uml: don't kill pid 0

A bit of defensive programming - during development, it ocassionally
happens that a call to init_new_context is missed, resulting in
context holding a host pid of zero.  When that address space is torn
down, destroy_context does a kill(0), which instantly kills the whole
UML without any errors whatsoever.

This patch add a check for pids less than 2, to also catch 1 and
negative pids.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 1 deletions

diff --git a/arch/um/kernel/skas/mmu.c b/arch/um/kernel/skas/mmu.c
index e8dc8540d444..78b3e9f69d57 100644
--- a/arch/um/kernel/skas/mmu.c
+++ b/arch/um/kernel/skas/mmu.c
@@ -164,8 +164,20 @@ void destroy_context(struct mm_struct *mm)
 
         if (proc_mm)
                 os_close_file(mmu->id.u.mm_fd);
-        else
+        else {
+                /*
+                 * If init_new_context wasn't called, this will be
+                 * zero, resulting in a kill(0), which will result in the
+                 * whole UML suddenly dying.  Also, cover negative and
+                 * 1 cases, since they shouldn't happen either.
+                 */
+                if (mmu->id.u.pid < 2) {
+                        printk(KERN_ERR "corrupt mm_context - pid = %d\n",
+                               mmu->id.u.pid);
+                        return;
+                }
                 os_kill_ptraced_process(mmu->id.u.pid, 1);
+        }
 
         if (skas_needs_stub)
                 free_page(mmu->id.stack);