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Date: Thu, 29 Jun 2017 03:04:59 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: davem@...emloft.net
Cc: ast@...com, kafai@...com, john.fastabend@...il.com,
netdev@...r.kernel.org, Daniel Borkmann <daniel@...earbox.net>
Subject: [PATCH net] bpf: prevent leaking pointer via xadd on unpriviledged
Leaking kernel addresses on unpriviledged is generally disallowed,
for example, verifier rejects the following:
0: (b7) r0 = 0
1: (18) r2 = 0xffff897e82304400
3: (7b) *(u64 *)(r1 +48) = r2
R2 leaks addr into ctx
Doing pointer arithmetic on them is also forbidden, so that they
don't turn into unknown value and then get leaked out. However,
there's xadd as a special case, where we don't check the src reg
for being a pointer register, e.g. the following will pass:
0: (b7) r0 = 0
1: (7b) *(u64 *)(r1 +48) = r0
2: (18) r2 = 0xffff897e82304400 ; map
4: (db) lock *(u64 *)(r1 +48) += r2
5: (95) exit
We could store the pointer into skb->cb, loose the type context,
and then read it out from there again to leak it eventually out
of a map value. Or more easily in a different variant, too:
0: (bf) r6 = r1
1: (7a) *(u64 *)(r10 -8) = 0
2: (bf) r2 = r10
3: (07) r2 += -8
4: (18) r1 = 0x0
6: (85) call bpf_map_lookup_elem#1
7: (15) if r0 == 0x0 goto pc+3
R0=map_value(ks=8,vs=8,id=0),min_value=0,max_value=0 R6=ctx R10=fp
8: (b7) r3 = 0
9: (7b) *(u64 *)(r0 +0) = r3
10: (db) lock *(u64 *)(r0 +0) += r6
11: (b7) r0 = 0
12: (95) exit
from 7 to 11: R0=inv,min_value=0,max_value=0 R6=ctx R10=fp
11: (b7) r0 = 0
12: (95) exit
Prevent this by checking xadd src reg for pointer types. Also
add a couple of test cases related to this.
Fixes: 1be7f75d1668 ("bpf: enable non-root eBPF programs")
Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
---
kernel/bpf/verifier.c | 5 +++
tools/testing/selftests/bpf/test_verifier.c | 66 +++++++++++++++++++++++++++++
2 files changed, 71 insertions(+)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 339c8a1..a8a7256 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -989,6 +989,11 @@ static int check_xadd(struct bpf_verifier_env *env, struct bpf_insn *insn)
if (err)
return err;
+ if (is_pointer_value(env, insn->src_reg)) {
+ verbose("R%d leaks addr into mem\n", insn->src_reg);
+ return -EACCES;
+ }
+
/* check whether atomic_add can read the memory */
err = check_mem_access(env, insn->dst_reg, insn->off,
BPF_SIZE(insn->code), BPF_READ, -1);
diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c
index cabb19b..0ff8c55 100644
--- a/tools/testing/selftests/bpf/test_verifier.c
+++ b/tools/testing/selftests/bpf/test_verifier.c
@@ -3749,6 +3749,72 @@ struct test_val {
.errstr = "invalid bpf_context access",
},
{
+ "leak pointer into ctx 1",
+ .insns = {
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_0,
+ offsetof(struct __sk_buff, cb[0])),
+ BPF_LD_MAP_FD(BPF_REG_2, 0),
+ BPF_STX_XADD(BPF_DW, BPF_REG_1, BPF_REG_2,
+ offsetof(struct __sk_buff, cb[0])),
+ BPF_EXIT_INSN(),
+ },
+ .fixup_map1 = { 2 },
+ .errstr_unpriv = "R2 leaks addr into mem",
+ .result_unpriv = REJECT,
+ .result = ACCEPT,
+ },
+ {
+ "leak pointer into ctx 2",
+ .insns = {
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_0,
+ offsetof(struct __sk_buff, cb[0])),
+ BPF_STX_XADD(BPF_DW, BPF_REG_1, BPF_REG_10,
+ offsetof(struct __sk_buff, cb[0])),
+ BPF_EXIT_INSN(),
+ },
+ .errstr_unpriv = "R10 leaks addr into mem",
+ .result_unpriv = REJECT,
+ .result = ACCEPT,
+ },
+ {
+ "leak pointer into ctx 3",
+ .insns = {
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_LD_MAP_FD(BPF_REG_2, 0),
+ BPF_STX_MEM(BPF_DW, BPF_REG_1, BPF_REG_2,
+ offsetof(struct __sk_buff, cb[0])),
+ BPF_EXIT_INSN(),
+ },
+ .fixup_map1 = { 1 },
+ .errstr_unpriv = "R2 leaks addr into ctx",
+ .result_unpriv = REJECT,
+ .result = ACCEPT,
+ },
+ {
+ "leak pointer into map val",
+ .insns = {
+ BPF_MOV64_REG(BPF_REG_6, BPF_REG_1),
+ BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+ BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
+ BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -8),
+ BPF_LD_MAP_FD(BPF_REG_1, 0),
+ BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0,
+ BPF_FUNC_map_lookup_elem),
+ BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 3),
+ BPF_MOV64_IMM(BPF_REG_3, 0),
+ BPF_STX_MEM(BPF_DW, BPF_REG_0, BPF_REG_3, 0),
+ BPF_STX_XADD(BPF_DW, BPF_REG_0, BPF_REG_6, 0),
+ BPF_MOV64_IMM(BPF_REG_0, 0),
+ BPF_EXIT_INSN(),
+ },
+ .fixup_map1 = { 4 },
+ .errstr_unpriv = "R6 leaks addr into mem",
+ .result_unpriv = REJECT,
+ .result = ACCEPT,
+ },
+ {
"helper access to map: full range",
.insns = {
BPF_MOV64_REG(BPF_REG_2, BPF_REG_10),
--
1.9.3
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