linux-hardening - Re: [RFC PATCH 6/6] powerpc/64s: Enable CONFIG_VMAP

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Date:   Sat, 5 Nov 2022 17:07:29 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andrew Donnellan <ajd@...ux.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
CC:     "ruscur@...sell.cc" <ruscur@...sell.cc>,
        "cmr@...escreens.de" <cmr@...escreens.de>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [RFC PATCH 6/6] powerpc/64s: Enable CONFIG_VMAP_STACK



Le 04/11/2022 à 18:27, Andrew Donnellan a écrit :
> Enable CONFIG_VMAP_STACK for book3s64.
> 
> To do this, we need to make some slight adjustments to set the stack SLB
> entry up for vmalloc rather than linear.
> 
> For now, only enable if KVM_BOOK3S_64_HV is disabled (there's some real mode
> handlers we need to fix there).

There is one missing point : with VMAP_STACK, a stack overflow will 
generate a page fault. You have to handle it at interrupt entry, before 
going back to virtual mode, otherwise it will fault forever.

See how it is done in arch/powerpc/kernel/head_32.h, in macro 
EXCEPTION_PROLOG_1

> 
> Signed-off-by: Andrew Donnellan <ajd@...ux.ibm.com>
> ---
>   arch/powerpc/kernel/process.c          |  4 ++++
>   arch/powerpc/mm/book3s64/slb.c         | 11 +++++++++--
>   arch/powerpc/platforms/Kconfig.cputype |  1 +
>   3 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index 07917726c629..cadf2db5a2a8 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1685,7 +1685,11 @@ static void setup_ksp_vsid(struct task_struct *p, unsigned long sp)
>   {
>   #ifdef CONFIG_PPC_64S_HASH_MMU
>   	unsigned long sp_vsid;
> +#ifdef CONFIG_VMAP_STACK
> +	unsigned long llp = mmu_psize_defs[mmu_vmalloc_psize].sllp;
> +#else /* CONFIG_VMAP_STACK */
>   	unsigned long llp = mmu_psize_defs[mmu_linear_psize].sllp;
> +#endif /* CONFIG_VMAP_STACK */

I think you could use IS_ENABLED() instead of an ifdef:

	unsigned long llp;

	if (IS_ENABLED(CONFIG_VMAP_STACK))
		llp = mmu_psize_defs[mmu_vmalloc_psize].sllp;
	else
		llp = mmu_psize_defs[mmu_linear_psize].sllp;

>   
>   	if (radix_enabled())
>   		return;
> diff --git a/arch/powerpc/mm/book3s64/slb.c b/arch/powerpc/mm/book3s64/slb.c
> index 6956f637a38c..0e21f0eaa7bb 100644
> --- a/arch/powerpc/mm/book3s64/slb.c
> +++ b/arch/powerpc/mm/book3s64/slb.c
> @@ -541,7 +541,7 @@ void slb_set_size(u16 size)
>   void slb_initialize(void)
>   {
>   	unsigned long linear_llp, vmalloc_llp, io_llp;
> -	unsigned long lflags;
> +	unsigned long lflags, kstack_flags;
>   	static int slb_encoding_inited;
>   #ifdef CONFIG_SPARSEMEM_VMEMMAP
>   	unsigned long vmemmap_llp;
> @@ -582,11 +582,18 @@ void slb_initialize(void)
>   	 * get_paca()->kstack hasn't been initialized yet.
>   	 * For secondary cpus, we need to bolt the kernel stack entry now.
>   	 */
> +
> +#ifdef CONFIG_VMAP_STACK
> +	kstack_flags = SLB_VSID_KERNEL | vmalloc_llp;
> +#else
> +	kstack_flags = SLB_VSID_KERNEL | linear_llp;
> +#endif

Same, should be

	if (IS_ENABLED(CONFIG_VMAP_STACK))
		kstack_flags = SLB_VSID_KERNEL | vmalloc_llp;
	else
		kstack_flags = SLB_VSID_KERNEL | linear_llp;

>   	slb_shadow_clear(KSTACK_INDEX);
>   	if (raw_smp_processor_id() != boot_cpuid &&
>   	    (get_paca()->kstack & slb_esid_mask(mmu_kernel_ssize)) > PAGE_OFFSET)
>   		create_shadowed_slbe(get_paca()->kstack,
> -				     mmu_kernel_ssize, lflags, KSTACK_INDEX);
> +				     mmu_kernel_ssize, kstack_flags,
> +				     KSTACK_INDEX);
>   
>   	asm volatile("isync":::"memory");
>   }
> diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
> index 0c4eed9aea80..998317257797 100644
> --- a/arch/powerpc/platforms/Kconfig.cputype
> +++ b/arch/powerpc/platforms/Kconfig.cputype
> @@ -104,6 +104,7 @@ config PPC_BOOK3S_64
>   	select IRQ_WORK
>   	select PPC_64S_HASH_MMU if !PPC_RADIX_MMU
>   	select KASAN_VMALLOC if KASAN
> +	select HAVE_ARCH_VMAP_STACK if KVM_BOOK3S_64_HV = n

Is it different from

	select HAVE_ARCH_VMAP_STACK if !KVM_BOOK3S_64_HV ?

>   
>   config PPC_BOOK3E_64
>   	bool "Embedded processors"