[PATCH v8 31/33] x86/fred: BUG() when ERETU with %rsp not equal to that when the ring 3 event was just delivered

[Xin Li]

A FRED stack frame generated by a ring 3 event should never be messed up, and
the first thing we must make sure is that at the time an ERETU instruction is
executed, %rsp must have the same address as that when the user level event
was just delivered.

However we don't want to bother the normal code path of ERETU because it's on
the hotest code path, a good choice is to do this check when ERETU faults.

Suggested-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Tested-by: Shan Kang <shan.kang@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
 arch/x86/mm/extable.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index 9d82193adf3c..be297d4b137b 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -204,6 +204,14 @@ static bool ex_handler_eretu(const struct exception_table_entry *fixup,
 	unsigned short ss = uregs->ss;
 	unsigned short cs = uregs->cs;
 
+	/*
+	 * A FRED stack frame generated by a ring 3 event should never be
+	 * messed up, and the first thing we must make sure is that at the
+	 * time an ERETU instruction is executed, %rsp must have the same
+	 * address as that when the user level event was just delivered.
+	 */
+	BUG_ON(uregs != current->thread_info.user_pt_regs);
+
 	/*
 	 * Move the NMI bit from the invalid stack frame, which caused ERETU
 	 * to fault, to the fault handler's stack frame, thus to unblock NMI
-- 
2.34.1