/dev/mem: Add bounce buffer for copy-out

As done for /proc/kcore in

  commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")

this adds a bounce buffer when reading memory via /dev/mem. This
is needed to allow kernel text memory to be read out when built with
CONFIG_HARDENED_USERCOPY (which refuses to read out kernel text) and
without CONFIG_STRICT_DEVMEM (which would have refused to read any RAM
contents at all).

Since this build configuration isn't common (most systems with
CONFIG_HARDENED_USERCOPY also have CONFIG_STRICT_DEVMEM), this also tries
to inform Kconfig about the recommended settings.

This patch is modified from Brad Spengler/PaX Team's changes to /dev/mem
code in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.

Reported-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Fixes: f5509cc18daa ("mm: Hardened usercopy")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

2 files changed, 23 insertions, 5 deletions

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 6aefe5370e5b..052011bcf100 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -107,6 +107,8 @@ static ssize_t read_mem(struct file *file, char __user *buf,
         phys_addr_t p = *ppos;
         ssize_t read, sz;
         void *ptr;
+        char *bounce;
+        int err;
 
         if (p != *ppos)
                 return 0;
@@ -129,15 +131,22 @@ static ssize_t read_mem(struct file *file, char __user *buf,
         }
 #endif
 
+        bounce = kmalloc(PAGE_SIZE, GFP_KERNEL);
+        if (!bounce)
+                return -ENOMEM;
+
         while (count > 0) {
                 unsigned long remaining;
                 int allowed;
 
                 sz = size_inside_page(p, count);
 
+                err = -EPERM;
                 allowed = page_is_allowed(p >> PAGE_SHIFT);
                 if (!allowed)
-                        return -EPERM;
+                        goto failed;
+
+                err = -EFAULT;
                 if (allowed == 2) {
                         /* Show zeros for restricted memory. */
                         remaining = clear_user(buf, sz);
@@ -149,24 +158,32 @@ static ssize_t read_mem(struct file *file, char __user *buf,
                          */
                         ptr = xlate_dev_mem_ptr(p);
                         if (!ptr)
-                                return -EFAULT;
-
-                        remaining = copy_to_user(buf, ptr, sz);
+                                goto failed;
 
+                        err = probe_kernel_read(bounce, ptr, sz);
                         unxlate_dev_mem_ptr(p, ptr);
+                        if (err)
+                                goto failed;
+
+                        remaining = copy_to_user(buf, bounce, sz);
                 }
 
                 if (remaining)
-                        return -EFAULT;
+                        goto failed;
 
                 buf += sz;
                 p += sz;
                 count -= sz;
                 read += sz;
         }
+        kfree(bounce);
 
         *ppos += read;
         return read;
+
+failed:
+        kfree(bounce);
+        return err;
 }
 
 static ssize_t write_mem(struct file *file, const char __user *buf,
diff --git a/security/Kconfig b/security/Kconfig
index e8e449444e65..3b4effd8bbc2 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -143,6 +143,7 @@ config HARDENED_USERCOPY
         bool "Harden memory copies between kernel and userspace"
         depends on HAVE_HARDENED_USERCOPY_ALLOCATOR
         select BUG
+        imply STRICT_DEVMEM
         help
           This option checks for obviously wrong memory regions when
           copying memory to/from the kernel (via copy_to_user() and