From: Ilya Leoshkevich <iii@linux.ibm.com>
To: Daniel Borkmann <daniel@iogearbox.net>,
Alexei Starovoitov <ast@kernel.org>
Cc: bpf@vger.kernel.org, Heiko Carstens <heiko.carstens@de.ibm.com>,
Vasily Gorbik <gor@linux.ibm.com>, Andrey Ignatov <rdna@fb.com>,
Ilya Leoshkevich <iii@linux.ibm.com>
Subject: [PATCH bpf] bpf: allow narrow loads of bpf_sysctl fields with offset > 0
Date: Mon, 28 Oct 2019 13:29:02 +0100 [thread overview]
Message-ID: <20191028122902.9763-1-iii@linux.ibm.com> (raw)
"ctx:file_pos sysctl:read read ok narrow" works on s390 by accident: it
reads the wrong byte, which happens to have the expected value of 0.
Improve the test by seeking to the 4th byte and expecting 4 instead of
0.
This makes the latent problem apparent: the test attempts to read the
first byte of bpf_sysctl.file_pos, assuming this is the least-significant
byte, which is not the case on big-endian machines: a non-zero offset is
needed.
The point of the test is to verify narrow loads, so we cannot cheat our
way out by simply using BPF_W. The existence of the test means that such
loads have to be supported, most likely because llvm can generate them.
Fix the test by adding a big-endian variant, which uses an offset to
access the least-significant byte of bpf_sysctl.file_pos.
This reveals the final problem: verifier rejects accesses to bpf_sysctl
fields with offset > 0. Such accesses are already allowed for a wide
range of structs: __sk_buff, bpf_sock_addr and sk_msg_md to name a few.
Extend this support to bpf_sysctl by using bpf_ctx_range instead of
offsetof when matching field offsets.
Fixes: 7b146cebe30c ("bpf: Sysctl hook")
Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx")
Fixes: 9a1027e52535 ("selftests/bpf: Test file_pos field in bpf_sysctl ctx")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
kernel/bpf/cgroup.c | 4 ++--
tools/testing/selftests/bpf/test_sysctl.c | 8 +++++++-
2 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index ddd8addcdb5c..a3eaf08e7dd3 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1311,12 +1311,12 @@ static bool sysctl_is_valid_access(int off, int size, enum bpf_access_type type,
return false;
switch (off) {
- case offsetof(struct bpf_sysctl, write):
+ case bpf_ctx_range(struct bpf_sysctl, write):
if (type != BPF_READ)
return false;
bpf_ctx_record_field_size(info, size_default);
return bpf_ctx_narrow_access_ok(off, size, size_default);
- case offsetof(struct bpf_sysctl, file_pos):
+ case bpf_ctx_range(struct bpf_sysctl, file_pos):
if (type == BPF_READ) {
bpf_ctx_record_field_size(info, size_default);
return bpf_ctx_narrow_access_ok(off, size, size_default);
diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c
index a320e3844b17..7c6e5b173f33 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -161,9 +161,14 @@ static struct sysctl_test tests[] = {
.descr = "ctx:file_pos sysctl:read read ok narrow",
.insns = {
/* If (file_pos == X) */
+#if __BYTE_ORDER == __LITTLE_ENDIAN
BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_1,
offsetof(struct bpf_sysctl, file_pos)),
- BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 0, 2),
+#else
+ BPF_LDX_MEM(BPF_B, BPF_REG_7, BPF_REG_1,
+ offsetof(struct bpf_sysctl, file_pos) + 3),
+#endif
+ BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 4, 2),
/* return ALLOW; */
BPF_MOV64_IMM(BPF_REG_0, 1),
@@ -176,6 +181,7 @@ static struct sysctl_test tests[] = {
.attach_type = BPF_CGROUP_SYSCTL,
.sysctl = "kernel/ostype",
.open_flags = O_RDONLY,
+ .seek = 4,
.result = SUCCESS,
},
{
--
2.23.0
next reply other threads:[~2019-10-28 12:29 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-28 12:29 Ilya Leoshkevich [this message]
2019-10-28 23:59 ` [PATCH bpf] bpf: allow narrow loads of bpf_sysctl fields with offset > 0 Andrey Ignatov
2019-10-29 4:36 ` Andrii Nakryiko
2019-10-29 14:19 ` Ilya Leoshkevich
2019-10-29 15:16 ` Andrey Ignatov
2019-10-29 17:39 ` Andrii Nakryiko
2019-10-30 19:54 ` Alexei Starovoitov
2019-10-29 15:19 ` Andrey Ignatov
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