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Subject[ 074/108] x86-32: Fix endless loop when processing signals for kernel tasks
3.0-stable review patch.  If anyone has any objections, please let me know.

------------------
From: Dmitry Adamushko <dmitry.adamushko@gmail.com>

commit 29a2e2836ff9ea65a603c89df217f4198973a74f upstream.

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.

Signed-off-by: Dmitry Adamushko <dmitry.adamushko@gmail.com>
Link: http://lkml.kernel.org/r/1332448765.2299.68.camel@dimm
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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




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