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    Subject[PATCH 4.15 43/84] bpf/cgroup: fix a verification error for a CGROUP_DEVICE type prog
    Date
    4.15-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    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>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    include/uapi/linux/bpf.h | 3 ++-
    kernel/bpf/cgroup.c | 15 +++++++++++++--
    2 files changed, 15 insertions(+), 3 deletions(-)

    --- 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;
    };
    --- a/kernel/bpf/cgroup.c
    +++ b/kernel/bpf/cgroup.c
    @@ -568,6 +568,8 @@ static bool cgroup_dev_is_valid_access(i
    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(i
    /* 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;
    }

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