Commit 06ef0ccb authored Dec 18, 2017 by Yonghong Song Committed by Daniel Borkmann Dec 19, 2017

bpf/cgroup: fix a verification error for a CGROUP_DEVICE type prog



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>

parent c060bc61

include/uapi/linux/bpf.h

+2 −1

Original line number	Diff line number	Diff line
		@@ -1012,7 +1012,8 @@ 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;
		};

kernel/bpf/cgroup.c

+13 −2

Original line number	Diff line number	Diff line
		@@ -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))

		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;
		}

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