[PATCH 5.10 107/130] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()

Greg Kroah-Hartman
2021-12-06T15:49:12Z

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