linux-kernel - Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm

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Message-ID: <6af85c04-0388-026a-e4ee-08d9aad6b86a@digikod.net>
Date:   Mon, 3 Apr 2023 20:06:45 +0200
From:   Mickaël Salaün <mic@...ikod.net>
To:     Casey Schaufler <casey@...aufler-ca.com>, paul@...l-moore.com,
        linux-security-module@...r.kernel.org
Cc:     jmorris@...ei.org, keescook@...omium.org,
        john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp,
        stephen.smalley.work@...il.com, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org
Subject: Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an
 lsm_ctx


On 03/04/2023 20:03, Casey Schaufler wrote:
> On 4/3/2023 2:47 AM, Mickaël Salaün wrote:
>>
>> On 15/03/2023 23:47, Casey Schaufler wrote:
>>> Add lsm_name_to_attr(), which translates a text string to a
>>> LSM_ATTR value if one is available.
>>>
>>> Add lsm_fill_user_ctx(), which fills a struct lsm_ctx, including
>>> the trailing attribute value.
>>>
>>> All are used in module specific components of LSM system calls.
>>>
>>> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
>>> ---
>>>    include/linux/security.h | 13 ++++++++++
>>>    security/lsm_syscalls.c  | 51 ++++++++++++++++++++++++++++++++++++++++
>>>    security/security.c      | 31 ++++++++++++++++++++++++
>>>    3 files changed, 95 insertions(+)
>>
>> [...]
>>
>>> diff --git a/security/security.c b/security/security.c
>>> index 2c57fe28c4f7..f7b814a3940c 100644
>>> --- a/security/security.c
>>> +++ b/security/security.c
>>> @@ -753,6 +753,37 @@ static int lsm_superblock_alloc(struct
>>> super_block *sb)
>>>        return 0;
>>>    }
>>>    +/**
>>> + * lsm_fill_user_ctx - Fill a user space lsm_ctx structure
>>> + * @ctx: an LSM context to be filled
>>> + * @context: the new context value
>>> + * @context_size: the size of the new context value
>>> + * @id: LSM id
>>> + * @flags: LSM defined flags
>>> + *
>>> + * Fill all of the fields in a user space lsm_ctx structure.
>>> + * Caller is assumed to have verified that @ctx has enough space
>>> + * for @context.
>>> + * Returns 0 on success, -EFAULT on a copyout error.
>>> + */
>>> +int lsm_fill_user_ctx(struct lsm_ctx __user *ctx, void *context,
>>> +              size_t context_size, u64 id, u64 flags)
>>> +{
>>> +    struct lsm_ctx local;
>>> +    void __user *vc = ctx;
>>> +
>>> +    local.id = id;
>>> +    local.flags = flags;
>>> +    local.ctx_len = context_size;
>>> +    local.len = context_size + sizeof(local);
>>> +    vc += sizeof(local);
>>> +    if (copy_to_user(ctx, &local, sizeof(local)))
>>> +        return -EFAULT;
>>> +    if (context_size > 0 && copy_to_user(vc, context, context_size))
>>> +        return -EFAULT;
>>
>> Can we do a single copy_to_user() call?
> 
> It would be possible, but would require allocating memory and copying
> the context. I don't see that as an improvement.
> 
>> That would avoid inconsistent user space data, could speed up a bit
>> the operation, and make the code easier to understand. To use the
>> stack, we need to know the maximum size of context_size for all use
>> cases, which seems reasonable and can be checked at build time (on
>> each LSM side, and potentially with specific context type passed as
>> enum instead of context_size) and run time (for this generic helper).
> 
> Knowning the maximum size of attributes for all LSMs and hard coding
> that here would make maintaining this code really painful.

Hmm, I forgot about variable-length strings, but maybe a reasonable 
common maximum size (that could fit on the stack) could be found?

> 
>>
>>
>>> +    return 0;
>>> +}
>>> +
>>>    /*
>>>     * The default value of the LSM hook is defined in
>>> linux/lsm_hook_defs.h and
>>>     * can be accessed with: