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[34.125.81.106]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-23fbe4fc7a4sm619335ad.120.2025.07.25.09.13.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Jul 2025 09:13:34 -0700 (PDT) Date: Fri, 25 Jul 2025 16:13:27 +0000 From: Sami Tolvanen To: Deepak Gupta , Will Deacon Cc: Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Masahiro Yamada , Nathan Chancellor , Nicolas Schier , Andrew Morton , David Hildenbrand , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Nick Desaulniers , Bill Wendling , Monk Chiang , Kito Cheng , Justin Stitt , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-mm@kvack.org, llvm@lists.linux.dev, rick.p.edgecombe@intel.com, broonie@kernel.org, cleger@rivosinc.com, apatel@ventanamicro.com, ajones@ventanamicro.com, conor.dooley@microchip.com, charlie@rivosinc.com, samuel.holland@sifive.com, bjorn@rivosinc.com, fweimer@redhat.com, jeffreyalaw@gmail.com, heinrich.schuchardt@canonical.com, andrew@sifive.com, ved@rivosinc.com Subject: Re: [PATCH 10/11] scs: generic scs code updated to leverage hw assisted shadow stack Message-ID: <20250725161327.GC1724026@google.com> References: <20250724-riscv_kcfi-v1-0-04b8fa44c98c@rivosinc.com> <20250724-riscv_kcfi-v1-10-04b8fa44c98c@rivosinc.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250724-riscv_kcfi-v1-10-04b8fa44c98c@rivosinc.com> 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 > --- > 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 > #include > #include > +#include > > #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 > #include > #include > +#include > > #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