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[v2,4.19,19/19] bpf: No need to simulate speculative domain for immediates

Message ID 20210528103810.22025-20-ovidiu.panait@windriver.com
State New
Headers show
Series bpf: fix verifier selftests, add CVE-2021-29155, CVE-2021-33200 fixes | expand

Commit Message

Ovidiu Panait May 28, 2021, 10:38 a.m. UTC 
From: Daniel Borkmann <daniel@iogearbox.net>

commit a7036191277f9fa68d92f2071ddc38c09b1e5ee5 upstream

In 801c6058d14a ("bpf: Fix leakage of uninitialized bpf stack under
speculation") we replaced masking logic with direct loads of immediates
if the register is a known constant. Given in this case we do not apply
any masking, there is also no reason for the operation to be truncated
under the speculative domain.

Therefore, there is also zero reason for the verifier to branch-off and
simulate this case, it only needs to do it for unknown but bounded scalars.
As a side-effect, this also enables few test cases that were previously
rejected due to simulation under zero truncation.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Piotr Krysiuk <piotras@gmail.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
---
 kernel/bpf/verifier.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
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Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a235342507a8..1f4c88ce58de 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2874,8 +2874,12 @@  static int sanitize_ptr_alu(struct bpf_verifier_env *env,
 	/* If we're in commit phase, we're done here given we already
 	 * pushed the truncated dst_reg into the speculative verification
 	 * stack.
+	 *
+	 * Also, when register is a known constant, we rewrite register-based
+	 * operation to immediate-based, and thus do not need masking (and as
+	 * a consequence, do not need to simulate the zero-truncation either).
 	 */
-	if (commit_window)
+	if (commit_window || off_is_imm)
 		return 0;
 
 	/* Simulate and find potential out-of-bounds access under