Fix CVE-2018-15594: x86/paravirt ensure Spectre-v2 mitigation patching works for paravirt guests #115
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Background
On paravirtual guests we could fail to rewrite certain indirect calls (e.g., CALLEE_SAVE paravirt functions like pv_queued_spin_unlock()), leaving indirect calls in place and weakening the Spectre-v2 mitigation that relies on avoiding indirect branches; this is tracked as CVE-2018-15594.
Changes
In arch/x86/kernel/paravirt.c, remove the incorrect clobber test in paravirt_patch_call() (the patch rewrites an indirect call into a direct call to the same function so clobbers cannot differ), and add WARN_ONCE diagnostics for the other patch failure case (insufficient room for the instruction) under CONFIG_RETPOLINE.
Security Impact
Restores intended Spectre-v2 hardening for paravirtual guests by ensuring indirect calls are properly patched, addressing CVE-2018-15594.
References
Upstream commit: 5800dc5c19f34e6e03b5adab1282535cb102fafd; CVE: CVE-2018-15594