[SRU Focal/Groovy 1/3] bpf: Simplify alu_limit masking for pointer arithmetic

[Thadeu Lima de Souza Cascardo]

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

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 99b1ae0f7f24..aaec49eb4f87 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4923,16 +4923,16 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg,
 		 */
 		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:
@@ -10147,7 +10147,7 @@ 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);
-- 
2.27.0