Re: [PATCH v9 08/11] Smack: implement setselfattr and getselfattr hooks

[Kees Cook <keescook@chromium.org>]

On Fri, Apr 21, 2023 at 10:42:56AM -0700, Casey Schaufler wrote:
> Implement Smack support for security_[gs]etselfattr.
> Refactor the setprocattr hook to avoid code duplication.
> 
> Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
> ---
>  security/smack/smack_lsm.c | 105 +++++++++++++++++++++++++++++++++++--
>  1 file changed, 100 insertions(+), 5 deletions(-)
> 
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 3cf862fcbe08..902b39c187bf 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -3552,6 +3552,41 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
>  	return;
>  }
>  
> +/**
> + * smack_getselfattr - Smack current process attribute
> + * @attr: which attribute to fetch
> + * @ctx: buffer to receive the result
> + * @size: available size in, actual size out
> + * @flags: unused
> + *
> + * Fill the passed user space @ctx with the details of the requested
> + * attribute.
> + *
> + * Returns 1, the number of attributes, on success, an error code otherwise.
> + */
> +static int smack_getselfattr(unsigned int __user attr,
> +			     struct lsm_ctx __user *ctx, size_t *size,
> +			     u32 __user flags)
> +{
> +	struct smack_known *skp = smk_of_current();
> +	int total;
> +	int slen;
> +	int rc = 1;
> +
> +	if (attr != LSM_ATTR_CURRENT)
> +		return -EOPNOTSUPP;
> +
> +	slen = strlen(skp->smk_known) + 1;
> +	total = ALIGN(slen + sizeof(*ctx), 8);
> +	if (total > *size)
> +		rc = -E2BIG;
> +	else
> +		lsm_fill_user_ctx(ctx, skp->smk_known, slen, LSM_ID_SMACK, 0);
> +
> +	*size = total;
> +	return rc;
> +}
> +
>  /**
>   * smack_getprocattr - Smack process attribute access
>   * @p: the object task
> @@ -3581,8 +3616,8 @@ static int smack_getprocattr(struct task_struct *p, const char *name, char **val
>  }
>  
>  /**
> - * smack_setprocattr - Smack process attribute setting
> - * @name: the name of the attribute in /proc/.../attr
> + * do_setattr - Smack process attribute setting
> + * @attr: the ID of the attribute
>   * @value: the value to set
>   * @size: the size of the value
>   *
> @@ -3591,7 +3626,7 @@ static int smack_getprocattr(struct task_struct *p, const char *name, char **val
>   *
>   * Returns the length of the smack label or an error code
>   */
> -static int smack_setprocattr(const char *name, void *value, size_t size)
> +static int do_setattr(u64 attr, void *value, size_t size)
>  {
>  	struct task_smack *tsp = smack_cred(current_cred());
>  	struct cred *new;
> @@ -3605,8 +3640,8 @@ static int smack_setprocattr(const char *name, void *value, size_t size)
>  	if (value == NULL || size == 0 || size >= SMK_LONGLABEL)
>  		return -EINVAL;
>  
> -	if (strcmp(name, "current") != 0)
> -		return -EINVAL;
> +	if (attr != LSM_ATTR_CURRENT)
> +		return -EOPNOTSUPP;
>  
>  	skp = smk_import_entry(value, size);
>  	if (IS_ERR(skp))
> @@ -3645,6 +3680,64 @@ static int smack_setprocattr(const char *name, void *value, size_t size)
>  	return size;
>  }
>  
> +/**
> + * smack_setselfattr - Set a Smack process attribute
> + * @attr: which attribute to set
> + * @ctx: buffer containing the data
> + * @size: size of @ctx
> + * @flags: unused
> + *
> + * Fill the passed user space @ctx with the details of the requested
> + * attribute.
> + *
> + * Returns 0 on success, an error code otherwise.
> + */
> +static int smack_setselfattr(unsigned int __user attr,
> +			     struct lsm_ctx __user *ctx, size_t __user size,

"size_t __user" isn't valid: it's not a pointer.

> +			     u32 __user flags)
> +{
> +	struct lsm_ctx *lctx;
> +	void *context;

Why is "context" used here at all? Can't this entirely be lctx?

> +	int rc;
> +
> +	context = kmalloc(size, GFP_KERNEL);
> +	if (context == NULL)
> +		return -ENOMEM;
> +
> +	lctx = (struct lsm_ctx *)context;
> +	if (copy_from_user(context, ctx, size))
> +		rc = -EFAULT;
> +	else if (lctx->ctx_len > size)
> +		rc = -EINVAL;
> +	else
> +		rc = do_setattr(attr, lctx + 1, lctx->ctx_len);

There is no "second lctx", please pass lctx->ctx instead.

> +
> +	kfree(context);
> +	if (rc > 0)
> +		return 0;
> +	return rc;
> +}
> +
> +/**
> + * smack_setprocattr - Smack process attribute setting
> + * @name: the name of the attribute in /proc/.../attr
> + * @value: the value to set
> + * @size: the size of the value
> + *
> + * Sets the Smack value of the task. Only setting self
> + * is permitted and only with privilege
> + *
> + * Returns the length of the smack label or an error code
> + */
> +static int smack_setprocattr(const char *name, void *value, size_t size)
> +{
> +	int attr = lsm_name_to_attr(name);
> +
> +	if (attr)
> +		return do_setattr(attr, value, size);
> +	return -EINVAL;
> +}
> +
>  /**
>   * smack_unix_stream_connect - Smack access on UDS
>   * @sock: one sock
> @@ -4955,6 +5048,8 @@ static struct security_hook_list smack_hooks[] __lsm_ro_after_init = {
>  
>  	LSM_HOOK_INIT(d_instantiate, smack_d_instantiate),
>  
> +	LSM_HOOK_INIT(getselfattr, smack_getselfattr),
> +	LSM_HOOK_INIT(setselfattr, smack_setselfattr),
>  	LSM_HOOK_INIT(getprocattr, smack_getprocattr),
>  	LSM_HOOK_INIT(setprocattr, smack_setprocattr),
>  
> -- 
> 2.39.2
> 

-- 
Kees Cook