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