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: 7c693f54 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS") Reported-by:José Oliveira <joseloliveira11@gmail.com> Reported-by:
Rodrigo Branco <rodrigo@kernelhacking.com> Signed-off-by:
KP Singh <kpsingh@kernel.org> Signed-off-by:
Borislav Petkov (AMD) <bp@alien8.de> Cc: stable@vger.kernel.org Link: https://lore.kernel.org/r/20230220120127.1975241-1-kpsingh@kernel.org Link: https://lore.kernel.org/r/20230221184908.2349578-1-kpsingh@kernel.org
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