[PATCH v29 25/32] x86/cet/shstk: Handle thread shadow stack

[Yu-cheng Yu]

For clone() with CLONE_VM, except vfork, the child and the parent must have
separate shadow stacks.  Thus, the kernel allocates, and frees on thread
exit a new shadow stack for the child.

Use stack_size passed from clone3() syscall for thread shadow stack size.
A compat-mode thread shadow stack size is further reduced to 1/4.  This
allows more threads to run in a 32-bit address space.

The earlier version of clone() did not have stack_size passed in.  In that
case, use RLIMIT_STACK size and cap to 4 GB.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
---
v29:
- WARN_ON_ONCE() when get_xsave_addr() returns NULL, and update comments.
  (Dave Hansen)

v28:
- Split out copy_thread() argument name changes to a new patch.
- Add compatibility for earlier clone(), which does not pass stack_size.
- Add comment for get_xsave_addr(), explain the handling of null return
  value.
---
 arch/x86/include/asm/cet.h         |  5 +++
 arch/x86/include/asm/mmu_context.h |  3 ++
 arch/x86/kernel/process.c          |  6 +++
 arch/x86/kernel/shstk.c            | 63 +++++++++++++++++++++++++++++-
 4 files changed, 76 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
index 6432baf4de1f..4314a41ab3c9 100644
--- a/arch/x86/include/asm/cet.h
+++ b/arch/x86/include/asm/cet.h
@@ -17,10 +17,15 @@ struct thread_shstk {
 
 #ifdef CONFIG_X86_SHADOW_STACK
 int shstk_setup(void);
+int shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
+			     unsigned long stack_size);
 void shstk_free(struct task_struct *p);
 void shstk_disable(void);
 #else
 static inline int shstk_setup(void) { return 0; }
+static inline int shstk_alloc_thread_stack(struct task_struct *p,
+					   unsigned long clone_flags,
+					   unsigned long stack_size) { return 0; }
 static inline void shstk_free(struct task_struct *p) {}
 static inline void shstk_disable(void) {}
 #endif
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 27516046117a..e1dd083261a5 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -12,6 +12,7 @@
 #include <asm/tlbflush.h>
 #include <asm/paravirt.h>
 #include <asm/debugreg.h>
+#include <asm/cet.h>
 
 extern atomic64_t last_mm_ctx_id;
 
@@ -146,6 +147,8 @@ do {						\
 #else
 #define deactivate_mm(tsk, mm)			\
 do {						\
+	if (!tsk->vfork_done)			\
+		shstk_free(tsk);		\
 	load_gs_index(0);			\
 	loadsegment(fs, 0);			\
 } while (0)
diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index e6e4d8bc9023..bade6a594d63 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -43,6 +43,7 @@
 #include <asm/io_bitmap.h>
 #include <asm/proto.h>
 #include <asm/frame.h>
+#include <asm/cet.h>
 
 #include "process.h"
 
@@ -103,6 +104,7 @@ void exit_thread(struct task_struct *tsk)
 
 	free_vm86(t);
 
+	shstk_free(tsk);
 	fpu__drop(fpu);
 }
 
@@ -200,6 +202,10 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
 	if (clone_flags & CLONE_SETTLS)
 		ret = set_new_tls(p, tls);
 
+	/* Allocate a new shadow stack for pthread */
+	if (!ret)
+		ret = shstk_alloc_thread_stack(p, clone_flags, stack_size);
+
 	if (!ret && unlikely(test_tsk_thread_flag(current, TIF_IO_BITMAP)))
 		io_bitmap_share(p);
 
diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index 5993aa8db338..7c1ca2476a5e 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -75,6 +75,61 @@ int shstk_setup(void)
 	return err;
 }
 
+int shstk_alloc_thread_stack(struct task_struct *tsk, unsigned long clone_flags,
+			     unsigned long stack_size)
+{
+	struct thread_shstk *shstk = &tsk->thread.shstk;
+	struct cet_user_state *state;
+	unsigned long addr;
+
+	if (!shstk->size)
+		return 0;
+
+	/*
+	 * Earlier clone() does not pass stack_size.  Use RLIMIT_STACK and
+	 * cap to 4 GB.
+	 */
+	if (!stack_size)
+		stack_size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
+
+	/*
+	 * For CLONE_VM, except vfork, the child needs a separate shadow
+	 * stack.
+	 */
+	if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
+		return 0;
+
+	/*
+	 * 'tsk' is configured with a shadow stack and the fpu.state is
+	 * up to date since it was just copied from the parent.  There
+	 * must be a valid non-init CET state location in the buffer.
+	 */
+	state = get_xsave_addr(&tsk->thread.fpu.state.xsave, XFEATURE_CET_USER);
+	if (WARN_ON_ONCE(!state))
+		return -EINVAL;
+
+	/*
+	 * Compat-mode pthreads share a limited address space.
+	 * If each function call takes an average of four slots
+	 * stack space, allocate 1/4 of stack size for shadow stack.
+	 */
+	if (in_compat_syscall())
+		stack_size /= 4;
+
+	stack_size = round_up(stack_size, PAGE_SIZE);
+	addr = alloc_shstk(stack_size);
+	if (IS_ERR_VALUE(addr)) {
+		shstk->base = 0;
+		shstk->size = 0;
+		return PTR_ERR((void *)addr);
+	}
+
+	state->user_ssp = (u64)(addr + stack_size);
+	shstk->base = addr;
+	shstk->size = stack_size;
+	return 0;
+}
+
 void shstk_free(struct task_struct *tsk)
 {
 	struct thread_shstk *shstk = &tsk->thread.shstk;
@@ -84,7 +139,13 @@ void shstk_free(struct task_struct *tsk)
 	    !shstk->base)
 		return;
 
-	if (!tsk->mm)
+	/*
+	 * When fork() with CLONE_VM fails, the child (tsk) already has a
+	 * shadow stack allocated, and exit_thread() calls this function to
+	 * free it.  In this case the parent (current) and the child share
+	 * the same mm struct.
+	 */
+	if (!tsk->mm || tsk->mm != current->mm)
 		return;
 
 	while (1) {
-- 
2.21.0