[PATCH V2 4/4] x86/entry: don't copy to tmp in fixup_bad_iret

[Lai Jiangshan <laijs@linux.alibaba.com>]

It is safe to do memcpy() in fixup_bad_iret() now.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/kernel/traps.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9d16672865f8..1a0253a80a4c 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -660,17 +660,23 @@ struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
 	 * (rather than just below the IRET frame) and we want to
 	 * pretend that the exception came from the IRET target.
 	 */
-	struct bad_iret_stack tmp, *new_stack =
+	struct bad_iret_stack *new_stack =
 		(struct bad_iret_stack *)__this_cpu_read(cpu_current_top_of_stack) - 1;
 
-	/* Copy the IRET target to the temporary storage. */
-	memcpy(&tmp.regs.ip, (void *)s->regs.sp, 5*8);
+	/*
+	 * Both the IRET frame and the saved pt_regs must be on the
+	 * entry stacks since iret to user is only issued on the
+	 * entry stacks. So they don't overlap with kernel stack and
+	 * memcpy() is safe to copy them.
+	 */
+	BUG_ON(((unsigned long)s - (unsigned long)new_stack) < PAGE_SIZE ||
+	       ((unsigned long)new_stack - (unsigned long)s) < PAGE_SIZE);
 
-	/* Copy the remainder of the stack from the current stack. */
-	memcpy(&tmp, s, offsetof(struct bad_iret_stack, regs.ip));
+	/* Copy the IRET target to the new stack. */
+	memcpy(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
 
-	/* Update the entry stack */
-	memcpy(new_stack, &tmp, sizeof(tmp));
+	/* Copy the remainder of the stack from the current stack. */
+	memcpy(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));
 
 	BUG_ON(!user_mode(&new_stack->regs));
 	return new_stack;
-- 
2.20.1