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Message-ID: <20250725161327.GC1724026@google.com>
Date: Fri, 25 Jul 2025 16:13:27 +0000
From: Sami Tolvanen <samitolvanen@...gle.com>
To: Deepak Gupta <debug@...osinc.com>, Will Deacon <will@...nel.org>
Cc: 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 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.
> 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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