Re: [PATCH] powerpc/powernv: fix null pointer dereference in pnv_get_random_long()

[Madhavan Srinivasan <maddy@linux.ibm.com>]

On 5/11/26 5:34 PM, Paul Menzel wrote:
> pnv_get_random_long() dereferences the per-CPU pnv_rng pointer without
> checking whether it has been initialized resulting in the oops below:
>
>      [    0.000000] Linux version 7.1.0-rc2+ (pmenzel@flughafenberlinbrandenburgwillybrandt.molgen.mpg.de) (gcc (Ubuntu 11.2.0-7ubuntu2) 11.2.0, GNU ld (GNU Binutils for Ubuntu) 2.37) #3 SMP PREEMPT Wed May  6 08:50:58 CEST 2026
>      […]
>      [   17.901992] Kernel attempted to read user page (0) - exploit attempt? (uid: 0)
>      [   17.902011] BUG: Kernel NULL pointer dereference on read at 0x00000000
>      [   17.902018] Faulting instruction address: 0xc0000000000e7138
>      [   17.902027] Oops: Kernel access of bad area, sig: 11 [#1]
>      [   17.902034] LE PAGE_SIZE=64K MMU=Hash  SMP NR_CPUS=2048 NUMA PowerNV
>      [   17.902045] Modules linked in: powernv_rng(+) bnx2x ofpart ibmpowernv bfq mdio cmdlinepart powernv_flash ipmi_powernv ipmi_devintf mtd ipmi_msghandler at24(+) vmx_crypto opal_prd sch_fq_codel nfsd parport_pc ppdev auth_rpcgss nfs_acl lp lockd grace parport sunrpc autofs4 btrfs xor libblake2b raid6_pq ast drm_shmem_helper drm_client_lib i2c_algo_bit drm_kms_helper drm ahci drm_panel_orientation_quirks libahci
>      [   17.902185] CPU: 147 UID: 0 PID: 2626 Comm: hwrng Not tainted 7.1.0-rc2+ #3 PREEMPTLAZY
>      [   17.902197] Hardware name: 8335-GCA POWER8 (raw) 0x4d0200 opal:skiboot-5.4.8-5787ad3 PowerNV
>      [   17.902204] NIP:  c0000000000e7138 LR: c00800001ec8013c CTR: c0000000000e70fc
>      [   17.902212] REGS: c000000092913c50 TRAP: 0300   Not tainted  (7.1.0-rc2+)
>      [   17.902222] MSR:  900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 44420220  XER: 20000000
>      [   17.902269] CFAR: c00800001ec8026c DAR: 0000000000000000 DSISR: 40000000 IRQMASK: 0
>                     GPR00: c00800001ec8013c c000000092913ef0 c000000001c18100 c00000002222d900
>                     GPR04: c00000002222d900 0000000000000080 0000000000000001 0000000000000000
>                     GPR08: 0000000000000000 c000000002212000 c0000000951e1780 c00800001ec80258
>                     GPR12: c0000000000e70fc c00000ffff6fd700 c0000000001d11c0 c00000001b99b9c0
>                     GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>                     GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>                     GPR24: 0000000000000000 c000000002fe6a58 0000000000000000 0000000000000000
>                     GPR28: c000000002fe6a20 0000000000000010 000000000000000f c00000002222d900
>      [   17.902406] NIP [c0000000000e7138] pnv_get_random_long+0x3c/0x114
>      [   17.902426] LR [c00800001ec8013c] powernv_rng_read+0x78/0xc4 [powernv_rng]
>      [   17.902444] Call Trace:
>      [   17.902448] [c000000092913ef0] [c000000092913f30] 0xc000000092913f30 (unreliable)
>      [   17.902463] [c000000092913f30] [c000000000decd58] hwrng_fillfn+0xd4/0x3dc
>      [   17.902484] [c000000092913f90] [c0000000001d1328] kthread+0x170/0x1a4
>      [   17.902498] [c000000092913fe0] [c00000000000d030] start_kernel_thread+0x14/0x18
>      [   17.902513] Code: 60000000 7d2000a6 71290010 418200bc e94d0908 812a0000 39290001 912a0000 e90d0030 3d220060 39299f00 7d08482a <e9280000> 7c0004ac e8e90000 0c070000
>      [   17.902569] ---[ end trace 0000000000000000 ]---
>      [   18.008801] pstore: backend (nvram) writing error (-1)
>
>      [   18.015458] note: hwrng[2626] exited with irqs disabled
>      [   18.015483] note: hwrng[2626] exited with preempt_count 1
>
> Commit f3eac426657d ("powerpc/powernv: wire up rng during setup_arch")
> introduced a lazy initialization path via pnv_get_random_long_early():
> per-CPU pointers are left NULL until slab becomes available and
> rng_create() completes.
>
> pnv_get_random_long() is an exported symbol called directly by the
> powernv_rng hwrng module (powernv_rng_read()), bypassing the
> ppc_md.get_random_seed guard that would otherwise ensure per-CPU data is
> ready.  If the hwrng fill thread runs on a CPU whose slot is still NULL,
> the function crashes dereferencing rng->regs at offset 0.
>
> Guard both branches with a NULL check and return 0 (no data) when the
> per-CPU pointer has not been set up yet.
>
> Testing on the IBM Power S822LC (8335-GCA POWER8 (raw) 0x4d0200
> opal:skiboot-5.4.8-5787ad3 PowerNV) is successful:
>
>      [   23.850775] powernv_rng: Registered powernv hwrng.
>
> Fixes: f3eac426657d ("powerpc/powernv: wire up rng during setup_arch")
> Link: https://lore.kernel.org/all/a159e81a-ccfd-440f-af68-6a56cca09cb2@molgen.mpg.de/
> Cc: Jason A. Donenfeld <Jason@zx2c4.com>
> Cc: stable@vger.kernel.org # v5.18
> Assisted-by: Claude Sonnet 4.6
> Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
> ---
> No idea, how to test, that the rng works as expected (and if, despite
> the missing message) it  didn’t work before.
>
>   arch/powerpc/platforms/powernv/rng.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
> index 7a4c38cd6a82..dc71eaf5d954 100644
> --- a/arch/powerpc/platforms/powernv/rng.c
> +++ b/arch/powerpc/platforms/powernv/rng.c
> @@ -87,12 +87,16 @@ int pnv_get_random_long(unsigned long *v)
>   
>   	if (mfmsr() & MSR_DR) {
>   		rng = get_cpu_var(pnv_rng);
> -		*v = rng_whiten(rng, in_be64(rng->regs));
> +		if (rng)
> +			*v = rng_whiten(rng, in_be64(rng->regs));
>   		put_cpu_var(rng);
> -	} else {
> -		rng = raw_cpu_read(pnv_rng);
> -		*v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));


Not sure whether I understand this. rng is not initialized, we will need 
raw_cpu_read before check right?


> +		return rng ? 1 : 0;
>   	}
> +
> +	rng = raw_cpu_read(pnv_rng);
> +	if (!rng)
> +		return 0;
> +	*v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));
>   	return 1;
>   }
>   EXPORT_SYMBOL_GPL(pnv_get_random_long);