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Subject[PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup
Date
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@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: xen-devel@lists.xenproject.org
Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
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
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