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Subject[PATCH AUTOSEL for 4.15 45/78] bpf/cgroup: fix a verification error for a CGROUP_DEVICE type prog
Date
From: Yonghong Song <yhs@fb.com>

[ Upstream commit 06ef0ccb5a36e1feba9b413ff59a04ecc4407c1c ]

The tools/testing/selftests/bpf test program
test_dev_cgroup fails with the following error
when compiled with llvm 6.0. (I did not try
with earlier versions.)

libbpf: load bpf program failed: Permission denied
libbpf: -- BEGIN DUMP LOG ---
libbpf:
0: (61) r2 = *(u32 *)(r1 +4)
1: (b7) r0 = 0
2: (55) if r2 != 0x1 goto pc+8
R0=inv0 R1=ctx(id=0,off=0,imm=0) R2=inv1 R10=fp0
3: (69) r2 = *(u16 *)(r1 +0)
invalid bpf_context access off=0 size=2
...

The culprit is the following statement in dev_cgroup.c:
short type = ctx->access_type & 0xFFFF;
This code is typical as the ctx->access_type is assigned
as below in kernel/bpf/cgroup.c:
struct bpf_cgroup_dev_ctx ctx = {
.access_type = (access << 16) | dev_type,
.major = major,
.minor = minor,
};

The compiler converts it to u16 access while
the verifier cgroup_dev_is_valid_access rejects
any non u32 access.

This patch permits the field access_type to be accessible
with type u16 and u8 as well.

Signed-off-by: Yonghong Song <yhs@fb.com>
Tested-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
include/uapi/linux/bpf.h | 3 ++-
kernel/bpf/cgroup.c | 15 +++++++++++++--
2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 4c223ab30293..04b2f613dd06 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -995,7 +995,8 @@ struct bpf_perf_event_value {
#define BPF_DEVCG_DEV_CHAR (1ULL << 1)

struct bpf_cgroup_dev_ctx {
- __u32 access_type; /* (access << 16) | type */
+ /* access_type encoded as (BPF_DEVCG_ACC_* << 16) | BPF_DEVCG_DEV_* */
+ __u32 access_type;
__u32 major;
__u32 minor;
};
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index b789ab78d28f..c1c0b60d3f2f 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -568,6 +568,8 @@ static bool cgroup_dev_is_valid_access(int off, int size,
enum bpf_access_type type,
struct bpf_insn_access_aux *info)
{
+ const int size_default = sizeof(__u32);
+
if (type == BPF_WRITE)
return false;

@@ -576,8 +578,17 @@ static bool cgroup_dev_is_valid_access(int off, int size,
/* The verifier guarantees that size > 0. */
if (off % size != 0)
return false;
- if (size != sizeof(__u32))
- return false;
+
+ switch (off) {
+ case bpf_ctx_range(struct bpf_cgroup_dev_ctx, access_type):
+ bpf_ctx_record_field_size(info, size_default);
+ if (!bpf_ctx_narrow_access_ok(off, size, size_default))
+ return false;
+ break;
+ default:
+ if (size != size_default)
+ return false;
+ }

return true;
}
--
2.14.1
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