The SYSCALL/SYSENTER/SYSRET paths need to use {SET,CLR}SSBSY. The IRET to
guest paths must not. In the SYSRET path, re-position the mov which loads rip
into %rcx so we can use %rcx for CLRSSBSY, rather than spilling another
register to the stack.
While we can in principle detect shadow stack corruption and a failure to
clear the supervisor token busy bit in the SYSRET path (by inspecting the
carry flag following CLRSSBSY), we cannot detect similar problems for the IRET
path (IRET is specified not to fault in this case).
We will double fault at some point later, when next trying to enter Xen, due
to an already-set supervisor shadow stack busy bit. As SYSRET is a uncommon
path anyway, avoid the added complexity for no appreciable gain.
The IST switch onto the primary stack is not great as we have an instruction
boundary with no shadow stack. This is the least bad option available.
These paths are not used before shadow stacks are properly established, so can
use alternatives to avoid extra runtime CET detection logic.
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
v2:
* Break comment deletion out to an earlier patch
* SETSSBSY on the SYSENTER path as well
* Don't spill %rax to the stack in the SYSRET path
---
xen/arch/x86/x86_64/compat/entry.S | 1 +
xen/arch/x86/x86_64/entry.S | 32 +++++++++++++++++++++++++++++++-
2 files changed, 32 insertions(+), 1 deletion(-)
diff --git a/xen/arch/x86/x86_64/compat/entry.S b/xen/arch/x86/x86_64/compat/entry.S
index 3cd375bd48..2ca81341a4 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -198,6 +198,7 @@ ENTRY(cr4_pv32_restore)
/* See lstar_enter for entry register state. */
ENTRY(cstar_enter)
+ ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
/* sti could live here when we don't switch page tables below. */
CR4_PV32_RESTORE
movq 8(%rsp),%rax /* Restore %rax. */
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 78ac0df49f..449ee468e4 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -191,9 +191,16 @@ restore_all_guest:
sarq $47,%rcx
incl %ecx
cmpl $1,%ecx
- movq 8(%rsp),%rcx # RIP
ja iret_exit_to_guest
+ /* Clear the supervisor shadow stack token busy bit. */
+.macro rag_clrssbsy
+ rdsspq %rcx
+ clrssbsy (%rcx)
+.endm
+ ALTERNATIVE "", rag_clrssbsy, X86_FEATURE_XEN_SHSTK
+
+ movq 8(%rsp), %rcx # RIP
cmpw $FLAT_USER_CS32,16(%rsp)# CS
movq 32(%rsp),%rsp # RSP
je 1f
@@ -226,6 +233,7 @@ iret_exit_to_guest:
* %ss must be saved into the space left by the trampoline.
*/
ENTRY(lstar_enter)
+ ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
/* sti could live here when we don't switch page tables below. */
movq 8(%rsp),%rax /* Restore %rax. */
movq $FLAT_KERNEL_SS,8(%rsp)
@@ -259,6 +267,7 @@ ENTRY(lstar_enter)
jmp test_all_events
ENTRY(sysenter_entry)
+ ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
/* sti could live here when we don't switch page tables below. */
pushq $FLAT_USER_SS
pushq $0
@@ -877,6 +886,27 @@ handle_ist_exception:
movl $UREGS_kernel_sizeof/8,%ecx
movq %rdi,%rsp
rep movsq
+
+ /* Switch Shadow Stacks */
+.macro ist_switch_shstk
+ rdsspq %rdi
+ clrssbsy (%rdi)
+ /*
+ * Switching supervisor shadow stacks is specially hard, as supervisor
+ * and restore tokens are incompatible.
+ *
+ * For now, we only need to switch on to an unused primary shadow
+ * stack, so use SETSSBSY for the purpose, exactly like the
+ * SYSCALL/SYSENTER entry.
+ *
+ * Ideally, we'd want to CLRSSBSY after switching stacks, but that
+ * will leave SSP zeroed so it not an option. Instead, we transiently
+ * have a zero SSP on this instruction boundary, and depend on IST for
+ * NMI/#MC protection.
+ */
+ setssbsy
+.endm
+ ALTERNATIVE "", ist_switch_shstk, X86_FEATURE_XEN_SHSTK
1:
#else
ASSERT_CONTEXT_IS_XEN
--
2.11.0