endless loop when processing signals for kernel-space tasks

From: Dmitry Adamushko <[email protected]>

The problem occurs on !CONFIG_VM86 kernels [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_notify_sig:
- 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 seen on MIPS.

Thanks in advance for any feedback!

index 79d97e6..7b784f4 100644
@@ -98,12 +98,6 @@
 #endif
 .endm
 
-#ifdef CONFIG_VM86
-#define resume_userspace_sig	check_userspace
-#else
-#define resume_userspace_sig	resume_userspace
-#endif
-
 /*
  * User gs save/restore
  *

@@ -327,10 +321,19 @@ ret_from_exception:
 	preempt_stop(CLBR_ANY)
 ret_from_intr:
 	GET_THREAD_INFO(%ebp)
-check_userspace:
+resume_userspace_sig:
+#ifdef CONFIG_VM86
 	movl PT_EFLAGS(%esp), %eax	# mix EFLAGS and CS
 	movb PT_CS(%esp), %al
 	andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax
+#else
+	/*
+	 * We can be coming here from a syscall done in the kernel space,
+	 * e.g. a failed kernel_execve().
+	 */
+	movl PT_CS(%esp), %eax
+	andl $SEGMENT_RPL_MASK, %eax
+#endif
 	cmpl $USER_RPL, %eax
 	jb resume_kernel		# not returning to v8086 or userspace
 

Commit-ID:  29a2e2836ff9ea65a603c89df217f4198973a74f
Gitweb:     http://git.kernel.org/tip/29a2e2836ff9ea65a603c89df217f4198973a74f
Author:     Dmitry Adamushko <[email protected]>
AuthorDate: Thu, 22 Mar 2012 21:39:25 +0100
Committer:  H. Peter Anvin <[email protected]>
CommitDate: Thu, 22 Mar 2012 13:50:25 -0700

x86-32: Fix endless loop when processing signals for kernel tasks

The problem occurs on !CONFIG_VM86 kernels [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_notify_sig:
- 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 seen on MIPS.

---
 arch/x86/kernel/entry_32.S |   17 ++++++++++-------
 1 files changed, 10 insertions(+), 7 deletions(-)

index 79d97e6..7b784f4 100644
@@ -98,12 +98,6 @@
 #endif
 .endm
 
-#ifdef CONFIG_VM86
-#define resume_userspace_sig	check_userspace
-#else
-#define resume_userspace_sig	resume_userspace
-#endif
-
 /*
  * User gs save/restore
  *

@@ -327,10 +321,19 @@ ret_from_exception:
 	preempt_stop(CLBR_ANY)
 ret_from_intr:
 	GET_THREAD_INFO(%ebp)
-check_userspace:
+resume_userspace_sig:
+#ifdef CONFIG_VM86
 	movl PT_EFLAGS(%esp), %eax	# mix EFLAGS and CS
 	movb PT_CS(%esp), %al
 	andl $(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %eax
+#else
+	/*
+	 * We can be coming here from a syscall done in the kernel space,
+	 * e.g. a failed kernel_execve().
+	 */
+	movl PT_CS(%esp), %eax
+	andl $SEGMENT_RPL_MASK, %eax
+#endif
 	cmpl $USER_RPL, %eax
 	jb resume_kernel		# not returning to v8086 or userspace