From: Jakub Kicinski Date: Mon, 16 Oct 2017 18:16:55 +0000 (-0700) Subject: bpf: disallow arithmetic operations on context pointer X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=28e33f9d78ee;p=GitHub%2FLineageOS%2Fandroid_kernel_motorola_exynos9610.git bpf: disallow arithmetic operations on context pointer Commit f1174f77b50c ("bpf/verifier: rework value tracking") removed the crafty selection of which pointer types are allowed to be modified. This is OK for most pointer types since adjust_ptr_min_max_vals() will catch operations on immutable pointers. One exception is PTR_TO_CTX which is now allowed to be offseted freely. The intent of aforementioned commit was to allow context access via modified registers. The offset passed to ->is_valid_access() verifier callback has been adjusted by the value of the variable offset. What is missing, however, is taking the variable offset into account when the context register is used. Or in terms of the code adding the offset to the value passed to the ->convert_ctx_access() callback. This leads to the following eBPF user code: r1 += 68 r0 = *(u32 *)(r1 + 8) exit being translated to this in kernel space: 0: (07) r1 += 68 1: (61) r0 = *(u32 *)(r1 +180) 2: (95) exit Offset 8 is corresponding to 180 in the kernel, but offset 76 is valid too. Verifier will "accept" access to offset 68+8=76 but then "convert" access to offset 8 as 180. Effective access to offset 248 is beyond the kernel context. (This is a __sk_buff example on a debug-heavy kernel - packet mark is 8 -> 180, 76 would be data.) Dereferencing the modified context pointer is not as easy as dereferencing other types, because we have to translate the access to reading a field in kernel structures which is usually at a different offset and often of a different size. To allow modifying the pointer we would have to make sure that given eBPF instruction will always access the same field or the fields accessed are "compatible" in terms of offset and size... Disallow dereferencing modified context pointers and add to selftests the test case described here. Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Signed-off-by: Jakub Kicinski Acked-by: Daniel Borkmann Acked-by: Alexei Starovoitov Acked-by: Edward Cree Signed-off-by: David S. Miller --- diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8b8d6ba39e23..20f3889c006e 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1116,7 +1116,12 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn /* ctx accesses must be at a fixed offset, so that we can * determine what type of data were returned. */ - if (!tnum_is_const(reg->var_off)) { + if (reg->off) { + verbose("dereference of modified ctx ptr R%d off=%d+%d, ctx+const is allowed, ctx+const+const is not\n", + regno, reg->off, off - reg->off); + return -EACCES; + } + if (!tnum_is_const(reg->var_off) || reg->var_off.value) { char tn_buf[48]; tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); @@ -1124,7 +1129,6 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn tn_buf, off, size); return -EACCES; } - off += reg->var_off.value; err = check_ctx_access(env, insn_idx, off, size, t, ®_type); if (!err && t == BPF_READ && value_regno >= 0) { /* ctx access returns either a scalar, or a diff --git a/tools/testing/selftests/bpf/test_verifier.c b/tools/testing/selftests/bpf/test_verifier.c index 26f3250bdcd2..3c7d3a45a3c5 100644 --- a/tools/testing/selftests/bpf/test_verifier.c +++ b/tools/testing/selftests/bpf/test_verifier.c @@ -6645,6 +6645,20 @@ static struct bpf_test tests[] = { .errstr = "BPF_END uses reserved fields", .result = REJECT, }, + { + "arithmetic ops make PTR_TO_CTX unusable", + .insns = { + BPF_ALU64_IMM(BPF_ADD, BPF_REG_1, + offsetof(struct __sk_buff, data) - + offsetof(struct __sk_buff, mark)), + BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, + offsetof(struct __sk_buff, mark)), + BPF_EXIT_INSN(), + }, + .errstr = "dereference of modified ctx ptr R1 off=68+8, ctx+const is allowed, ctx+const+const is not", + .result = REJECT, + .prog_type = BPF_PROG_TYPE_SCHED_CLS, + }, }; static int probe_filter_length(const struct bpf_insn *fp)