[PATCH] Fix for 32-bit execve() error path

From: David Gibson <david@gibson.dropbear.id.au>

The patch below fixes a bug in ppc64's 32-bit execve() path.  It duplicates
logic already in the normal fs/exec.c do_execve() to avoid dropping a NULL
mm.  The bprm.mm becomes NULL once the exec passes the "point of no
return".  Without this patch a failure past that point (e.g.  mmap()
failure) will cause an oops, with it just a killed process.

arch/ppc64/kernel/sys_ppc32.c

@@ -2084,7 +2084,8 @@ static int do_execve32(char * filename, u32 * argv, u32 * envp, struct pt_regs *
 		security_bprm_free(&bprm);
 
 out_mm:
-	mmdrop(bprm.mm);
+	if (bprm.mm)
+		mmdrop(bprm.mm);
 
 out_file:
 	if (bprm.file) {