[PATCH for 4.1,4.4] bpf: fix incorrect sign extension in check_alu_op()

Jann Horn
2018-03-19T16:55:58Z

commit 95a762e2c8c942780948091f8f2a4f32fce1ac6f upstream.

Distinguish between
BPF_ALU64|BPF_MOV|BPF_K (load 32-bit immediate, sign-extended to 64-bit)
and BPF_ALU|BPF_MOV|BPF_K (load 32-bit immediate, zero-padded to 64-bit);
only perform sign extension in the first case.

This patch differs from the mainline one because the verifier's internals
have changed in the meantime. Mainline tracks register values as 64-bit
values; however, 4.4 still stores tracked register values as 32-bit
values with sign extension. Therefore, in the case of a 32-bit op with
negative immediate, the value can't be tracked; leave the register as
UNKNOWN_VALUE (set by the preceding check_reg_arg() call).

I have manually tested this patch on top of 4.4.122. For the following BPF
bytecode:

BPF_MOV64_IMM(BPF_REG_1, 1),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 1, 1),
BPF_EXIT_INSN(),

BPF_MOV32_IMM(BPF_REG_1, 1),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, 1, 1),
BPF_EXIT_INSN(),

BPF_MOV64_IMM(BPF_REG_1, -1),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, -1, 1),
BPF_EXIT_INSN(),

BPF_MOV32_IMM(BPF_REG_1, -1),
BPF_JMP_IMM(BPF_JEQ, BPF_REG_1, -1, 2),
BPF_MOV32_IMM(BPF_REG_0, 42),
BPF_EXIT_INSN(),

BPF_MOV32_IMM(BPF_REG_0, 43),
BPF_EXIT_INSN()

Verifier output on 4.4.122 without this patch:

0: (b7) r1 = 1
1: (15) if r1 == 0x1 goto pc+1
3: (b4) (u32) r1 = (u32) 1
4: (15) if r1 == 0x1 goto pc+1
6: (b7) r1 = -1
7: (15) if r1 == 0xffffffff goto pc+1
9: (b4) (u32) r1 = (u32) -1
10: (15) if r1 == 0xffffffff goto pc+2
13: (b4) (u32) r0 = (u32) 43
14: (95) exit

Verifier output on 4.4.122+ with this patch:

0: (b7) r1 = 1
1: (15) if r1 == 0x1 goto pc+1
3: (b4) (u32) r1 = (u32) 1
4: (15) if r1 == 0x1 goto pc+1
6: (b7) r1 = -1
7: (15) if r1 == 0xffffffff goto pc+1
9: (b4) (u32) r1 = (u32) -1
10: (15) if r1 == 0xffffffff goto pc+2
R1=inv R10=fp
11: (b4) (u32) r0 = (u32) 42
12: (95) exit

from 10 to 13: R1=imm-1 R10=fp
13: (b4) (u32) r0 = (u32) 43
14: (95) exit

This patch should be applied to linux-4.4.y and linux-4.1.y.

Signed-off-by: Jann Horn <jannh@google.com>
---
kernel/bpf/verifier.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c14003840bc5..79e3c21a35d0 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1135,7 +1135,8 @@ static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn)
regs[insn->dst_reg].type = UNKNOWN_VALUE;
regs[insn->dst_reg].map_ptr = NULL;
}
- } else {
+ } else if (BPF_CLASS(insn->code) == BPF_ALU64 ||
+ insn->imm >= 0) {
/* case: R = imm
* remember the value we stored into this reg
*/
--
2.16.2.804.g6dcf76e118-goog