[PATCH] x86/stacktrace: Do not access user space memory unnecessarily

[Eiichi Tsukata]

Put the boundary check before it accesses user space to prevent unnecessary
access which might crash the machine.

Especially, ftrace preemptirq/irq_disable event with user stack trace
option can trigger SEGV in pid 1 which leads to panic.

Reproducer:

  CONFIG_PREEMPTIRQ_TRACEPOINTS=y
  # echo 1 > events/preemptirq/enable
  # echo userstacktrace > trace_options

Output:

  Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
  CPU: 1 PID: 1 Comm: systemd Not tainted 5.2.0-rc7+ #10
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
  Call Trace:
   dump_stack+0x67/0x90
   panic+0x100/0x2c6
   do_exit.cold+0x4e/0x101
   do_group_exit+0x3a/0xa0
   get_signal+0x14a/0x8e0
   do_signal+0x36/0x650
   exit_to_usermode_loop+0x92/0xb0
   prepare_exit_to_usermode+0x6f/0xb0
   retint_user+0x8/0x18
  RIP: 0033:0x55be7ad1c89f
  Code: Bad RIP value.
  RSP: 002b:00007ffe329a4b00 EFLAGS: 00010202
  RAX: 0000000000000768 RBX: 00007ffe329a4ba0 RCX: 00007ff0063aa469
  RDX: 00007ff0066761de RSI: 00007ffe329a4b20 RDI: 0000000000000768
  RBP: 000000000000000b R08: 0000000000000000 R09: 00007ffe329a4e2f
  R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000768
  R13: 0000000000000000 R14: 0000000000000004 R15: 000055be7b3d3560
  Kernel Offset: 0x2a000000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)

Fixes: 02b67518e2b1 ("tracing: add support for userspace stacktraces in tracing/iter_ctrl")
Signed-off-by: Eiichi Tsukata <devel@xxxxxxxxxxxx>
---
 arch/x86/kernel/stacktrace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index 2abf27d7df6b..6d0c608ffe34 100644
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -123,12 +123,12 @@ void arch_stack_walk_user(stack_trace_consume_fn consume_entry, void *cookie,
 	while (1) {
 		struct stack_frame_user frame;
 
+		if ((unsigned long)fp < regs->sp)
+			break;
 		frame.next_fp = NULL;
 		frame.ret_addr = 0;
 		if (!copy_stack_frame(fp, &frame))
 			break;
-		if ((unsigned long)fp < regs->sp)
-			break;
 		if (frame.ret_addr) {
 			if (!consume_entry(cookie, frame.ret_addr, false))
 				return;
-- 
2.21.0