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Subject[PATCH 4.19 086/103] bpf: improve verifier branch analysis
Date
4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

[ commit 4f7b3e82589e0de723780198ec7983e427144c0a upstream ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/verifier.c | 93 +++++++++++++++++++++++++++++++++++++------
1 file changed, 80 insertions(+), 13 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 341806668f03..3d093003c723 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3475,6 +3475,79 @@ 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.
@@ -3868,21 +3941,15 @@ 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;
--
2.19.1


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