On 03/19/2018 05:55 PM, Jann Horn wrote:
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
Acked-by: Daniel Borkmann daniel@iogearbox.net
Looks good, thanks Jann!