered in Xen through 4.12.x allowing x86 PV guest OS users to gain host OS privileges by leveraging race conditions in pagetable promotion and demotion operations. There were two commits that, according to the reporter, introduced the vulnerability: f024ee098476 ("KVM: PPC: Book3S HV: Pull out TM state save/restore into separate procedures") 87a11bb6a7f7 ("KVM: PPC: Book3S HV: Work around XER bug in fake suspend mode") The former landed in 4.8, the latter in 4.17. Because of this, an attacker with the ability run code in kernel space of a guest VM can cause the host kernel to panic. KVM in the Linux kernel on Power8 processors has a conflicting use of HSTATE_HOST_R1 to store r1 state in kvmppc_hv_entry plus in kvmppc_tm, leading to a stack corruption.
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