Re: [PATCH 2/2] s390/ipl: Fix NULL deref in dump_reipl without re-IPL parm block

[Mikhail Zaslonko <zaslonko@linux.ibm.com>]

On 13-Aug-26 13:06, Vasily Gorbik wrote:
> Unlike kdump, which passes the re-IPL parameter block through os_info,
> the stand-alone dump passes it through the IPL parm block address and
> checksum in lowcore.
> 
> Some IPL types, like HMC FTP boot or QEMU direct kernel boot, might not
> provide an IPL parameter block. In this case reipl_type_init() selects
> IPL_TYPE_UNKNOWN and reipl_block_actual remains NULL. Nevertheless,
> dump_reipl_run() unconditionally dereferences it when preparing the
> lowcore fields. This may happen to work by chance when address zero
> contains readable lowcore data. A zero IPL parameter block address is
> then stored in lowcore, causing the stand-alone dumper to enter disabled
> wait after completing the dump.
> 
> Explicitly store a zero IPL parameter block address and checksum when no
> re-IPL parameter block is available. This does not change the behavior:
> the stand-alone dumper completes the dump and halts, while valid re-IPL
> parameter blocks continue to be handled as before.
> 
> Fixes: 099b76513992 ("[S390] Automatic IPL after dump")
> Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
> ---
>  arch/s390/kernel/ipl.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/s390/kernel/ipl.c b/arch/s390/kernel/ipl.c
> index 7024fc413715..68fdd5616dfe 100644
> --- a/arch/s390/kernel/ipl.c
> +++ b/arch/s390/kernel/ipl.c
> @@ -1929,7 +1929,8 @@ static struct shutdown_action __refdata dump_action = {
>  static void dump_reipl_run(struct shutdown_trigger *trigger)
>  {
>  	struct lowcore *abs_lc;
> -	unsigned int csum;
> +	unsigned long ipib = 0;
> +	unsigned int csum = 0;
>  
>  	/*
>  	 * Set REIPL_CLEAR flag in os_info flags entry indicating
> @@ -1945,9 +1946,12 @@ static void dump_reipl_run(struct shutdown_trigger *trigger)
>  	    reipl_type == IPL_TYPE_UNKNOWN)
>  		os_info_flags |= OS_INFO_FLAG_REIPL_CLEAR;
>  	os_info_entry_add_data(OS_INFO_FLAGS_ENTRY, &os_info_flags, sizeof(os_info_flags));
> -	csum = (__force unsigned int)cksm(reipl_block_actual, reipl_block_actual->hdr.len, 0);
> +	if (reipl_block_actual) {
> +		ipib = __pa(reipl_block_actual);
> +		csum = (__force unsigned int)cksm(reipl_block_actual, reipl_block_actual->hdr.len, 0);
> +	}
>  	abs_lc = get_abs_lowcore();
> -	abs_lc->ipib = __pa(reipl_block_actual);
> +	abs_lc->ipib = ipib;
>  	abs_lc->ipib_checksum = csum;
>  	put_abs_lowcore(abs_lc);
>  	dump_run(trigger);

Reviewed-by: Mikhail Zaslonko <zaslonko@linux.ibm.com>