um: Implement probe_kernel_read()

commit f75b1b1bedfb498cc43a992ce4d7ed8df3b1e770 upstream.

UML needs it's own probe_kernel_read() to handle kernel
mode faults correctly.
The implementation uses mincore() on the host side to detect
whether a page is owned by the UML kernel process.

This fixes also a possible crash when sysrq-t is used.
Starting with 3.10 sysrq-t calls probe_kernel_read() to
read details from the kernel workers. As kernel worker are
completely async pointers may turn NULL while reading them.

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: <stian@nixia.no>
Cc: <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

4 files changed, 78 insertions, 1 deletions

diff --git a/arch/um/include/shared/os.h b/arch/um/include/shared/os.h
index 95feaa47a2fb..c70a234a3f8c 100644
--- a/arch/um/include/shared/os.h
+++ b/arch/um/include/shared/os.h
@@ -200,6 +200,7 @@ extern int os_unmap_memory(void *addr, int len);
 extern int os_drop_memory(void *addr, int length);
 extern int can_drop_memory(void);
 extern void os_flush_stdout(void);
+extern int os_mincore(void *addr, unsigned long len);
 
 /* execvp.c */
 extern int execvp_noalloc(char *buf, const char *file, char *const argv[]);
diff --git a/arch/um/kernel/Makefile b/arch/um/kernel/Makefile
index babe21826e3e..d8b78a03855c 100644
--- a/arch/um/kernel/Makefile
+++ b/arch/um/kernel/Makefile
@@ -13,7 +13,7 @@ clean-files :=
 obj-y = config.o exec.o exitcode.o irq.o ksyms.o mem.o \
         physmem.o process.o ptrace.o reboot.o sigio.o \
         signal.o smp.o syscall.o sysrq.o time.o tlb.o trap.o \
-        um_arch.o umid.o skas/
+        um_arch.o umid.o maccess.o skas/
 
 obj-$(CONFIG_BLK_DEV_INITRD) += initrd.o
 obj-$(CONFIG_GPROF)     += gprof_syms.o
diff --git a/arch/um/kernel/maccess.c b/arch/um/kernel/maccess.c
new file mode 100644
index 000000000000..1f3d5c4910d1
--- /dev/null
+++ b/arch/um/kernel/maccess.c
@@ -0,0 +1,24 @@
+/*
+ * Copyright (C) 2013 Richard Weinberger <richrd@nod.at>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#include <linux/uaccess.h>
+#include <linux/kernel.h>
+#include <os.h>
+
+long probe_kernel_read(void *dst, const void *src, size_t size)
+{
+        void *psrc = (void *)rounddown((unsigned long)src, PAGE_SIZE);
+
+        if ((unsigned long)src < PAGE_SIZE || size <= 0)
+                return -EFAULT;
+
+        if (os_mincore(psrc, size + src - psrc) <= 0)
+                return -EFAULT;
+
+        return __probe_kernel_read(dst, src, size);
+}
diff --git a/arch/um/os-Linux/process.c b/arch/um/os-Linux/process.c
index b8f34c9e53ae..67b9c8f5a89e 100644
--- a/arch/um/os-Linux/process.c
+++ b/arch/um/os-Linux/process.c
@@ -4,6 +4,7 @@
  */
 
 #include <stdio.h>
+#include <stdlib.h>
 #include <unistd.h>
 #include <errno.h>
 #include <signal.h>
@@ -232,6 +233,57 @@ out:
         return ok;
 }
 
+static int os_page_mincore(void *addr)
+{
+        char vec[2];
+        int ret;
+
+        ret = mincore(addr, UM_KERN_PAGE_SIZE, vec);
+        if (ret < 0) {
+                if (errno == ENOMEM || errno == EINVAL)
+                        return 0;
+                else
+                        return -errno;
+        }
+
+        return vec[0] & 1;
+}
+
+int os_mincore(void *addr, unsigned long len)
+{
+        char *vec;
+        int ret, i;
+
+        if (len <= UM_KERN_PAGE_SIZE)
+                return os_page_mincore(addr);
+
+        vec = calloc(1, (len + UM_KERN_PAGE_SIZE - 1) / UM_KERN_PAGE_SIZE);
+        if (!vec)
+                return -ENOMEM;
+
+        ret = mincore(addr, UM_KERN_PAGE_SIZE, vec);
+        if (ret < 0) {
+                if (errno == ENOMEM || errno == EINVAL)
+                        ret = 0;
+                else
+                        ret = -errno;
+
+                goto out;
+        }
+
+        for (i = 0; i < ((len + UM_KERN_PAGE_SIZE - 1) / UM_KERN_PAGE_SIZE); i++) {
+                if (!(vec[i] & 1)) {
+                        ret = 0;
+                        goto out;
+                }
+        }
+
+        ret = 1;
+out:
+        free(vec);
+        return ret;
+}
+
 void init_new_thread_signals(void)
 {
         set_handler(SIGSEGV);