linux-kernel - Re: [PATCH] powerpc: don't select KFENCE on platform PPC_FSL

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Date:   Fri, 24 Sep 2021 08:41:57 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Liu Shixin <liushixin2@...wei.com>, Marco Elver <elver@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc: don't select KFENCE on platform PPC_FSL_BOOK3E



Le 24/09/2021 à 08:39, Liu Shixin a écrit :
> On platform PPC_FSL_BOOK3E, all lowmem is managed by tlbcam. That means
> we didn't really map the kfence pool with page granularity. Therefore,
> if KFENCE is enabled, the system will hit the following panic:

Could you please explain a bit more what the problem is ?

KFENCE has been implemented with the same logic as DEBUG_PAGEALLOC.

DEBUG_PAGEALLOC is enabled on FSL_BOOK3E.

In MMU_setup(), __map_without_ltlbs is set to 1 when KFENCE is enabled.

__map_without_ltlbs should disable the use of tlbcam.


So what's wrong really ?

Does DEBUG_PAGEALLOC work on FSL_BOOK3E ?

Thanks
Christophe

> 
>      BUG: Kernel NULL pointer dereference on read at 0x00000000
>      Faulting instruction address: 0xc01de598
>      Oops: Kernel access of bad area, sig: 11 [#1]
>      BE PAGE_SIZE=4K SMP NR_CPUS=4 MPC8544 DS
>      Dumping ftrace buffer:
>         (ftrace buffer empty)
>      Modules linked in:
>      CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.12.0-rc3+ #298
>      NIP:  c01de598 LR: c08ae9c4 CTR: 00000000
>      REGS: c0b4bea0 TRAP: 0300   Not tainted  (5.12.0-rc3+)
>      MSR:  00021000 <CE,ME>  CR: 24000228  XER: 20000000
>      DEAR: 00000000 ESR: 00000000
>      GPR00: c08ae9c4 c0b4bf60 c0ad64e0 ef720000 00021000 00000000 00000000 00000200
>      GPR08: c0ad5000 00000000 00000000 00000004 00000000 008fbb30 00000000 00000000
>      GPR16: 00000000 00000000 00000000 00000000 c0000000 00000000 00000000 00000000
>      GPR24: c08ca004 c08ca004 c0b6a0e0 c0b60000 c0b58f00 c0850000 c08ca000 ef720000
>      NIP [c01de598] kfence_protect+0x44/0x6c
>      LR [c08ae9c4] kfence_init+0xfc/0x2a4
>      Call Trace:
>      [c0b4bf60] [efffe160] 0xefffe160 (unreliable)
>      [c0b4bf70] [c08ae9c4] kfence_init+0xfc/0x2a4
>      [c0b4bfb0] [c0894d3c] start_kernel+0x3bc/0x574
>      [c0b4bff0] [c0000470] set_ivor+0x14c/0x188
>      Instruction dump:
>      7c0802a6 8109d594 546a653a 90010014 54630026 39200000 7d48502e 2c0a0000
>      41820010 554a0026 5469b53a 7d295214 <81490000> 38831000 554a003c 91490000
>      random: get_random_bytes called from print_oops_end_marker+0x40/0x78 with crng_init=0
>      ---[ end trace 0000000000000000 ]---
> 
> Signed-off-by: Liu Shixin <liushixin2@...wei.com>
> ---
>   arch/powerpc/Kconfig | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index d46db0bfb998..cffd57bcb5e4 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -185,7 +185,7 @@ config PPC
>   	select HAVE_ARCH_KASAN			if PPC32 && PPC_PAGE_SHIFT <= 14
>   	select HAVE_ARCH_KASAN_VMALLOC		if PPC32 && PPC_PAGE_SHIFT <= 14
>   	select HAVE_ARCH_KGDB
> -	select HAVE_ARCH_KFENCE			if PPC32
> +	select HAVE_ARCH_KFENCE			if PPC32 && !PPC_FSL_BOOK3E
>   	select HAVE_ARCH_MMAP_RND_BITS
>   	select HAVE_ARCH_MMAP_RND_COMPAT_BITS	if COMPAT
>   	select HAVE_ARCH_NVRAM_OPS
>