x86-64/CET: Extend ucontext_t to save shadow stack

This patch adds a field to ucontext_t to save shadow stack:

1. getcontext and swapcontext are updated to save the caller's shadow
stack pointer and return addresses.
2. setcontext and swapcontext are updated to restore shadow stack and
jump to new context directly.
3. makecontext is updated to allocate a new shadow stack and set the
caller's return address to __start_context.

Since makecontext allocates a new shadow stack when making a new
context and kernel allocates a new shadow stack for clone/fork/vfork
syscalls, we track the current shadow stack base.  In setcontext and
swapcontext, if the target shadow stack base is the same as the current
shadow stack base, we unwind the shadow stack.  Otherwise it is a stack
switch and we look for a restore token.

We enable shadow stack at run-time only if program and all used shared
objects, including dlopened ones, are shadow stack enabled, which means
that they must be compiled with GCC 8 or above and glibc 2.28 or above.
We need to save and restore shadow stack only if shadow stack is enabled.
When caller of getcontext, setcontext, swapcontext and makecontext is
compiled with smaller ucontext_t, shadow stack won't be enabled at
run-time.  We check if shadow stack is enabled before accessing the
extended field in ucontext_t.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

	* sysdeps/unix/sysv/linux/x86/sys/ucontext.h (ucontext_t): Add
	__ssp.
	* sysdeps/unix/sysv/linux/x86_64/__start_context.S: Include
	<asm/prctl.h> and "ucontext_i.h" when shadow stack is enabled.
	(__push___start_context): New.
	* sysdeps/unix/sysv/linux/x86_64/getcontext.S: Include
	<asm/prctl.h>.
	(__getcontext): Record the current shadow stack base.  Save the
	caller's shadow stack pointer and base.
	* sysdeps/unix/sysv/linux/x86_64/makecontext.c: Include
	<pthread.h>, <libc-pointer-arith.h> and <sys/prctl.h>.
	(__push___start_context): New prototype.
	(__makecontext): Call __push___start_context to allocate a new
	shadow stack, push __start_context onto the new stack as well
	as the new shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/setcontext.S: Include
	<asm/prctl.h>.
	(__setcontext): Restore the target shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/swapcontext.S: Include
	<asm/prctl.h>.
	(__swapcontext): Record the current shadow stack base.  Save
	the caller's shadow stack pointer and base.  Restore the target
	shadow stack.
	* sysdeps/unix/sysv/linux/x86_64/sysdep.h
	(STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT): New.
	* sysdeps/unix/sysv/linux/x86_64/ucontext_i.sym (oSSP): New.

1 files changed, 92 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/x86_64/setcontext.S b/sysdeps/unix/sysv/linux/x86_64/setcontext.S
index b42af8e291..0afdf8ce0a 100644
--- a/sysdeps/unix/sysv/linux/x86_64/setcontext.S
+++ b/sysdeps/unix/sysv/linux/x86_64/setcontext.S
@@ -18,6 +18,7 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <sysdep.h>
+#include <asm/prctl.h>
 
 #include "ucontext_i.h"
 
@@ -79,6 +80,97 @@ ENTRY(__setcontext)
 	movq	oR14(%rdx), %r14
 	movq	oR15(%rdx), %r15
 
+#if SHSTK_ENABLED
+	/* Check if shadow stack is enabled.  */
+	testl	$X86_FEATURE_1_SHSTK, %fs:FEATURE_1_OFFSET
+	jz	L(no_shstk)
+
+	/* If the base of the target shadow stack is the same as the
+	   base of the current shadow stack, we unwind the shadow
+	   stack.  Otherwise it is a stack switch and we look for a
+	   restore token.  */
+	movq	oSSP(%rdx), %rsi
+	movq	%rsi, %rdi
+
+	/* Get the base of the target shadow stack.  */
+	movq	(oSSP + 8)(%rdx), %rcx
+	cmpq	%fs:SSP_BASE_OFFSET, %rcx
+	je	L(unwind_shadow_stack)
+
+L(find_restore_token_loop):
+	/* Look for a restore token.  */
+	movq	-8(%rsi), %rax
+	andq	$-8, %rax
+	cmpq	%rsi, %rax
+	je	L(restore_shadow_stack)
+
+	/* Try the next slot.  */
+	subq	$8, %rsi
+	jmp	L(find_restore_token_loop)
+
+L(restore_shadow_stack):
+	/* Pop return address from the shadow stack since setcontext
+	   will not return.  */
+	movq	$1, %rax
+	incsspq	%rax
+
+	/* Use the restore stoken to restore the target shadow stack.  */
+	rstorssp -8(%rsi)
+
+	/* Save the restore token on the old shadow stack.  NB: This
+	   restore token may be checked by setcontext or swapcontext
+	   later.  */
+	saveprevssp
+
+	/* Record the new shadow stack base that was switched to.  */
+	movq	(oSSP + 8)(%rdx), %rax
+	movq	%rax, %fs:SSP_BASE_OFFSET
+
+L(unwind_shadow_stack):
+	rdsspq	%rcx
+	subq	%rdi, %rcx
+	je	L(skip_unwind_shadow_stack)
+	negq	%rcx
+	shrq	$3, %rcx
+	movl	$255, %esi
+L(loop):
+	cmpq	%rsi, %rcx
+	cmovb	%rcx, %rsi
+	incsspq	%rsi
+	subq	%rsi, %rcx
+	ja	L(loop)
+
+L(skip_unwind_shadow_stack):
+	movq	oRSI(%rdx), %rsi
+	movq	oRDI(%rdx), %rdi
+	movq	oRCX(%rdx), %rcx
+	movq	oR8(%rdx), %r8
+	movq	oR9(%rdx), %r9
+
+	/* Get the return address set with getcontext.  */
+	movq	oRIP(%rdx), %r10
+
+	/* Setup finally %rdx.  */
+	movq	oRDX(%rdx), %rdx
+
+	/* Check if return address is valid for the case when setcontext
+	   is invoked from __start_context with linked context.  */
+	rdsspq	%rax
+	cmpq	(%rax), %r10
+	/* Clear RAX to indicate success.  NB: Don't use xorl to keep
+	   EFLAGS for jne.  */
+	movl	$0, %eax
+	jne	L(jmp)
+	/* Return to the new context if return address valid.  */
+	pushq	%r10
+	ret
+
+L(jmp):
+	/* Jump to the new context directly.  */
+	jmp	*%r10
+
+L(no_shstk):
+#endif
 	/* The following ret should return to the address set with
 	getcontext.  Therefore push the address on the stack.  */
 	movq	oRIP(%rdx), %rcx