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SubjectRe: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm_ctx
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On 03/04/2023 11:47, 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@schaufler-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? 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).

Well, actually the context_size should be inferred from id, and the
"local" size should be defined and check at build time against all
context ID sizes.

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

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