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Subject[PATCH 5.3 032/135] bpf: Allow narrow loads of bpf_sysctl fields with offset > 0
Date
From: Ilya Leoshkevich <iii@linux.ibm.com>

[ Upstream commit 7541c87c9b7a7e07c84481f37f2c19063b44469b ]

"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>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrey Ignatov <rdna@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20191028122902.9763-1-iii@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
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 0a00eaca6fae6..9ad7cd3267f52 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1302,12 +1302,12 @@ 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 a3bebd7c68ddc..c938f1767ca72 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -158,9 +158,14 @@ 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),
@@ -173,6 +178,7 @@ static struct sysctl_test tests[] = {
.attach_type = BPF_CGROUP_SYSCTL,
.sysctl = "kernel/ostype",
.open_flags = O_RDONLY,
+ .seek = 4,
.result = SUCCESS,
},
{
--
2.20.1


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