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

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Message-ID: <bbd6abb7-7443-0f34-788b-0e441dc5afee@schaufler-ca.com>
Date:   Mon, 3 Apr 2023 11:03:01 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mickaël Salaün <mic@...ikod.net>,
        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, Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an
 lsm_ctx

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.

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