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

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Message-ID: <c79a66f4-53c0-66f5-4539-4994365aa656@schaufler-ca.com>
Date:   Thu, 30 Mar 2023 13:42:44 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     linux-security-module@...r.kernel.org, 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,
        mic@...ikod.net, Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an
 lsm_ctx

On 3/29/2023 6:13 PM, Paul Moore wrote:
> On Wed, Mar 15, 2023 at 6:50 PM Casey Schaufler <casey@...aufler-ca.com> 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/lsm_syscalls.c b/security/lsm_syscalls.c
>> index 6efbe244d304..55d849ad5d6e 100644
>> --- a/security/lsm_syscalls.c
>> +++ b/security/lsm_syscalls.c
>> @@ -17,6 +17,57 @@
>>  #include <linux/lsm_hooks.h>
>>  #include <uapi/linux/lsm.h>
>>
>> +struct attr_map {
>> +       char *name;
>> +       u64 attr;
>> +};
>> +
>> +static const struct attr_map lsm_attr_names[] = {
>> +       {
>> +               .name = "current",
>> +               .attr = LSM_ATTR_CURRENT,
>> +       },
>> +       {
>> +               .name = "exec",
>> +               .attr = LSM_ATTR_EXEC,
>> +       },
>> +       {
>> +               .name = "fscreate",
>> +               .attr = LSM_ATTR_FSCREATE,
>> +       },
>> +       {
>> +               .name = "keycreate",
>> +               .attr = LSM_ATTR_KEYCREATE,
>> +       },
>> +       {
>> +               .name = "prev",
>> +               .attr = LSM_ATTR_PREV,
>> +       },
>> +       {
>> +               .name = "sockcreate",
>> +               .attr = LSM_ATTR_SOCKCREATE,
>> +       },
>> +};
>> +
>> +/**
>> + * lsm_name_to_attr - map an LSM attribute name to its ID
>> + * @name: name of the attribute
>> + *
>> + * Look the given @name up in the table of know attribute names.
>> + *
>> + * Returns the LSM attribute value associated with @name, or 0 if
>> + * there is no mapping.
>> + */
>> +u64 lsm_name_to_attr(const char *name)
>> +{
>> +       int i;
>> +
>> +       for (i = 0; i < ARRAY_SIZE(lsm_attr_names); i++)
>> +               if (!strcmp(name, lsm_attr_names[i].name))
>> +                       return lsm_attr_names[i].attr;
> I'm pretty sure this is the only place where @lsm_attr_names is used,
> right?  If true, when coupled with the idea that these syscalls are
> going to close the door on new LSM attributes in procfs I think we can
> just put the mapping directly in this function via a series of
> if-statements.

Ick. You're not wrong, but the hard coded if-statement approach goes
against all sorts of coding principles. I'll do it, but I can't say I
like it.

>> +       return 0;
>> +}
>> +
>>  /**
>>   * sys_lsm_set_self_attr - Set current task's security module attribute
>>   * @attr: which attribute to set
>> 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);
> See my prior comments about void pointer math.
>
>> +       if (copy_to_user(ctx, &local, sizeof(local)))
>> +               return -EFAULT;
>> +       if (context_size > 0 && copy_to_user(vc, context, context_size))
>> +               return -EFAULT;
> Should we handle the padding in this function?

This function fills in a lsm_ctx. The padding, if any, is in addition to
the lsm_ctx, not part of it.

>> +       return 0;
>> +}
> --
> paul-moore.com