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Commit 4f7b3e82 authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by Daniel Borkmann
Browse files

bpf: improve verifier branch analysis 



pathological bpf programs may try to force verifier to explode in
the number of branch states:
  20: (d5) if r1 s<= 0x24000028 goto pc+0
  21: (b5) if r0 <= 0xe1fa20 goto pc+2
  22: (d5) if r1 s<= 0x7e goto pc+0
  23: (b5) if r0 <= 0xe880e000 goto pc+0
  24: (c5) if r0 s< 0x2100ecf4 goto pc+0
  25: (d5) if r1 s<= 0xe880e000 goto pc+1
  26: (c5) if r0 s< 0xf4041810 goto pc+0
  27: (d5) if r1 s<= 0x1e007e goto pc+0
  28: (b5) if r0 <= 0xe86be000 goto pc+0
  29: (07) r0 += 16614
  30: (c5) if r0 s< 0x6d0020da goto pc+0
  31: (35) if r0 >= 0x2100ecf4 goto pc+0

Teach verifier to recognize always taken and always not taken branches.
This analysis is already done for == and != comparison.
Expand it to all other branches.

It also helps real bpf programs to be verified faster:
                       before  after
bpf_lb-DLB_L3.o         2003    1940
bpf_lb-DLB_L4.o         3173    3089
bpf_lb-DUNKNOWN.o       1080    1065
bpf_lxc-DDROP_ALL.o     29584   28052
bpf_lxc-DUNKNOWN.o      36916   35487
bpf_netdev.o            11188   10864
bpf_overlay.o           6679    6643
bpf_lcx_jit.o           39555   38437

Reported-by: default avatarAnatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Acked-by: default avatarEdward Cree <ecree@solarflare.com>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
parent c3494801

kernel/bpf/verifier.c

+80 −13
Original line number Diff line number Diff line
@@ -3751,6 +3751,79 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate, 
	}

}



/* compute branch direction of the expression "if (reg opcode val) goto target;"

 * and return:

 *  1 - branch will be taken and "goto target" will be executed

 *  0 - branch will not be taken and fall-through to next insn

 * -1 - unknown. Example: "if (reg < 5)" is unknown when register value range [0,10]

 */

static int is_branch_taken(struct bpf_reg_state *reg, u64 val, u8 opcode)

{

	if (__is_pointer_value(false, reg))

		return -1;



	switch (opcode) {

	case BPF_JEQ:

		if (tnum_is_const(reg->var_off))

			return !!tnum_equals_const(reg->var_off, val);

		break;

	case BPF_JNE:

		if (tnum_is_const(reg->var_off))

			return !tnum_equals_const(reg->var_off, val);

		break;

	case BPF_JGT:

		if (reg->umin_value > val)

			return 1;

		else if (reg->umax_value <= val)

			return 0;

		break;

	case BPF_JSGT:

		if (reg->smin_value > (s64)val)

			return 1;

		else if (reg->smax_value < (s64)val)

			return 0;

		break;

	case BPF_JLT:

		if (reg->umax_value < val)

			return 1;

		else if (reg->umin_value >= val)

			return 0;

		break;

	case BPF_JSLT:

		if (reg->smax_value < (s64)val)

			return 1;

		else if (reg->smin_value >= (s64)val)

			return 0;

		break;

	case BPF_JGE:

		if (reg->umin_value >= val)

			return 1;

		else if (reg->umax_value < val)

			return 0;

		break;

	case BPF_JSGE:

		if (reg->smin_value >= (s64)val)

			return 1;

		else if (reg->smax_value < (s64)val)

			return 0;

		break;

	case BPF_JLE:

		if (reg->umax_value <= val)

			return 1;

		else if (reg->umin_value > val)

			return 0;

		break;

	case BPF_JSLE:

		if (reg->smax_value <= (s64)val)

			return 1;

		else if (reg->smin_value > (s64)val)

			return 0;

		break;

	}



	return -1;

}



/* Adjusts the register min/max values in the case that the dst_reg is the

 * variable register that we are working on, and src_reg is a constant or we're

 * simply doing a BPF_K check.
@@ -4152,21 +4225,15 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env, 


	dst_reg = &regs[insn->dst_reg];



	/* detect if R == 0 where R was initialized to zero earlier */

	if (BPF_SRC(insn->code) == BPF_K &&

	    (opcode == BPF_JEQ || opcode == BPF_JNE) &&

	    dst_reg->type == SCALAR_VALUE &&

	    tnum_is_const(dst_reg->var_off)) {

		if ((opcode == BPF_JEQ && dst_reg->var_off.value == insn->imm) ||

		    (opcode == BPF_JNE && dst_reg->var_off.value != insn->imm)) {

			/* if (imm == imm) goto pc+off;

			 * only follow the goto, ignore fall-through

			 */

	if (BPF_SRC(insn->code) == BPF_K) {

		int pred = is_branch_taken(dst_reg, insn->imm, opcode);



		if (pred == 1) {

			 /* only follow the goto, ignore fall-through */

			*insn_idx += insn->off;

			return 0;

		} else {

			/* if (imm != imm) goto pc+off;

			 * only follow fall-through branch, since

		} else if (pred == 0) {

			/* only follow fall-through branch, since

			 * that's where the program will go

			 */

			return 0;

tools/testing/selftests/bpf/test_verifier.c

+2 −2
Original line number Diff line number Diff line
@@ -8576,7 +8576,7 @@ static struct bpf_test tests[] = { 
			BPF_JMP_IMM(BPF_JA, 0, 0, -7),

		},

		.fixup_map_hash_8b = { 4 },

		.errstr = "R0 invalid mem access 'inv'",

		.errstr = "unbounded min value",

		.result = REJECT,

	},

	{
@@ -10547,7 +10547,7 @@ static struct bpf_test tests[] = { 
		"check deducing bounds from const, 5",

		.insns = {

			BPF_MOV64_IMM(BPF_REG_0, 0),

			BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 0, 1),

			BPF_JMP_IMM(BPF_JSGE, BPF_REG_0, 1, 1),

			BPF_ALU64_REG(BPF_SUB, BPF_REG_0, BPF_REG_1),

			BPF_EXIT_INSN(),

		},