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x86/speculation: Allow enabling STIBP with legacy IBRS
When plain IBRS is enabled (not enhanced IBRS), the logic in spectre_v2_user_select_mitigation() determines that STIBP is not needed. The IBRS bit implicitly protects against cross-thread branch target injection. However, with legacy IBRS, the IBRS bit is cleared on returning to userspace for performance reasons which leaves userspace threads vulnerable to cross-thread branch target injection against which STIBP protects. Exclude IBRS from the spectre_v2_in_ibrs_mode() check to allow for enabling STIBP (through seccomp/prctl() by default or always-on, if selected by spectre_v2_user kernel cmdline parameter). [ bp: Massage. ] Fixes: 7c693f5 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS") Reported-by: José Oliveira <[email protected]> Reported-by: Rodrigo Branco <[email protected]> Signed-off-by: KP Singh <[email protected]> Signed-off-by: Borislav Petkov (AMD) <[email protected]> Cc: [email protected] Link: https://lore.kernel.org/r/[email protected] Link: https://lore.kernel.org/r/[email protected]
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