x86/shstk: Handle signals for shadow stack

When a signal is handled, the context is pushed to the stack before
handling it. For shadow stacks, since the shadow stack only tracks return
addresses, there isn't any state that needs to be pushed. However, there
are still a few things that need to be done. These things are visible to
userspace and which will be kernel ABI for shadow stacks.

One is to make sure the restorer address is written to shadow stack, since
the signal handler (if not changing ucontext) returns to the restorer, and
the restorer calls sigreturn. So add the restorer on the shadow stack
before handling the signal, so there is not a conflict when the signal
handler returns to the restorer.

The other thing to do is to place some type of checkable token on the
thread's shadow stack before handling the signal and check it during
sigreturn. This is an extra layer of protection to hamper attackers
calling sigreturn manually as in SROP-like attacks.

For this token the shadow stack data format defined earlier can be used.
Have the data pushed be the previous SSP. In the future the sigreturn
might want to return back to a different stack. Storing the SSP (instead
of a restore offset or something) allows for future functionality that
may want to restore to a different stack.

So, when handling a signal push
 - the SSP pointing in the shadow stack data format
 - the restorer address below the restore token.

In sigreturn, verify SSP is stored in the data format and pop the shadow
stack.

Co-developed-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-32-rick.p.edgecombe%40intel.com

1 files changed, 5 insertions, 0 deletions

diff --git a/arch/x86/include/asm/shstk.h b/arch/x86/include/asm/shstk.h
index d4a5c7b10cb5..ecb23a8ca47d 100644
--- a/arch/x86/include/asm/shstk.h
+++ b/arch/x86/include/asm/shstk.h
@@ -6,6 +6,7 @@
 #include <linux/types.h>
 
 struct task_struct;
+struct ksignal;
 
 #ifdef CONFIG_X86_USER_SHADOW_STACK
 struct thread_shstk {
@@ -18,6 +19,8 @@ void reset_thread_features(void);
 unsigned long shstk_alloc_thread_stack(struct task_struct *p, unsigned long clone_flags,
 				       unsigned long stack_size);
 void shstk_free(struct task_struct *p);
+int setup_signal_shadow_stack(struct ksignal *ksig);
+int restore_signal_shadow_stack(void);
 #else
 static inline long shstk_prctl(struct task_struct *task, int option,
 			       unsigned long arg2) { return -EINVAL; }
@@ -26,6 +29,8 @@ static inline unsigned long 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 int setup_signal_shadow_stack(struct ksignal *ksig) { return 0; }
+static inline int restore_signal_shadow_stack(void) { return 0; }
 #endif /* CONFIG_X86_USER_SHADOW_STACK */
 
 #endif /* __ASSEMBLY__ */