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Message-ID: <aIOz_wxxCr8tPgXz@debug.ba.rivosinc.com>
Date: Fri, 25 Jul 2025 09:42:39 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Sami Tolvanen <samitolvanen@...gle.com>
Cc: Will Deacon <will@...nel.org>, Paul Walmsley <paul.walmsley@...ive.com>,
Palmer Dabbelt <palmer@...belt.com>,
Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
Masahiro Yamada <masahiroy@...nel.org>,
Nathan Chancellor <nathan@...nel.org>,
Nicolas Schier <nicolas.schier@...ux.dev>,
Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Michal Hocko <mhocko@...e.com>,
Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
Bill Wendling <morbo@...gle.com>,
Monk Chiang <monk.chiang@...ive.com>,
Kito Cheng <kito.cheng@...ive.com>,
Justin Stitt <justinstitt@...gle.com>,
linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
linux-kbuild@...r.kernel.org, linux-mm@...ck.org,
llvm@...ts.linux.dev, rick.p.edgecombe@...el.com,
broonie@...nel.org, cleger@...osinc.com, apatel@...tanamicro.com,
ajones@...tanamicro.com, conor.dooley@...rochip.com,
charlie@...osinc.com, samuel.holland@...ive.com, bjorn@...osinc.com,
fweimer@...hat.com, jeffreyalaw@...il.com,
heinrich.schuchardt@...onical.com, andrew@...ive.com,
ved@...osinc.com
Subject: Re: [PATCH 10/11] scs: generic scs code updated to leverage hw
assisted shadow stack
On Fri, Jul 25, 2025 at 04:13:27PM +0000, Sami Tolvanen wrote:
>On Thu, Jul 24, 2025 at 04:37:03PM -0700, Deepak Gupta wrote:
>> If shadow stack have memory protections from underlying cpu, use those
>> protections. arches can define PAGE_KERNEL_SHADOWSTACK to vmalloc such shadow
>> stack pages. Hw assisted shadow stack pages grow downwards like regular
>> stack. Clang based software shadow call stack grows low to high address.
>
>Is this the case for all the current hardware shadow stack
>implementations? If not, we might want a separate config for the
>shadow stack direction instead.
Is there something like this for regular stack as well?
I could copy same mechanism.
>
>> Thus this patch addresses some of those needs due to opposite direction
>> of shadow stack. Furthermore, hw shadow stack can't be memset because memset
>> uses normal stores. Lastly to store magic word at base of shadow stack, arch
>> specific shadow stack store has to be performed.
>>
>> Signed-off-by: Deepak Gupta <debug@...osinc.com>
>> ---
>> include/linux/scs.h | 26 +++++++++++++++++++++++++-
>> kernel/scs.c | 38 +++++++++++++++++++++++++++++++++++---
>> 2 files changed, 60 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/scs.h b/include/linux/scs.h
>> index 4ab5bdc898cf..6ceee07c2d1a 100644
>> --- a/include/linux/scs.h
>> +++ b/include/linux/scs.h
>> @@ -12,6 +12,7 @@
>> #include <linux/poison.h>
>> #include <linux/sched.h>
>> #include <linux/sizes.h>
>> +#include <asm/scs.h>
>>
>> #ifdef CONFIG_SHADOW_CALL_STACK
>>
>> @@ -37,22 +38,45 @@ static inline void scs_task_reset(struct task_struct *tsk)
>> * Reset the shadow stack to the base address in case the task
>> * is reused.
>> */
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + task_scs_sp(tsk) = task_scs(tsk) + SCS_SIZE;
>> +#else
>> task_scs_sp(tsk) = task_scs(tsk);
>> +#endif
>> }
>>
>> static inline unsigned long *__scs_magic(void *s)
>> {
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + return (unsigned long *)(s);
>> +#else
>> return (unsigned long *)(s + SCS_SIZE) - 1;
>> +#endif
>> }
>>
>> static inline bool task_scs_end_corrupted(struct task_struct *tsk)
>> {
>> unsigned long *magic = __scs_magic(task_scs(tsk));
>> - unsigned long sz = task_scs_sp(tsk) - task_scs(tsk);
>> + unsigned long sz;
>> +
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + sz = (task_scs(tsk) + SCS_SIZE) - task_scs_sp(tsk);
>> +#else
>> + sz = task_scs_sp(tsk) - task_scs(tsk);
>> +#endif
>>
>> return sz >= SCS_SIZE - 1 || READ_ONCE_NOCHECK(*magic) != SCS_END_MAGIC;
>> }
>>
>> +static inline void __scs_store_magic(unsigned long *s, unsigned long magic_val)
>> +{
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + arch_scs_store(s, magic_val);
>> +#else
>> + *__scs_magic(s) = magic_val;
>> +#endif
>> +}
>> +
>
>I'm not a huge fan of all the ifdefs. We could clean this up by
>allowing architectures to simply override some these functions, or at
>least use if (IS_ENABLED(CONFIG...)) instead. Will, any thoughts about
>this?
>
>> DECLARE_STATIC_KEY_FALSE(dynamic_scs_enabled);
>>
>> static inline bool scs_is_dynamic(void)
>> diff --git a/kernel/scs.c b/kernel/scs.c
>> index d7809affe740..5910c0a8eabd 100644
>> --- a/kernel/scs.c
>> +++ b/kernel/scs.c
>> @@ -11,6 +11,7 @@
>> #include <linux/scs.h>
>> #include <linux/vmalloc.h>
>> #include <linux/vmstat.h>
>> +#include <asm-generic/set_memory.h>
>>
>> #ifdef CONFIG_DYNAMIC_SCS
>> DEFINE_STATIC_KEY_FALSE(dynamic_scs_enabled);
>> @@ -32,19 +33,31 @@ static void *__scs_alloc(int node)
>> {
>> int i;
>> void *s;
>> + pgprot_t prot = PAGE_KERNEL;
>> +
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + prot = PAGE_KERNEL_SHADOWSTACK;
>> +#endif
>
>I would rather define the shadow stack protection flags in the header
>file and allow them to be overridden in asm/scs.h.
>
>> for (i = 0; i < NR_CACHED_SCS; i++) {
>> s = this_cpu_xchg(scs_cache[i], NULL);
>> if (s) {
>> s = kasan_unpoison_vmalloc(s, SCS_SIZE,
>> KASAN_VMALLOC_PROT_NORMAL);
>> +/*
>> + * If software shadow stack, its safe to memset. Else memset is not
>> + * possible on hw protected shadow stack. memset constitutes stores and
>> + * stores to shadow stack memory are disallowed and will fault.
>> + */
>> +#ifndef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> memset(s, 0, SCS_SIZE);
>> +#endif
>
>This could also be moved to a static inline function that
>architectures can override if they have hardware shadow stacks that
>cannot be cleared at this point.
>
>> goto out;
>> }
>> }
>>
>> s = __vmalloc_node_range(SCS_SIZE, 1, VMALLOC_START, VMALLOC_END,
>> - GFP_SCS, PAGE_KERNEL, 0, node,
>> + GFP_SCS, prot, 0, node,
>> __builtin_return_address(0));
>>
>> out:
>> @@ -59,7 +72,7 @@ void *scs_alloc(int node)
>> if (!s)
>> return NULL;
>>
>> - *__scs_magic(s) = SCS_END_MAGIC;
>> + __scs_store_magic(__scs_magic(s), SCS_END_MAGIC);
>>
>> /*
>> * Poison the allocation to catch unintentional accesses to
>> @@ -87,6 +100,16 @@ void scs_free(void *s)
>> return;
>>
>> kasan_unpoison_vmalloc(s, SCS_SIZE, KASAN_VMALLOC_PROT_NORMAL);
>> + /*
>> + * Hardware protected shadow stack is not writeable by regular stores
>> + * Thus adding this back to free list will raise faults by vmalloc
>> + * It needs to be writeable again. It's good sanity as well because
>> + * then it can't be inadvertently accesses and if done, it will fault.
>> + */
>> +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
>> + set_memory_rw((unsigned long)s, (SCS_SIZE/PAGE_SIZE));
>> +#endif
>
>Another candidate for an arch-specific function to reduce the number
>of ifdefs in the generic code.
>
>Sami
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