[PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

[Andy Lutomirski <luto@xxxxxxxxxx>]

The SYSENTER frame setup was nonsense.  It worked by accident
because the normal code into which the Xen asm jumped
(entry_SYSENTER_32/compat) threw away SP without touching the stack.
entry_SYSENTER_compat was recently modified such that it relied on
having a valid stack pointer, so now the Xen asm needs to invoke it
with a valid stack.

Fix it up like SYSCALL: use the Xen-provided frame and skip the bare
metal prologue.

Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/entry/entry_64_compat.S |  1 +
 arch/x86/xen/xen-asm_64.S        | 20 ++++++++++++++++----
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 7b9d8150f652..381a6de7de9c 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -79,6 +79,7 @@ SYM_CODE_START(entry_SYSENTER_compat)
        pushfq                          /* pt_regs->flags (except IF = 0) */
        pushq   $__USER32_CS            /* pt_regs->cs */
        pushq   $0                      /* pt_regs->ip = 0 (placeholder) */
+SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
        pushq   %rax                    /* pt_regs->orig_ax */
        pushq   %rdi                    /* pt_regs->di */
        pushq   %rsi                    /* pt_regs->si */
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index 5d252aaeade8..e1e1c7eafa60 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -161,10 +161,22 @@ SYM_FUNC_END(xen_syscall32_target)
 
 /* 32-bit compat sysenter target */
 SYM_FUNC_START(xen_sysenter_target)
-       mov 0*8(%rsp), %rcx
-       mov 1*8(%rsp), %r11
-       mov 5*8(%rsp), %rsp
-       jmp entry_SYSENTER_compat
+       /*
+        * NB: Xen is polite and clears TF from EFLAGS for us.  This means
+        * that we don't need to guard against single step exceptions here.
+        */
+       popq %rcx
+       popq %r11
+
+       /*
+        * Neither Xen nor the kernel really knows what the old SS and
+        * CS were.  The kernel expects __USER32_DS and __USER32_CS, so
+        * report those values even though Xen will guess its own values.
+        */
+       movq $__USER32_DS, 4*8(%rsp)
+       movq $__USER32_CS, 1*8(%rsp)
+
+       jmp entry_SYSENTER_compat_after_hwframe
 SYM_FUNC_END(xen_sysenter_target)
 
 #else /* !CONFIG_IA32_EMULATION */
-- 
2.25.4