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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: Anatoly Trosinenko <anatoly.trosinenko@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Alexei Starovoitov 2018-12-03 22:46:05 -08:00 committed by Daniel Borkmann
parent c3494801cd
commit 4f7b3e8258
2 changed files with 82 additions and 15 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

93
kernel/bpf/verifier.c
View File

@ -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;

4
tools/testing/selftests/bpf/test_verifier.c
View File

@ -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(),
},