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This changes the semantics of BPF_NOSPEC (previously a v4-only barrier) to always emit a speculation barrier that works against both Spectre v1 AND v4. If mitigation is not needed on an architecture, the backend should set bpf_jit_bypass_spec_v4/v1(). As of now, this commit only has the user-visible implication that unpriv BPF's performance on PowerPC is reduced. This is the case because we have to emit additional v1 barrier instructions for BPF_NOSPEC now. This commit is required for a future commit to allow us to rely on BPF_NOSPEC for Spectre v1 mitigation. As of this commit, the feature that nospec acts as a v1 barrier is unused. Commitf5e81d1117("bpf: Introduce BPF nospec instruction for mitigating Spectre v4") noted that mitigation instructions for v1 and v4 might be different on some archs. While this would potentially offer improved performance on PowerPC, it was dismissed after the following considerations: * Only having one barrier simplifies the verifier and allows us to easily rely on v4-induced barriers for reducing the complexity of v1-induced speculative path verification. * For the architectures that implemented BPF_NOSPEC, only PowerPC has distinct instructions for v1 and v4. Even there, some insns may be shared between the barriers for v1 and v4 (e.g., 'ori 31,31,0' and 'sync'). If this is still found to impact performance in an unacceptable way, BPF_NOSPEC can be split into BPF_NOSPEC_V1 and BPF_NOSPEC_V4 later. As an optimization, we can already skip v1/v4 insns from being emitted for PowerPC with this setup if bypass_spec_v1/v4 is set. Vulnerability-status for BPF_NOSPEC-based Spectre mitigations (v4 as of this commit, v1 in the future) is therefore: * x86 (32-bit and 64-bit), ARM64, and PowerPC (64-bit): Mitigated - This patch implements BPF_NOSPEC for these architectures. The previous v4-only version was supported since commitf5e81d1117("bpf: Introduce BPF nospec instruction for mitigating Spectre v4") and commitb7540d6250("powerpc/bpf: Emit stf barrier instruction sequences for BPF_NOSPEC"). * LoongArch: Not Vulnerable - Commita6f6a95f25("LoongArch, bpf: Fix jit to skip speculation barrier opcode") is the only other past commit related to BPF_NOSPEC and indicates that the insn is not required there. * MIPS: Vulnerable (if unprivileged BPF is enabled) - Commit a6f6a95f2580 ("LoongArch, bpf: Fix jit to skip speculation barrier opcode") indicates that it is not vulnerable, but this contradicts the kernel and Debian documentation. Therefore, I assume that there exist vulnerable MIPS CPUs (but maybe not from Loongson?). In the future, BPF_NOSPEC could be implemented for MIPS based on the GCC speculation_barrier [1]. For now, we rely on unprivileged BPF being disabled by default. * Other: Unknown - To the best of my knowledge there is no definitive information available that indicates that any other arch is vulnerable. They are therefore left untouched (BPF_NOSPEC is not implemented, but bypass_spec_v1/v4 is also not set). I did the following testing to ensure the insn encoding is correct: * ARM64: * 'dsb nsh; isb' was successfully tested with the BPF CI in [2] * 'sb' locally using QEMU v7.2.15 -cpu max (emitted sb insn is executed for example with './test_progs -t verifier_array_access') * PowerPC: The following configs were tested locally with ppc64le QEMU v8.2 '-machine pseries -cpu POWER9': * STF_BARRIER_EIEIO + CONFIG_PPC_BOOK32_64 * STF_BARRIER_SYNC_ORI (forced on) + CONFIG_PPC_BOOK32_64 * STF_BARRIER_FALLBACK (forced on) + CONFIG_PPC_BOOK32_64 * CONFIG_PPC_E500 (forced on) + STF_BARRIER_EIEIO * CONFIG_PPC_E500 (forced on) + STF_BARRIER_SYNC_ORI (forced on) * CONFIG_PPC_E500 (forced on) + STF_BARRIER_FALLBACK (forced on) * CONFIG_PPC_E500 (forced on) + STF_BARRIER_NONE (forced on) Most of those cobinations should not occur in practice, but I was not able to get an PPC e6500 rootfs (for testing PPC_E500 without forcing it on). In any case, this should ensure that there are no unexpected conflicts between the insns when combined like this. Individual v1/v4 barriers were already emitted elsewhere. Hari's ack is for the PowerPC changes only. [1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=29b74545531f6afbee9fc38c267524326dbfbedf ("MIPS: Add speculation_barrier support") [2] https://github.com/kernel-patches/bpf/pull/8576 Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de> Acked-by: Hari Bathini <hbathini@linux.ibm.com> Cc: Henriette Herzog <henriette.herzog@rub.de> Cc: Maximilian Ott <ott@cs.fau.de> Cc: Milan Stephan <milan.stephan@fau.de> Link: https://lore.kernel.org/r/20250603211703.337860-1-luis.gerhorst@fau.de Signed-off-by: Alexei Starovoitov <ast@kernel.org>