Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm_ctx

[Mickaël Salaün]

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@xxxxxxxxxxxxxxxx>
> > > ---
> > >    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: