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>
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@ -1012,7 +1012,8 @@ struct bpf_perf_event_value {
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#define BPF_DEVCG_DEV_CHAR (1ULL << 1)
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struct bpf_cgroup_dev_ctx {
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__u32 access_type; /* (access << 16) | type */
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/* access_type encoded as (BPF_DEVCG_ACC_* << 16) | BPF_DEVCG_DEV_* */
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__u32 access_type;
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__u32 major;
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__u32 minor;
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};
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@ -568,6 +568,8 @@ static bool cgroup_dev_is_valid_access(int off, int size,
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enum bpf_access_type type,
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struct bpf_insn_access_aux *info)
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{
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const int size_default = sizeof(__u32);
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if (type == BPF_WRITE)
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return false;
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@ -576,8 +578,17 @@ static bool cgroup_dev_is_valid_access(int off, int size,
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/* The verifier guarantees that size > 0. */
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if (off % size != 0)
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return false;
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if (size != sizeof(__u32))
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return false;
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switch (off) {
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case bpf_ctx_range(struct bpf_cgroup_dev_ctx, access_type):
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bpf_ctx_record_field_size(info, size_default);
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if (!bpf_ctx_narrow_access_ok(off, size, size_default))
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return false;
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break;
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default:
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if (size != size_default)
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return false;
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}
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return true;
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}
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