[tip: x86/urgent] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     ffae641f57476369b4d503402b37ebe489d23395
Gitweb:        https://git.kernel.org/tip/ffae641f57476369b4d503402b37ebe489d23395
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Fri, 26 Jun 2020 10:21:13 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Wed, 01 Jul 2020 10:00:26 +02:00

x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

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.

Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/947880c41ade688ff4836f665d0c9fcaa9bd1201.1593191971.git.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 7b9d815..381a6de 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 5d252aa..e1e1c7e 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 */