[PATCH] lsm: handle the NULL buffer case in lsm_fill_user_ctx()

[Serge E. Hallyn]

On Wed, Mar 13, 2024 at 10:22:03PM -0400, Paul Moore wrote:
> Passing a NULL buffer into the lsm_get_self_attr() syscall is a valid
> way to quickly determine the minimum size of the buffer needed to for
> the syscall to return all of the LSM attributes to the caller.
> Unfortunately we/I broke that behavior in commit d7cf3412a9f6
> ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
> such that it returned an error to the caller; this patch restores the
> original desired behavior of using the NULL buffer as a quick way to
> correctly size the attribute buffer.
> 
> Cc: stable at vger.kernel.org
> Fixes: d7cf3412a9f6 ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
> Signed-off-by: Paul Moore <paul at paul-moore.com>
> ---
>  security/security.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/security/security.c b/security/security.c
> index 5b2e0a15377d..7e118858b545 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -780,7 +780,9 @@ static int lsm_superblock_alloc(struct super_block *sb)
>   * @id: LSM id
>   * @flags: LSM defined flags
>   *
> - * Fill all of the fields in a userspace lsm_ctx structure.
> + * Fill all of the fields in a userspace lsm_ctx structure.  If @uctx is NULL
> + * simply calculate the required size to output via @utc_len and return
> + * success.
>   *
>   * Returns 0 on success, -E2BIG if userspace buffer is not large enough,
>   * -EFAULT on a copyout error, -ENOMEM if memory can't be allocated.
> @@ -799,6 +801,10 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
>  		goto out;
>  	}
>  
> +	/* no buffer - return success/0 and set @uctx_len to the req size */
> +	if (!uctx)
> +		goto out;

If the user just passes in *uctx_len=0, then they will get -E2BIG
but still will get the length in *uctx_len.

To use it this new way, they have to first set *uctx_len to a
value larger than nctx_len could possibly be, else they'll...
still get -E2BIG.

So I'm not sure this patch has value.

>  	nctx = kzalloc(nctx_len, GFP_KERNEL);
>  	if (nctx == NULL) {
>  		rc = -ENOMEM;
> -- 
> 2.44.0
>