[RFC][PATCH 1/3] x86_64: Add gap to int3 to allow for call emulation

From: Steven Rostedt
Date: Tue May 07 2019 - 13:45:10 EST

From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>

To allow an int3 handler to emulate a call instruction, it must be able to
push a return address onto the stack. Add a gap to the stack to allow the
int3 handler to push the return address and change the return from int3 to
jump straight to the emulated called function target.

Link: http://lkml.kernel.org/r/20181130183917.hxmti5josgq4clti@treble
Link: http://lkml.kernel.org/r/20190502162133.GX2623@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Note, this is needed to allow Live Kernel Patching to not miss calling a
patched function when tracing is enabled. -- Steven Rostedt

Cc: stable@xxxxxxxxxxxxxxx
Fixes: b700e7f03df5 ("livepatch: kernel: add support for live patching")
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
arch/x86/entry/entry_64.S | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..00df6b135ab1 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -879,7 +879,7 @@ apicinterrupt IRQ_WORK_VECTOR irq_work_interrupt smp_irq_work_interrupt
* @paranoid == 2 is special: the stub will never switch stacks. This is for
* #DF: if the thread stack is somehow unusable, we'll still get a useful OOPS.
-.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1
+.macro idtentry sym do_sym has_error_code:req paranoid=0 shift_ist=-1 create_gap=0
UNWIND_HINT_IRET_REGS offset=\has_error_code*8

@@ -899,6 +899,20 @@ ENTRY(\sym)
jnz .Lfrom_usermode_switch_stack_\@

+ .if \create_gap == 1
+ /*
+ * If coming from kernel space, create a 6-word gap to allow the static
+ * call #BP handler to emulate a call instruction.
+ */
+ testb $3, CS-ORIG_RAX(%rsp)
+ jnz .Lfrom_usermode_no_gap_\@
+ .rept 6
+ pushq 5*8(%rsp)
+ .endr
+ .endif
.if \paranoid
call paranoid_entry
@@ -1130,7 +1144,7 @@ apicinterrupt3 HYPERV_STIMER0_VECTOR \
#endif /* CONFIG_HYPERV */

idtentry debug do_debug has_error_code=0 paranoid=1 shift_ist=DEBUG_STACK
-idtentry int3 do_int3 has_error_code=0
+idtentry int3 do_int3 has_error_code=0 create_gap=1
idtentry stack_segment do_stack_segment has_error_code=1