This improves the expressiveness of unprivileged BPF by inserting speculation barriers instead of rejecting the programs.
The approach was previously presented at LPC'24 [1] and RAID'24 [2].
To mitigate the Spectre v1 (PHT) vulnerability, the kernel rejects potentially-dangerous unprivileged BPF programs as of commit 9183671af6db ("bpf: Fix leakage under speculation on mispredicted branches"). In [2], we have analyzed 364 object files from open source projects (Linux Samples and Selftests, BCC, Loxilb, Cilium, libbpf Examples, Parca, and Prevail) and found that this affects 31% to 54% of programs.
To resolve this in the majority of cases this patchset adds a fall-back for mitigating Spectre v1 using speculation barriers. The kernel still optimistically attempts to verify all speculative paths but uses speculation barriers against v1 when unsafe behavior is detected. This allows for more programs to be accepted without disabling the BPF Spectre mitigations (e.g., by setting cpu_mitigations_off()).
For this, it relies on the fact that speculation barriers prevent all later instructions if the speculation was not correct:
* On x86_64, lfence acts as full speculation barrier, not only as a load fence [3]:
An LFENCE instruction or a serializing instruction will ensure that no later instructions execute, even speculatively, until all prior instructions complete locally. [...] Inserting an LFENCE instruction after a bounds check prevents later operations from executing before the bound check completes.
This was experimentally confirmed in [4].
* ARM's SB speculation barrier instruction also affects "any instruction that appears later in the program order than the barrier" [5].
In [1] we have measured the overhead of this approach relative to having mitigations off and including the upstream Spectre v4 mitigations. For event tracing and stack-sampling profilers, we found that mitigations increase BPF program execution time by 0% to 62%. For the Loxilb network load balancer, we have measured a 14% slowdown in SCTP performance but no significant slowdown for TCP. This overhead only applies to programs that were previously rejected.
I reran the expressiveness-evaluation with v6.14 and made sure the main results still match those from [1] and [2] (which used v6.5).
Main design decisions are:
* Do not use separate bytecode insns for v1 and v4 barriers. This simplifies the verifier significantly and has the only downside that performance on PowerPC is not as high as it could be.
* Allow archs to still disable v1/v4 mitigations separately by setting bpf_jit_bypass_spec_v1/v4(). This has the benefit that archs can benefit from improved BPF expressiveness / performance if they are not vulnerable (e.g., ARM64 for v4 in the kernel).
* Do not remove the empty BPF_NOSPEC implementation for backends for which it is unknown whether they are vulnerable to Spectre v1.
[1] https://lpc.events/event/18/contributions/1954/ ("Mitigating Spectre-PHT using Speculation Barriers in Linux eBPF") [2] https://arxiv.org/pdf/2405.00078 ("VeriFence: Lightweight and Precise Spectre Defenses for Untrusted Linux Kernel Extensions") [3] https://www.intel.com/content/www/us/en/developer/articles/technical/softwar... ("Managed Runtime Speculative Execution Side Channel Mitigations") [4] https://dl.acm.org/doi/pdf/10.1145/3359789.3359837 ("Speculator: a tool to analyze speculative execution attacks and mitigations" - Section 4.6 "Stopping Speculative Execution") [5] https://developer.arm.com/documentation/ddi0597/2020-12/Base-Instructions/SB... ("SB - Speculation Barrier - Arm Armv8-A A32/T32 Instruction Set Architecture (2020-12)")
Changes:
* v1 -> v2: - Drop former commits 9 ("bpf: Return PTR_ERR from push_stack()") and 11 ("bpf: Fall back to nospec for spec path verification") as suggested by Alexei. This series therefore no longer changes push_stack() to return PTR_ERR. - Add detailed explanation of how lfence works internally and how it affects the algorithm. - Add tests checking that nospec instructions are inserted in expected locations using __xlated_unpriv as suggested by Eduard (also, include a fix for __xlated_unpriv) - Add a test for the mitigations from the description of commit 9183671af6db ("bpf: Fix leakage under speculation on mispredicted branches") - Remove unused variables from do_check[_insn]() as suggested by Eduard. - Remove INSN_IDX_MODIFIED to improve readability as suggested by Eduard. This also causes the nospec_result-check to run (and fail) for jumping-ops. Add a warning to assert that this check must never succeed in that case. - Add details on the safety of patch 10 ("bpf: Allow nospec-protected var-offset stack access") based on the feedback on v1. - Rebase to bpf-next-250420 - Link to v1: https://lore.kernel.org/all/20250313172127.1098195-1-luis.gerhorst@fau.de/
* RFC -> v1: - rebase to bpf-next-250313 - tests: mark expected successes/new errors - add bpt_jit_bypass_spec_v1/v4() to avoid #ifdef in bpf_bypass_spec_v1/v4() - ensure that nospec with v1-support is implemented for archs for which GCC supports speculation barriers, except for MIPS - arm64: emit speculation barrier - powerpc: change nospec to include v1 barrier - discuss potential security (archs that do not impl. BPF nospec) and performance (only PowerPC) regressions - Linkt to RFC: https://lore.kernel.org/bpf/20250224203619.594724-1-luis.gerhorst@fau.de/
Luis Gerhorst (11): selftests/bpf: Fix caps for __xlated/jited_unpriv bpf: Move insn if/else into do_check_insn() bpf: Return -EFAULT on misconfigurations bpf: Return -EFAULT on internal errors bpf, arm64, powerpc: Add bpf_jit_bypass_spec_v1/v4() bpf, arm64, powerpc: Change nospec to include v1 barrier bpf: Rename sanitize_stack_spill to nospec_result bpf: Fall back to nospec for Spectre v1 selftests/bpf: Add test for Spectre v1 mitigation bpf: Allow nospec-protected var-offset stack access bpf: Fall back to nospec for sanitization-failures
arch/arm64/net/bpf_jit.h | 5 + arch/arm64/net/bpf_jit_comp.c | 28 +- arch/powerpc/net/bpf_jit_comp64.c | 79 ++- include/linux/bpf.h | 11 +- include/linux/bpf_verifier.h | 3 +- include/linux/filter.h | 2 +- kernel/bpf/core.c | 32 +- kernel/bpf/verifier.c | 648 ++++++++++-------- tools/testing/selftests/bpf/progs/bpf_misc.h | 4 + .../selftests/bpf/progs/verifier_and.c | 8 +- .../selftests/bpf/progs/verifier_bounds.c | 66 +- .../bpf/progs/verifier_bounds_deduction.c | 45 +- .../selftests/bpf/progs/verifier_map_ptr.c | 20 +- .../selftests/bpf/progs/verifier_movsx.c | 16 +- .../selftests/bpf/progs/verifier_unpriv.c | 65 +- .../bpf/progs/verifier_value_ptr_arith.c | 101 ++- tools/testing/selftests/bpf/test_loader.c | 14 +- .../selftests/bpf/verifier/dead_code.c | 3 +- tools/testing/selftests/bpf/verifier/jmp32.c | 33 +- tools/testing/selftests/bpf/verifier/jset.c | 10 +- 20 files changed, 765 insertions(+), 428 deletions(-)
base-commit: 8582d9ab3efdebb88e0cd8beed8e0b9de76443e7