CVE-2020-8834

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_{save,restore}_tm, leading to a stack corruption. Because of this,
an attacker with the ability run code in kernel space of a guest VM can
cause the host kernel to panic. 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[SO] bug in fake suspend mode”) The
former landed in 4.8, the latter in 4.17. 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”)

Bugs

• <https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1867717&gt;

Notes