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

[Mickaël Salaün]

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@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? 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: