Re: [PATCH v2] s390/debug: Fix NULL pointer dereference in debug_info_copy()

[Heiko Carstens <hca@linux.ibm.com>]

On Wed, Sep 16, 2026 at 06:06:26PM +0200, Mikhail Zaslonko wrote:
> When debug_register_static() fails, it clears areas, active_pages and
> active_entries but leaves the area bounds unchanged. Copying such an
> area, either by opening its view file or via debug_dump(), makes
> debug_info_copy() dereference the NULL pointers.
> 
> Skip the copy loop when the source has no areas.
> 
> Closes: https://lore.kernel.org/r/20260903132123.12F271F00A3F@smtp.kernel.org
> Fixes: d72541f94512 ("s390/debug: add early tracing support")
> Signed-off-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>
> ---
> v2:
> - Different approach: check the areas pointer in debug_info_copy()
>   instead of zeroing nr_areas/pages_per_area in debug_register_static().
>   Zeroing nr_areas made debug_info_alloc() return ZERO_SIZE_PTR for
>   active_pages/active_entries, which debug_to_act_entry() dereferences
>   before debug_prev_entry() checks id->areas.
>   Checking the pointer also matches what all other readers in this file
>   already do.
> - v1: https://lore.kernel.org/all/20260910104525.3788362-1-zaslonko@linux.ibm.com/
> 
>  arch/s390/kernel/debug.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kernel/debug.c b/arch/s390/kernel/debug.c
> index b5bf8284dbfc..d611d0eed1f6 100644
> --- a/arch/s390/kernel/debug.c
> +++ b/arch/s390/kernel/debug.c
> @@ -434,7 +434,8 @@ static debug_info_t *debug_info_copy(debug_info_t *in, int mode)
>  		debug_info_free(rc);
>  	} while (1);
>  
> -	if (mode == NO_AREAS)
> +	/* debug_register_static() failure leaves areas NULL, bounds intact */
> +	if (mode == NO_AREAS || !in->areas)

This is just a more or less random indicator that debug_register_static()
failed. But this seems to be the easiest way forward. Alternatively an
explicit flag in debug_info_t would make this more obvious.

Anyway:
Acked-by: Heiko Carstens <hca@linux.ibm.com>

Waiting for Peter's reply.