Re: [PATCH v27 23/31] x86/cet/shstk: Add user-mode shadow stack support
From: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Date: 2021-07-19 19:02:44
Also in:
linux-arch, linux-doc, linux-mm, lkml
On Fri, 2021-05-21 at 15:12 -0700, Yu-cheng Yu wrote:
quoted hunk ↗ jump to hunk
Introduce basic shadow stack enabling/disabling/allocation routines. A task's shadow stack is allocated from memory with VM_SHADOW_STACK flag and has a fixed size of min(RLIMIT_STACK, 4GB). Signed-off-by: Yu-cheng Yu <redacted> Cc: Kees Cook <redacted> --- v27: - Change 'struct cet_status' to 'struct thread_shstk', and change member types from unsigned long to u64. - Re-order local variables in reverse order of length. - WARN_ON_ONCE() when vm_munmap() fails. arch/x86/include/asm/cet.h | 30 +++++++ arch/x86/include/asm/processor.h | 5 ++ arch/x86/kernel/Makefile | 1 + arch/x86/kernel/shstk.c | 130 +++++++++++++++++++++++++++++++ 4 files changed, 166 insertions(+) create mode 100644 arch/x86/include/asm/cet.h create mode 100644 arch/x86/kernel/shstk.cdiff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h new file mode 100644 index 000000000000..6432baf4de1f --- /dev/null +++ b/arch/x86/include/asm/cet.h@@ -0,0 +1,30 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +#ifndef _ASM_X86_CET_H +#define _ASM_X86_CET_H + +#ifndef __ASSEMBLY__ +#include <linux/types.h> + +struct task_struct; + +/* + * Per-thread CET status + */ +struct thread_shstk { + u64 base; + u64 size; +}; + +#ifdef CONFIG_X86_SHADOW_STACK +int shstk_setup(void); +void shstk_free(struct task_struct *p); +void shstk_disable(void); +#else +static inline int shstk_setup(void) { return 0; } +static inline void shstk_free(struct task_struct *p) {} +static inline void shstk_disable(void) {} +#endif + +#endif /* __ASSEMBLY__ */ + +#endif /* _ASM_X86_CET_H */diff --git a/arch/x86/include/asm/processor.hb/arch/x86/include/asm/processor.h index 556b2b17c3e2..7eb56a837cfa 100644--- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h@@ -27,6 +27,7 @@ struct vm86;#include <asm/unwind_hints.h> #include <asm/vmxfeatures.h> #include <asm/vdso/processor.h> +#include <asm/cet.h> #include <linux/personality.h> #include <linux/cache.h>@@ -518,6 +519,10 @@ struct thread_struct {unsigned int sig_on_uaccess_err:1; +#ifdef CONFIG_X86_SHADOW_STACK + struct thread_shstk shstk; +#endif + /* Floating point and extended processor state */ struct fpu fpu; /*diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile index 0f66682ac02a..b0e599102277 100644 --- a/arch/x86/kernel/Makefile +++ b/arch/x86/kernel/Makefile@@ -149,6 +149,7 @@ obj-$(CONFIG_UNWINDER_FRAME_POINTER) +=unwind_frame.o obj-$(CONFIG_UNWINDER_GUESS) += unwind_guess.o obj-$(CONFIG_AMD_MEM_ENCRYPT) += sev.o +obj-$(CONFIG_X86_SHADOW_STACK) += shstk.o ### # 64 bit specific files ifeq ($(CONFIG_X86_64),y)diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c new file mode 100644 index 000000000000..5ea2b494e9f9 --- /dev/null +++ b/arch/x86/kernel/shstk.c@@ -0,0 +1,130 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * shstk.c - Intel shadow stack support + * + * Copyright (c) 2021, Intel Corporation. + * Yu-cheng Yu <yu-cheng.yu@intel.com> + */ + +#include <linux/types.h> +#include <linux/mm.h> +#include <linux/mman.h> +#include <linux/slab.h> +#include <linux/uaccess.h> +#include <linux/sched/signal.h> +#include <linux/compat.h> +#include <linux/sizes.h> +#include <linux/user.h> +#include <asm/msr.h> +#include <asm/fpu/internal.h> +#include <asm/fpu/xstate.h> +#include <asm/fpu/types.h> +#include <asm/cet.h> + +static void start_update_msrs(void) +{ + fpregs_lock(); + if (test_thread_flag(TIF_NEED_FPU_LOAD)) + __fpregs_load_activate(); +} + +static void end_update_msrs(void) +{ + fpregs_unlock(); +} + +static unsigned long alloc_shstk(unsigned long size) +{ + int flags = MAP_ANONYMOUS | MAP_PRIVATE; + struct mm_struct *mm = current->mm; + unsigned long addr, populate; + + mmap_write_lock(mm); + addr = do_mmap(NULL, 0, size, PROT_READ, flags,VM_SHADOW_STACK, 0, + &populate, NULL); + mmap_write_unlock(mm); + + return addr; +} + +int shstk_setup(void) +{ + struct thread_shstk *shstk = ¤t->thread.shstk; + unsigned long addr, size; + + if (!cpu_feature_enabled(X86_FEATURE_SHSTK)) + return -EOPNOTSUPP;
The only caller of this will skip it if !cpu_feature_enabled(X86_FEATURE_SHSTK), so this is dead logic. Same pattern in the IBT patch.
+
+ size = round_up(min_t(unsigned long long,
rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
+ addr = alloc_shstk(size);
+ if (IS_ERR_VALUE(addr))
+ return PTR_ERR((void *)addr);
+
+ shstk->base = addr;
+ shstk->size = size;
+
+ start_update_msrs();
+ wrmsrl(MSR_IA32_PL3_SSP, addr + size);
+ wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);
+ end_update_msrs();
+
+ return 0;
+}
+
+void shstk_free(struct task_struct *tsk)
+{
+ struct thread_shstk *shstk = &tsk->thread.shstk;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !shstk->size ||
+ !shstk->base)
+ return;
+
+ if (!tsk->mm)
+ return;
+
+ while (1) {
+ int r;
+
+ r = vm_munmap(shstk->base, shstk->size);
+
+ /*
+ * vm_munmap() returns -EINTR when mmap_lock is held
by
+ * something else, and that lock should not be held
for a
+ * long time. Retry it for the case.
+ */
+ if (r == -EINTR) {
+ cond_resched();
+ continue;
+ }
+
+ /*
+ * For all other types of vm_munmap() failure, either
the
+ * system is out of memory or there is bug.
+ */
+ WARN_ON_ONCE(r);
+ break;
+ }
+
+ shstk->base = 0;
+ shstk->size = 0;
+}
+
+void shstk_disable(void)
+{
+ struct thread_shstk *shstk = ¤t->thread.shstk;
+ u64 msr_val;
+
+ if (!cpu_feature_enabled(X86_FEATURE_SHSTK) ||
+ !shstk->size ||
+ !shstk->base)
+ return;
+
+ start_update_msrs();
+ rdmsrl(MSR_IA32_U_CET, msr_val);
+ wrmsrl(MSR_IA32_U_CET, msr_val & ~CET_SHSTK_EN);
+ wrmsrl(MSR_IA32_PL3_SSP, 0);
+ end_update_msrs();
+
+ shstk_free(current);
+}