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, 95 insertions, 0 deletions

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index e22928c63ffc..f02e8ea4f1b5 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c
@@ -232,6 +232,101 @@ static int get_shstk_data(unsigned long *data, unsigned long __user *addr)
 	return 0;
 }
 
+static int shstk_push_sigframe(unsigned long *ssp)
+{
+	unsigned long target_ssp = *ssp;
+
+	/* Token must be aligned */
+	if (!IS_ALIGNED(target_ssp, 8))
+		return -EINVAL;
+
+	*ssp -= SS_FRAME_SIZE;
+	if (put_shstk_data((void *__user)*ssp, target_ssp))
+		return -EFAULT;
+
+	return 0;
+}
+
+static int shstk_pop_sigframe(unsigned long *ssp)
+{
+	unsigned long token_addr;
+	int err;
+
+	err = get_shstk_data(&token_addr, (unsigned long __user *)*ssp);
+	if (unlikely(err))
+		return err;
+
+	/* Restore SSP aligned? */
+	if (unlikely(!IS_ALIGNED(token_addr, 8)))
+		return -EINVAL;
+
+	/* SSP in userspace? */
+	if (unlikely(token_addr >= TASK_SIZE_MAX))
+		return -EINVAL;
+
+	*ssp = token_addr;
+
+	return 0;
+}
+
+int setup_signal_shadow_stack(struct ksignal *ksig)
+{
+	void __user *restorer = ksig->ka.sa.sa_restorer;
+	unsigned long ssp;
+	int err;
+
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
+	    !features_enabled(ARCH_SHSTK_SHSTK))
+		return 0;
+
+	if (!restorer)
+		return -EINVAL;
+
+	ssp = get_user_shstk_addr();
+	if (unlikely(!ssp))
+		return -EINVAL;
+
+	err = shstk_push_sigframe(&ssp);
+	if (unlikely(err))
+		return err;
+
+	/* Push restorer address */
+	ssp -= SS_FRAME_SIZE;
+	err = write_user_shstk_64((u64 __user *)ssp, (u64)restorer);
+	if (unlikely(err))
+		return -EFAULT;
+
+	fpregs_lock_and_load();
+	wrmsrl(MSR_IA32_PL3_SSP, ssp);
+	fpregs_unlock();
+
+	return 0;
+}
+
+int restore_signal_shadow_stack(void)
+{
+	unsigned long ssp;
+	int err;
+
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) ||
+	    !features_enabled(ARCH_SHSTK_SHSTK))
+		return 0;
+
+	ssp = get_user_shstk_addr();
+	if (unlikely(!ssp))
+		return -EINVAL;
+
+	err = shstk_pop_sigframe(&ssp);
+	if (unlikely(err))
+		return err;
+
+	fpregs_lock_and_load();
+	wrmsrl(MSR_IA32_PL3_SSP, ssp);
+	fpregs_unlock();
+
+	return 0;
+}
+
 void shstk_free(struct task_struct *tsk)
 {
 	struct thread_shstk *shstk = &tsk->thread.shstk;