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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: References: <20250724-riscv_kcfi-v1-0-04b8fa44c98c@rivosinc.com> <20250724-riscv_kcfi-v1-10-04b8fa44c98c@rivosinc.com> <20250725161327.GC1724026@google.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; format=flowed Content-Disposition: inline In-Reply-To: Sorry forgot to respond to rest of the comments. On Fri, Jul 25, 2025 at 09:42:39AM -0700, Deepak Gupta wrote: >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 >>>--- >>> 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? Yes I don't like it either. I'll do something about it in next iteration. >> >>> 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. Yes that's good idea. I'll do that. >> >>> 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. Make sense. >> >>> 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. Yes I'll do these changes in next iteration. >> >>Sami