netdev - [PATCH bpf-next] bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var

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Date:   Fri, 29 Oct 2021 09:31:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     davem@...emloft.net
Cc:     daniel@...earbox.net, andrii@...nel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...com
Subject: [PATCH bpf-next] bpf: Fix propagation of bounds from 64-bit min/max into 32-bit and var_off.

From: Alexei Starovoitov <ast@...nel.org>

Before this fix:
166: (b5) if r2 <= 0x1 goto pc+22
from 166 to 189: R2=invP(id=1,umax_value=1,var_off=(0x0; 0xffffffff))

After this fix:
166: (b5) if r2 <= 0x1 goto pc+22
from 166 to 189: R2=invP(id=1,umax_value=1,var_off=(0x0; 0x1))

While processing BPF_JLE the reg_set_min_max() would set true_reg->umax_value = 1
and call __reg_combine_64_into_32(true_reg).

Without the fix it would not pass the condition:
if (__reg64_bound_u32(reg->umin_value) && __reg64_bound_u32(reg->umax_value))

since umin_value == 0 at this point.
Before commit 10bf4e83167c the umin was incorrectly ingored.
The commit 10bf4e83167c fixed the correctness issue, but pessimized
propagation of 64-bit min max into 32-bit min max and corresponding var_off.

Fixes: 10bf4e83167c ("bpf: Fix propagation of 32 bit unsigned bounds from 64 bit bounds")
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
---
 kernel/bpf/verifier.c                               | 2 +-
 tools/testing/selftests/bpf/verifier/array_access.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 3c8aa7df1773..29671ed49ee8 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1425,7 +1425,7 @@ static bool __reg64_bound_s32(s64 a)
 
 static bool __reg64_bound_u32(u64 a)
 {
-	return a > U32_MIN && a < U32_MAX;
+	return a >= U32_MIN && a <= U32_MAX;
 }
 
 static void __reg_combine_64_into_32(struct bpf_reg_state *reg)
diff --git a/tools/testing/selftests/bpf/verifier/array_access.c b/tools/testing/selftests/bpf/verifier/array_access.c
index 1b1c798e9248..1b138cd2b187 100644
--- a/tools/testing/selftests/bpf/verifier/array_access.c
+++ b/tools/testing/selftests/bpf/verifier/array_access.c
@@ -186,7 +186,7 @@
 	},
 	.fixup_map_hash_48b = { 3 },
 	.errstr_unpriv = "R0 leaks addr",
-	.errstr = "R0 unbounded memory access",
+	.errstr = "invalid access to map value, value_size=48 off=44 size=8",
 	.result_unpriv = REJECT,
 	.result = REJECT,
 	.flags = F_NEEDS_EFFICIENT_UNALIGNED_ACCESS,
-- 
2.30.2