By itself, this is useless. It gives us the ability to run some final
code before exit that cannnot run on the kernel stack. This could
include a CR3 switch a la KAISER or some kernel stack erasing, for
example. (Or even weird things like *changing* which kernel stack
gets used as an ASLR-strengthening mechanism.)
The SYSRET32 path is not covered yet. It could be in the future or
we could just ignore it and force the slow path if needed.
Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
arch/x86/entry/entry_64.S | 55 +++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 51 insertions(+), 4 deletions(-)
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 7d47199f405f..426b8c669d6a 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -330,8 +330,24 @@ syscall_return_via_sysret:
popq %rsi /* skip rcx */
popq %rdx
popq %rsi
+
+ /*
+ * Now all regs are restored except RSP and RDI.
+ * Save old stack pointer and switch to trampoline stack.
+ */
+ movq %rsp, %rdi
+ movq PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp
+
+ pushq RSP-RDI(%rdi) /* RSP */
+ pushq (%rdi) /* RDI */
+
+ /*
+ * We are on the trampoline stack. All regs except RDI are live.
+ * We can do future final exit work right here.
+ */
+
popq %rdi
- movq RSP-ORIG_RAX(%rsp), %rsp
+ popq %rsp
USERGS_SYSRET64
END(entry_SYSCALL_64)
@@ -633,10 +649,41 @@ GLOBAL(swapgs_restore_regs_and_return_to_usermode)
ud2
1:
#endif
- SWAPGS
POP_EXTRA_REGS
- POP_C_REGS
- addq $8, %rsp /* skip regs->orig_ax */
+ popq %r11
+ popq %r10
+ popq %r9
+ popq %r8
+ popq %rax
+ popq %rcx
+ popq %rdx
+ popq %rsi
+
+ /*
+ * The stack is now user RDI, orig_ax, RIP, CS, EFLAGS, RSP, SS.
+ * Save old stack pointer and switch to trampoline stack.
+ */
+ movq %rsp, %rdi
+ movq PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp
+
+ /* Copy the IRET frame to the trampoline stack. */
+ pushq 6*8(%rdi) /* SS */
+ pushq 5*8(%rdi) /* RSP */
+ pushq 4*8(%rdi) /* EFLAGS */
+ pushq 3*8(%rdi) /* CS */
+ pushq 2*8(%rdi) /* RIP */
+
+ /* Push user RDI on the trampoline stack. */
+ pushq (%rdi)
+
+ /*
+ * We are on the trampoline stack. All regs except RDI are live.
+ * We can do future final exit work right here.
+ */
+
+ /* Restore RDI. */
+ popq %rdi
+ SWAPGS
INTERRUPT_RETURN
--
2.13.6
