MIPS: Fix endless loop when processing signals for kernel tasks

The problem occurs [1] when a kernel-mode task returns from a system
call with a pending signal.

A real-life scenario is a child of 'khelper' returning from a failed
kernel_execve() in ____call_usermodehelper() [ kernel/kmod.c ].
kernel_execve() fails due to a pending SIGKILL, which is the result of
"kill -9 -1" (at least, busybox's init does it upon reboot).

The loop is as follows:

* syscall_exit_work:
 - work_pending:            // start_of_the_loop
 - work_notifysig:
   - do_notify_resume()
     - do_signal()
       - if (!user_mode(regs)) return;
 - resume_userspace         // TIF_SIGPENDING is still set
 - work_pending             // so we call work_pending => goto
                            // start_of_the_loop

More information can be found in another LKML thread:
http://www.serverphorums.com/read.php?12,457826

[1] The problem was also reproduced on !CONFIG_VM86 x86, and the
following fix was accepted.

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=29a2e2836ff9ea65a603c89df217f4198973a74f

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3571/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 6 insertions, 1 deletions

diff --git a/arch/mips/kernel/entry.S b/arch/mips/kernel/entry.S
index a6c133212003..9b00362f32f6 100644
--- a/arch/mips/kernel/entry.S
+++ b/arch/mips/kernel/entry.S
@@ -36,6 +36,11 @@ FEXPORT(ret_from_exception)
 FEXPORT(ret_from_irq)
         LONG_S  s0, TI_REGS($28)
 FEXPORT(__ret_from_irq)
+/*
+ * We can be coming here from a syscall done in the kernel space,
+ * e.g. a failed kernel_execve().
+ */
+resume_userspace_check:
         LONG_L  t0, PT_STATUS(sp)               # returning to kernel mode?
         andi    t0, t0, KU_USER
         beqz    t0, resume_kernel
@@ -162,7 +167,7 @@ work_notifysig:				# deal with pending signals and
         move    a0, sp
         li      a1, 0
         jal     do_notify_resume        # a2 already loaded
-        j       resume_userspace
+        j       resume_userspace_check
 
 FEXPORT(syscall_exit_partial)
         local_irq_disable               # make sure need_resched doesn't