[xen master] x86/boot: Use RSTORSSP to establish SSP

[patchbot@xxxxxxx]

commit 3037d8b7eb3e9c3c9e4a0a257115557c300795fc
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon May 26 18:48:26 2025 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Dec 5 23:00:32 2025 +0000

    x86/boot: Use RSTORSSP to establish SSP
    
    Under FRED, SETSSBSY is disallowed, and we want to be setting up FRED prior 
to
    setting up shadow stacks.  As we still need Supervisor Tokens in IDT mode, 
we
    need mode-specific logic to establish SSP.
    
    In FRED mode, write a Restore Token, RSTORSSP it, and discard the resulting
    Previous-SSP token.
    
    No change outside of FRED mode.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/boot/x86_64.S | 23 +++++++++++++++++++++--
 xen/arch/x86/setup.c       | 25 ++++++++++++++++++++++++-
 2 files changed, 45 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/boot/x86_64.S b/xen/arch/x86/boot/x86_64.S
index 11a7e9d3bd..9705d03f84 100644
--- a/xen/arch/x86/boot/x86_64.S
+++ b/xen/arch/x86/boot/x86_64.S
@@ -53,17 +53,21 @@ ENTRY(__high_start)
         mov     %rcx, STACK_CPUINFO_FIELD(cr4)(%r15)
         mov     %rcx, %cr4
 
-        /* WARNING! call/ret now fatal (iff SHSTK) until SETSSBSY loads SSP */
+        /* WARNING! CALL/RET now fatal (iff SHSTK) until SETSSBSY/RSTORSSP 
loads SSP */
 
 #if defined(CONFIG_XEN_SHSTK)
         test    $CET_SHSTK_EN, %al
         jz      .L_ap_cet_done
 
-        /* Derive the supervisor token address from %rsp. */
+        /* Derive the token address from %rsp. */
         mov     %rsp, %rdx
         and     $~(STACK_SIZE - 1), %rdx
         or      $(PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8, %rdx
 
+        /* Establishing SSP differs between IDT or FRED mode. */
+        bt      $32 /* ilog2(X86_CR4_FRED) */, %rcx
+        jc      .L_fred_shstk
+
         /*
          * Write a new Supervisor Token.  It doesn't matter the first time a
          * CPU boots, but for S3 resume or hotplug this clears the busy bit so
@@ -71,6 +75,21 @@ ENTRY(__high_start)
          */
         wrssq   %rdx, (%rdx)
         setssbsy
+        jmp     .L_ap_cet_done
+
+.L_fred_shstk:
+
+        /*
+         * Write a Restore Token, value: &token + 8 + 64BIT (bit 0) at the
+         * base of the shstk (which isn't in use yet).
+         */
+        lea     9(%rdx), %rdi
+        wrssq   %rdi, (%rdx)
+        rstorssp (%rdx)
+
+        /* Discard the Previous-SSP Token from the shstk. */
+        mov     $2, %edx
+        incsspd %edx
 
 #endif /* CONFIG_XEN_SHSTK */
 .L_ap_cet_done:
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 872a8c63f9..44da5efa1d 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -49,6 +49,7 @@
 #include <asm/prot-key.h>
 #include <asm/pv/domain.h>
 #include <asm/setup.h>
+#include <asm/shstk.h>
 #include <asm/smp.h>
 #include <asm/spec_ctrl.h>
 #include <asm/stubs.h>
@@ -908,7 +909,29 @@ static void __init noreturn reinit_bsp_stack(void)
     if ( cpu_has_xen_shstk )
     {
         wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
-        asm volatile ("setssbsy" ::: "memory");
+
+        /*
+         * IDT and FRED differ by a Supervisor Token on the shadow stack.
+         *
+         * In IDT mode, we use SETSSBSY (itself using MSR_PL0_SSP, configured
+         * previously) to mark the Supervisor Token as Busy.  In FRED mode,
+         * there is no token, so we need to create a temporary Restore Token
+         * to establish SSP.
+         */
+        if ( opt_fred )
+        {
+            unsigned long *token =
+                (void *)stack + (PRIMARY_SHSTK_SLOT + 1) * PAGE_SIZE - 8;
+
+            wrss((unsigned long)token + 9, token);
+            asm volatile ( "rstorssp %0" : "+m" (*token) );
+            /*
+             * We need to discard the resulting Previous-SSP Token, but
+             * reset_stack_and_jump() will do that for us.
+             */
+        }
+        else
+            asm volatile ( "setssbsy" ::: "memory" );
     }
 
     reset_stack_and_jump(init_done);
--
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