From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Lai Jiangshan <laijs@linux.alibaba.com>,
        Borislav Petkov <bp@suse.de>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.10 107/130] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()
Date:   Mon,  6 Dec 2021 15:57:04 +0100
Message-Id: <20211206145603.335959196@linuxfoundation.org>
In-Reply-To: <20211206145559.607158688@linuxfoundation.org>
References: <20211206145559.607158688@linuxfoundation.org>
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From: Lai Jiangshan <laijs@linux.alibaba.com>

[ Upstream commit 5c8f6a2e316efebb3ba93d8c1af258155dcf5632 ]

In the native case, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is the
trampoline stack. But XEN pv doesn't use trampoline stack, so
PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is also the kernel stack.

In that case, source and destination stacks are identical, which means
that reusing swapgs_restore_regs_and_return_to_usermode() in XEN pv
would cause %rsp to move up to the top of the kernel stack and leave the
IRET frame below %rsp.

This is dangerous as it can be corrupted if #NMI / #MC hit as either of
these events occurring in the middle of the stack pushing would clobber
data on the (original) stack.

And, with  XEN pv, swapgs_restore_regs_and_return_to_usermode() pushing
the IRET frame on to the original address is useless and error-prone
when there is any future attempt to modify the code.

 [ bp: Massage commit message. ]

Fixes: 7f2590a110b8 ("x86/entry/64: Use a per-CPU trampoline stack for IDT entries")
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lkml.kernel.org/r/20211126101209.8613-4-jiangshanlai@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/x86/entry/entry_64.S |  4 ++++
 arch/x86/xen/xen-asm.S    | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index a806d68b96990..de541ea2788eb 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -575,6 +575,10 @@ SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
 	ud2
 1:
 #endif
+#ifdef CONFIG_XEN_PV
+	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
+#endif
+
 	POP_REGS pop_rdi=0
 
 	/*
diff --git a/arch/x86/xen/xen-asm.S b/arch/x86/xen/xen-asm.S
index 53cf8aa35032d..011ec649f3886 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -19,6 +19,7 @@
 
 #include <linux/init.h>
 #include <linux/linkage.h>
+#include <../entry/calling.h>
 
 /*
  * Enable events.  This clears the event mask and tests the pending
@@ -235,6 +236,25 @@ SYM_CODE_START(xen_sysret64)
 	jmp hypercall_iret
 SYM_CODE_END(xen_sysret64)
 
+/*
+ * XEN pv doesn't use trampoline stack, PER_CPU_VAR(cpu_tss_rw + TSS_sp0) is
+ * also the kernel stack.  Reusing swapgs_restore_regs_and_return_to_usermode()
+ * in XEN pv would cause %rsp to move up to the top of the kernel stack and
+ * leave the IRET frame below %rsp, which is dangerous to be corrupted if #NMI
+ * interrupts. And swapgs_restore_regs_and_return_to_usermode() pushing the IRET
+ * frame at the same address is useless.
+ */
+SYM_CODE_START(xenpv_restore_regs_and_return_to_usermode)
+	UNWIND_HINT_REGS
+	POP_REGS
+
+	/* stackleak_erase() can work safely on the kernel stack. */
+	STACKLEAK_ERASE_NOCLOBBER
+
+	addq	$8, %rsp	/* skip regs->orig_ax */
+	jmp xen_iret
+SYM_CODE_END(xenpv_restore_regs_and_return_to_usermode)
+
 /*
  * Xen handles syscall callbacks much like ordinary exceptions, which
  * means we have:
-- 
2.33.0