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    SubjectRe: [RFC][PATCH 4/4] security: Enforce limitations on return values from LSMs
    On Tue, Nov 15, 2022 at 12:58 PM Roberto Sassu
    <roberto.sassu@huaweicloud.com> wrote:
    >
    > From: Roberto Sassu <roberto.sassu@huawei.com>
    >
    > LSMs should not be able to return arbitrary return values, as the callers
    > of the LSM infrastructure might not be ready to handle unexpected values
    > (e.g. positive values that are first converted to a pointer with ERR_PTR,
    > and then evaluated with IS_ERR()).
    >
    > Modify call_int_hook() to call is_ret_value_allowed(), so that the return
    > value from each LSM for a given hook is checked. If for the interval the
    > return value falls into the corresponding flag is not set, change the
    > return value to the default value, just for the current LSM.
    >
    > A misbehaving LSM would not have impact on the decision of other LSMs, as
    > the loop terminates whenever the return value is not zero.
    >
    > Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
    > ---
    > security/security.c | 34 ++++++++++++++++++++++++++++++++++
    > 1 file changed, 34 insertions(+)

    Casey touched on some of this in his reply to patch 0/4, but basically
    I see this as a BPF LSM specific problem and not a generalized LSM
    issue that should be addressed at the LSM layer. Especially if the
    solution involves incurring additional processing for every LSM hook
    instantiation, regardless if a BPF LSM is present. Reading your
    overall patchset description I believe that you understand this too.

    If you want to somehow instrument the LSM hook definitions (what I
    believe to be the motivation behind patch 3/4) to indicate valid
    return values for use by the BPF verifier, I think we could entertain
    that, or at least discuss it further, but I'm not inclined to support
    any runtime overhead at the LSM layer for a specific LSM.

    --
    paul-moore.com

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