CVE-2024-50115 — KVM: nSVM: Ignore nCR3[4:0] when loading PDPTEs from memory

Affected: Linux

In the Linux kernel, the following vulnerability has been resolved:

KVM: nSVM: Ignore nCR3[4:0] when loading PDPTEs from memory

Ignore nCR3[4:0] when loading PDPTEs from memory for nested SVM, as bits 4:0 of CR3 are ignored when PAE paging is used, and thus VMRUN doesn't enforce 32-byte alignment of nCR3.

In the absolute worst case scenario, failure to ignore bits 4:0 can result in an out-of-bounds read, e.g. if the target page is at the end of a memslot, and the VMM isn't using guard pages.

Per the APM:

The CR3 register points to the base address of the page-directory-pointer table. The page-directory-pointer table is aligned on a 32-byte boundary, with the low 5 address bits 4:0 assumed to be 0.

And the SDM's much more explicit:

4:0 Ignored

Note, KVM gets this right when loading PDPTRs, it's only the nSVM flow that is broken.

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CVE-2024-50115 — KVM: nSVM: Ignore nCR3[4:0] when loading PDPTEs from memory