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Subject[tip: x86/urgent] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 5c8f6a2e316efebb3ba93d8c1af258155dcf5632
Gitweb: https://git.kernel.org/tip/5c8f6a2e316efebb3ba93d8c1af258155dcf5632
Author: Lai Jiangshan <laijs@linux.alibaba.com>
AuthorDate: Fri, 26 Nov 2021 18:11:23 +08:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Fri, 03 Dec 2021 19:21:15 +01:00

x86/xen: Add xenpv_restore_regs_and_return_to_usermode()

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
---
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 f9e1c06..97b1f84 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -574,6 +574,10 @@ 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 220dd96..444d824 100644
--- a/arch/x86/xen/xen-asm.S
+++ b/arch/x86/xen/xen-asm.S
@@ -20,6 +20,7 @@

#include <linux/init.h>
#include <linux/linkage.h>
+#include <../entry/calling.h>

.pushsection .noinstr.text, "ax"
/*
@@ -193,6 +194,25 @@ SYM_CODE_START(xen_iret)
SYM_CODE_END(xen_iret)

/*
+ * 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:
* - kernel gs
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