diff mbox series

[bpf-next,11/11] bpf: Fall back to nospec for spec path verification

Message ID 20250313175312.1120183-2-luis.gerhorst@fau.de (mailing list archive)
State New
Delegated to: BPF
Headers show
Series bpf: Mitigate Spectre v1 using barriers | expand

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bpf/vmtest-bpf-next-PR success PR summary
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netdev/ynl success Generated files up to date; no warnings/errors; no diff in generated;
netdev/fixes_present success Fixes tag not required for -next series
netdev/header_inline success No static functions without inline keyword in header files
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netdev/check_selftest success No net selftest shell script
netdev/verify_fixes success No Fixes tag
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netdev/checkpatch warning WARNING: line length of 90 exceeds 80 columns
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Commit Message

Luis Gerhorst March 13, 2025, 5:53 p.m. UTC 
This trades verification complexity for runtime overheads due to the
nospec inserted because of the EINVAL.

With increased limits this allows applying mitigations to large BPF
progs such as the Parca Continuous Profiler's prog. However, this
requires a jump-seq limit of 256k. In any case, the same principle
should apply to smaller programs therefore include it even if the limit
stays at 8k for now. Most programs in [1] only require a limit of 32k.

[1] https://arxiv.org/pdf/2405.00078 ("VeriFence: Lightweight and
    Precise Spectre Defenses for Untrusted Linux Kernel Extensions")

Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Acked-by: Henriette Herzog <henriette.herzog@rub.de>
Cc: Maximilian Ott <ott@cs.fau.de>
Cc: Milan Stephan <milan.stephan@fau.de>
---
 kernel/bpf/verifier.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)
diff mbox series

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 03af82f52a02..49c7e2608ccd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -187,6 +187,7 @@  struct bpf_verifier_stack_elem {
 };
 
 #define BPF_COMPLEXITY_LIMIT_JMP_SEQ	8192
+#define BPF_COMPLEXITY_LIMIT_SPEC_V1_VERIFICATION	(BPF_COMPLEXITY_LIMIT_JMP_SEQ / 2)
 #define BPF_COMPLEXITY_LIMIT_STATES	64
 
 #define BPF_MAP_KEY_POISON	(1ULL << 63)
@@ -2010,6 +2011,19 @@  static struct bpf_verifier_state *push_stack(struct bpf_verifier_env *env,
 	struct bpf_verifier_stack_elem *elem;
 	int err;
 
+	if (!env->bypass_spec_v1 &&
+	    cur->speculative &&
+	    env->stack_size > BPF_COMPLEXITY_LIMIT_SPEC_V1_VERIFICATION) {
+		/* Avoiding nested speculative path verification because we are
+		 * close to exceeding the jump sequence complexity limit. Will
+		 * instead insert a speculation barrier which will impact
+		 * performace. To improve performance, authors should reduce the
+		 * program's complexity. Barrier will be inserted in
+		 * do_check().
+		 */
+		return ERR_PTR(-EINVAL);
+	}
+
 	elem = kzalloc(sizeof(struct bpf_verifier_stack_elem), GFP_KERNEL);
 	if (!elem) {
 		err = -ENOMEM;