x86_64: Add gap to int3 to allow for call emulation

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@hirez.programming.kicks-ass.net

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

Cc: stable@vger.kernel.org
Fixes: b700e7f03df5 ("livepatch: kernel: add support for live patching")
Tested-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Nicolai Stange <nstange@suse.de>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..27fcc6fbdd52 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
 ENTRY(\sym)
         UNWIND_HINT_IRET_REGS offset=\has_error_code*8
 
@@ -899,6 +899,20 @@ ENTRY(\sym)
         jnz     .Lfrom_usermode_switch_stack_\@
         .endif
 
+        .if \create_gap == 1
+        /*
+         * If coming from kernel space, create a 6-word gap to allow the
+         * int3 handler to emulate a call instruction.
+         */
+        testb   $3, CS-ORIG_RAX(%rsp)
+        jnz     .Lfrom_usermode_no_gap_\@
+        .rept   6
+        pushq   5*8(%rsp)
+        .endr
+        UNWIND_HINT_IRET_REGS offset=8
+.Lfrom_usermode_no_gap_\@:
+        .endif
+
         .if \paranoid
         call    paranoid_entry
         .else
@@ -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
 
 #ifdef CONFIG_XEN_PV