From: Daniel Borkmann
mainline inclusion
from mainline-v5.13-rc4
commit a7036191277f9fa68d92f2071ddc38c09b1e5ee5
category: bugfix
issue: #I421HX
CVE: CVE-2021-33200
--------------------------------
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
Reviewed-by: Piotr Krysiuk
Acked-by: Alexei Starovoitov
Signed-off-by: Yang Yingliang
Reviewed-by: Kuohai Xu
Reviewed-by: Xiu Jianfeng
Signed-off-by: Yang Yingliang
Signed-off-by: Yu Changchun
---
kernel/bpf/verifier.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2c37b5f27..740732f6b 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
--
2.25.1
