On Mon, Feb 27, 2023 at 07:05:40AM +0100, KP Singh wrote:
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).
Fixes: 7c693f54c873 ("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 Cc: stable@vger.kernel.org Signed-off-by: KP Singh kpsingh@kernel.org
arch/x86/kernel/cpu/bugs.c | 26 +++++++++++++++++++------- 1 file changed, 19 insertions(+), 7 deletions(-)
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