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Date: Thu, 8 Sep 2016 01:03:42 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: davem@...emloft.net
Cc: alexei.starovoitov@...il.com, tgraf@...g.ch,
netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net-next] bpf: fix range propagation on direct packet access
LLVM can generate code that tests for direct packet access via
skb->data/data_end in a way that currently gets rejected by the
verifier, example:
[...]
7: (61) r3 = *(u32 *)(r6 +80)
8: (61) r9 = *(u32 *)(r6 +76)
9: (bf) r2 = r9
10: (07) r2 += 54
11: (3d) if r3 >= r2 goto pc+12
R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv R6=ctx
R9=pkt(id=0,off=0,r=0) R10=fp
12: (18) r4 = 0xffffff7a
14: (05) goto pc+430
[...]
from 11 to 24: R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv
R6=ctx R9=pkt(id=0,off=0,r=0) R10=fp
24: (7b) *(u64 *)(r10 -40) = r1
25: (b7) r1 = 0
26: (63) *(u32 *)(r6 +56) = r1
27: (b7) r2 = 40
28: (71) r8 = *(u8 *)(r9 +20)
invalid access to packet, off=20 size=1, R9(id=0,off=0,r=0)
The reason why this gets rejected despite a proper test is that we
currently call find_good_pkt_pointers() only in case where we detect
tests like rX > pkt_end, where rX is of type pkt(id=Y,off=Z,r=0) and
derived, for example, from a register of type pkt(id=Y,off=0,r=0)
pointing to skb->data. find_good_pkt_pointers() then fills the range
in the current branch to pkt(id=Y,off=0,r=Z) on success.
For above case, we need to extend that to recognize pkt_end >= rX
pattern and mark the other branch that is taken on success with the
appropriate pkt(id=Y,off=0,r=Z) type via find_good_pkt_pointers().
Since eBPF operates on BPF_JGT (>) and BPF_JGE (>=), these are the
only two practical options to test for from what LLVM could have
generated, since there's no such thing as BPF_JLT (<) or BPF_JLE (<=)
that we would need to take into account as well.
After the fix:
[...]
7: (61) r3 = *(u32 *)(r6 +80)
8: (61) r9 = *(u32 *)(r6 +76)
9: (bf) r2 = r9
10: (07) r2 += 54
11: (3d) if r3 >= r2 goto pc+12
R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv R6=ctx
R9=pkt(id=0,off=0,r=0) R10=fp
12: (18) r4 = 0xffffff7a
14: (05) goto pc+430
[...]
from 11 to 24: R1=inv R2=pkt(id=0,off=54,r=54) R3=pkt_end R4=inv
R6=ctx R9=pkt(id=0,off=0,r=54) R10=fp
24: (7b) *(u64 *)(r10 -40) = r1
25: (b7) r1 = 0
26: (63) *(u32 *)(r6 +56) = r1
27: (b7) r2 = 40
28: (71) r8 = *(u8 *)(r9 +20)
29: (bf) r1 = r8
30: (25) if r8 > 0x3c goto pc+47
R1=inv56 R2=imm40 R3=pkt_end R4=inv R6=ctx R8=inv56
R9=pkt(id=0,off=0,r=54) R10=fp
31: (b7) r1 = 1
[...]
Verifier test cases are also added in this work, one that demonstrates
the mentioned example here and one that tries a bad packet access for
the current/fall-through branch (the one with types pkt(id=X,off=Y,r=0),
pkt(id=X,off=0,r=0)), then a case with good and bad accesses, and two
with both test variants (>, >=).
Fixes: 969bf05eb3ce ("bpf: direct packet access")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
Acked-by: Alexei Starovoitov <ast@...nel.org>
---
Targetting at net-next as this should be just fine.
kernel/bpf/verifier.c | 55 +++++++++++++++++-------
samples/bpf/test_verifier.c | 102 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 142 insertions(+), 15 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 48c2705..90493a6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1637,21 +1637,42 @@ static int check_alu_op(struct verifier_env *env, struct bpf_insn *insn)
return 0;
}
-static void find_good_pkt_pointers(struct verifier_env *env,
- struct reg_state *dst_reg)
+static void find_good_pkt_pointers(struct verifier_state *state,
+ const struct reg_state *dst_reg)
{
- struct verifier_state *state = &env->cur_state;
struct reg_state *regs = state->regs, *reg;
int i;
- /* r2 = r3;
- * r2 += 8
- * if (r2 > pkt_end) goto somewhere
- * r2 == dst_reg, pkt_end == src_reg,
- * r2=pkt(id=n,off=8,r=0)
- * r3=pkt(id=n,off=0,r=0)
- * find register r3 and mark its range as r3=pkt(id=n,off=0,r=8)
- * so that range of bytes [r3, r3 + 8) is safe to access
+
+ /* LLVM can generate two kind of checks:
+ *
+ * Type 1:
+ *
+ * r2 = r3;
+ * r2 += 8;
+ * if (r2 > pkt_end) goto <handle exception>
+ * <access okay>
+ *
+ * Where:
+ * r2 == dst_reg, pkt_end == src_reg
+ * r2=pkt(id=n,off=8,r=0)
+ * r3=pkt(id=n,off=0,r=0)
+ *
+ * Type 2:
+ *
+ * r2 = r3;
+ * r2 += 8;
+ * if (pkt_end >= r2) goto <access okay>
+ * <handle exception>
+ *
+ * Where:
+ * pkt_end == dst_reg, r2 == src_reg
+ * r2=pkt(id=n,off=8,r=0)
+ * r3=pkt(id=n,off=0,r=0)
+ *
+ * Find register r3 and mark its range as r3=pkt(id=n,off=0,r=8)
+ * so that range of bytes [r3, r3 + 8) is safe to access.
*/
+
for (i = 0; i < MAX_BPF_REG; i++)
if (regs[i].type == PTR_TO_PACKET && regs[i].id == dst_reg->id)
regs[i].range = dst_reg->off;
@@ -1668,8 +1689,8 @@ static void find_good_pkt_pointers(struct verifier_env *env,
static int check_cond_jmp_op(struct verifier_env *env,
struct bpf_insn *insn, int *insn_idx)
{
- struct reg_state *regs = env->cur_state.regs, *dst_reg;
- struct verifier_state *other_branch;
+ struct verifier_state *other_branch, *this_branch = &env->cur_state;
+ struct reg_state *regs = this_branch->regs, *dst_reg;
u8 opcode = BPF_OP(insn->code);
int err;
@@ -1750,13 +1771,17 @@ static int check_cond_jmp_op(struct verifier_env *env,
} else if (BPF_SRC(insn->code) == BPF_X && opcode == BPF_JGT &&
dst_reg->type == PTR_TO_PACKET &&
regs[insn->src_reg].type == PTR_TO_PACKET_END) {
- find_good_pkt_pointers(env, dst_reg);
+ find_good_pkt_pointers(this_branch, dst_reg);
+ } else if (BPF_SRC(insn->code) == BPF_X && opcode == BPF_JGE &&
+ dst_reg->type == PTR_TO_PACKET_END &&
+ regs[insn->src_reg].type == PTR_TO_PACKET) {
+ find_good_pkt_pointers(other_branch, ®s[insn->src_reg]);
} else if (is_pointer_value(env, insn->dst_reg)) {
verbose("R%d pointer comparison prohibited\n", insn->dst_reg);
return -EACCES;
}
if (log_level)
- print_verifier_state(&env->cur_state);
+ print_verifier_state(this_branch);
return 0;
}
diff --git a/samples/bpf/test_verifier.c b/samples/bpf/test_verifier.c
index 78c6f13..1f6cc9b 100644
--- a/samples/bpf/test_verifier.c
+++ b/samples/bpf/test_verifier.c
@@ -1529,6 +1529,108 @@ static struct bpf_test tests[] = {
.prog_type = BPF_PROG_TYPE_SCHED_CLS,
},
{
+ "direct packet access: test5 (pkt_end >= reg, good access)",
+ .insns = {
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+ offsetof(struct __sk_buff, data)),
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+ offsetof(struct __sk_buff, data_end)),
+ BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),
+ BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_0, 2),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_EXIT_INSN(),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .result = ACCEPT,
+ .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+ },
+ {
+ "direct packet access: test6 (pkt_end >= reg, bad access)",
+ .insns = {
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+ offsetof(struct __sk_buff, data)),
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+ offsetof(struct __sk_buff, data_end)),
+ BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),
+ BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_0, 3),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_EXIT_INSN(),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .errstr = "invalid access to packet",
+ .result = REJECT,
+ .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+ },
+ {
+ "direct packet access: test7 (pkt_end >= reg, both accesses)",
+ .insns = {
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+ offsetof(struct __sk_buff, data)),
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+ offsetof(struct __sk_buff, data_end)),
+ BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),
+ BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_0, 3),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_EXIT_INSN(),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .errstr = "invalid access to packet",
+ .result = REJECT,
+ .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+ },
+ {
+ "direct packet access: test8 (double test, variant 1)",
+ .insns = {
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+ offsetof(struct __sk_buff, data)),
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+ offsetof(struct __sk_buff, data_end)),
+ BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),
+ BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_0, 4),
+ BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_3, 1),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_EXIT_INSN(),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .result = ACCEPT,
+ .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+ },
+ {
+ "direct packet access: test9 (double test, variant 2)",
+ .insns = {
+ BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
+ offsetof(struct __sk_buff, data)),
+ BPF_LDX_MEM(BPF_W, BPF_REG_3, BPF_REG_1,
+ offsetof(struct __sk_buff, data_end)),
+ BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 8),
+ BPF_JMP_REG(BPF_JGE, BPF_REG_3, BPF_REG_0, 2),
+ BPF_MOV64_IMM(BPF_REG_0, 1),
+ BPF_EXIT_INSN(),
+ BPF_JMP_REG(BPF_JGT, BPF_REG_0, BPF_REG_3, 1),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_LDX_MEM(BPF_B, BPF_REG_0, BPF_REG_2, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .result = ACCEPT,
+ .prog_type = BPF_PROG_TYPE_SCHED_CLS,
+ },
+ {
"helper access to packet: test1, valid packet_ptr range",
.insns = {
BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1,
--
1.9.3
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