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[SRU,Bionic,1/3] bpf: Simplify alu_limit masking for pointer arithmetic

Message ID 20210324004337.170117-2-cascardo@canonical.com
State New
Headers show
Series (LP: #1920995) selftests: bpf verifier fails after sanitize_ptr_alu fixes | expand

Commit Message

Thadeu Lima de Souza Cascardo March 24, 2021, 12:43 a.m. UTC 
From: Piotr Krysiuk <piotras@gmail.com>

BugLink: https://bugs.launchpad.net/bugs/1920995

Instead of having the mov32 with aux->alu_limit - 1 immediate, move this
operation to retrieve_ptr_limit() instead to simplify the logic and to
allow for subsequent sanity boundary checks inside retrieve_ptr_limit().
This avoids in future that at the time of the verifier masking rewrite
we'd run into an underflow which would not sign extend due to the nature
of mov32 instruction.

Signed-off-by: Piotr Krysiuk <piotras@gmail.com>
Co-developed-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
(cherry picked from commit b5871dca250cd391885218b99cc015aca1a51aea net.git)
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
---
 kernel/bpf/verifier.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)
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Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4dfe5c6ba576..a9086c4a03b5 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1994,16 +1994,16 @@  static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
 	case PTR_TO_STACK:
 		off = ptr_reg->off + ptr_reg->var_off.value;
 		if (mask_to_left)
-			*ptr_limit = MAX_BPF_STACK + off + 1;
+			*ptr_limit = MAX_BPF_STACK + off;
 		else
-			*ptr_limit = -off;
+			*ptr_limit = -off - 1;
 		return 0;
 	case PTR_TO_MAP_VALUE:
 		if (mask_to_left) {
-			*ptr_limit = ptr_reg->umax_value + ptr_reg->off + 1;
+			*ptr_limit = ptr_reg->umax_value + ptr_reg->off;
 		} else {
 			off = ptr_reg->smin_value + ptr_reg->off;
-			*ptr_limit = ptr_reg->map_ptr->value_size - off;
+			*ptr_limit = ptr_reg->map_ptr->value_size - off - 1;
 		}
 		return 0;
 	default:
@@ -4924,7 +4924,7 @@  static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			off_reg = issrc ? insn->src_reg : insn->dst_reg;
 			if (isneg)
 				*patch++ = BPF_ALU64_IMM(BPF_MUL, off_reg, -1);
-			*patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit - 1);
+			*patch++ = BPF_MOV32_IMM(BPF_REG_AX, aux->alu_limit);
 			*patch++ = BPF_ALU64_REG(BPF_SUB, BPF_REG_AX, off_reg);
 			*patch++ = BPF_ALU64_REG(BPF_OR, BPF_REG_AX, off_reg);
 			*patch++ = BPF_ALU64_IMM(BPF_NEG, BPF_REG_AX, 0);