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[xen master] x86/entry: Adjust guest paths to be shadow stack compatible

commit 43b98e71903845c727028d2e84b90e5905ff8d66
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Apr 24 14:34:44 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 29 23:09:46 2020 +0100

    x86/entry: Adjust guest paths to be shadow stack compatible
    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 
    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 
    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 
    use alternatives to avoid extra runtime CET detection logic.
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
 xen/arch/x86/x86_64/compat/entry.S |  1 +
 xen/arch/x86/x86_64/entry.S        | 36 ++++++++++++++++++++++++++++++++++--
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/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. */
+        ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
         /* sti could live here when we don't switch page tables below. */
         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 e68d680791..6470266d2f 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
+        ja    .Lrestore_rcx_iret_exit_to_guest
+        /* Clear the supervisor shadow stack token busy bit. */
+.macro rag_clrssbsy
+        rdsspq %rcx
+        clrssbsy (%rcx)
+        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
@@ -201,6 +208,8 @@ restore_all_guest:
 1:      sysretl
+        movq  8(%rsp), %rcx           # RIP
 /* No special register assumptions. */
         andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
@@ -226,6 +235,7 @@ iret_exit_to_guest:
  * %ss must be saved into the space left by the trampoline.
+        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 +269,7 @@ ENTRY(lstar_enter)
         jmp   test_all_events
+        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 +888,27 @@ 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
+        ALTERNATIVE "", ist_switch_shstk, X86_FEATURE_XEN_SHSTK
generated by git-patchbot for /home/xen/git/xen.git#master



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