Re: [PATCH 2/2] lsm: fix size queries for getselfattr with NULL buffer

[Paul Moore <paul@paul-moore.com>]

On Jun 19, 2026 Bradley Morgan <include@grrlz.net> wrote:
> 
> The lsm_get_self_attr() syscall allows callers to pass in a NULL context
> buffer to find out the size of the output needed. That path still
> compared the computed entry size against the caller provided size first,
> so a NULL buffer with size 0 incorrectly returned -E2BIG rather than
> reporting the required size.
> 
> Only enforce the available buffer length after checking for the NULL
> buffer. Cover the zero length sizing query in the self test.
> 
> Fixes: d7cf3412a9f6 ("lsm: consolidate buffer size handling into lsm_fill_user_ctx()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Bradley Morgan <include@grrlz.net>
> ---
>  security/security.c                                  | 8 ++++----
>  tools/testing/selftests/lsm/lsm_get_self_attr_test.c | 5 ++---
>  2 files changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/security/security.c b/security/security.c
> index 71aea8fdf014..fa0d7e036249 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -406,15 +406,15 @@ int lsm_fill_user_ctx(struct lsm_ctx __user *uctx, u32 *uctx_len,
>  	int rc = 0;
>  
>  	nctx_len = ALIGN(struct_size(nctx, ctx, val_len), sizeof(void *));
> +	/* no buffer - return success/0 and set @uctx_len to the req size */
> +	if (!uctx)
> +		goto out;
> +
>  	if (nctx_len > *uctx_len) {
>  		rc = -E2BIG;
>  		goto out;
>  	}
>  
> -	/* no buffer - return success/0 and set @uctx_len to the req size */
> -	if (!uctx)
> -		goto out;
> -
>  	nctx = kzalloc(nctx_len, GFP_KERNEL);
>  	if (nctx == NULL) {
>  		rc = -ENOMEM;

I'm not sure the existing code is necessarily wrong.  The existing code
allows for an E2BIG length check against the current necessary size, so
one can still do a length check regardless of if a buffer is passed.
Even in the NULL/E2BIG case, the necessary buffer size should still be
returned to the caller so that the process can size their allocation
appropriately.

Or am I missing something else?

--
paul-moore.com