mm: Tighten x86 /dev/mem with zeroing reads
commit a4866aa812518ed1a37d8ea0c881dc946409de94 upstream.
Under CONFIG_STRICT_DEVMEM, reading System RAM through /dev/mem is
disallowed. However, on x86, the first 1MB was always allowed for BIOS
and similar things, regardless of it actually being System RAM. It was
possible for heap to end up getting allocated in low 1MB RAM, and then
read by things like x86info or dd, which would trip hardened usercopy:
usercopy: kernel memory exposure attempt detected from ffff880000090000 (dma-kmalloc-256) (4096 bytes)
This changes the x86 exception for the low 1MB by reading back zeros for
System RAM areas instead of blindly allowing them. More work is needed to
extend this to mmap, but currently mmap doesn't go through usercopy, so
hardened usercopy won't Oops the kernel.
Reported-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Tested-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Brad Spengler <spender@grsecurity.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 6d9cc2d..7e4a9d1 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -60,6 +60,10 @@
#endif
#ifdef CONFIG_STRICT_DEVMEM
+static inline int page_is_allowed(unsigned long pfn)
+{
+ return devmem_is_allowed(pfn);
+}
static inline int range_is_allowed(unsigned long pfn, unsigned long size)
{
u64 from = ((u64)pfn) << PAGE_SHIFT;
@@ -75,6 +79,10 @@
return 1;
}
#else
+static inline int page_is_allowed(unsigned long pfn)
+{
+ return 1;
+}
static inline int range_is_allowed(unsigned long pfn, unsigned long size)
{
return 1;
@@ -122,23 +130,31 @@
while (count > 0) {
unsigned long remaining;
+ int allowed;
sz = size_inside_page(p, count);
- if (!range_is_allowed(p >> PAGE_SHIFT, count))
+ allowed = page_is_allowed(p >> PAGE_SHIFT);
+ if (!allowed)
return -EPERM;
+ if (allowed == 2) {
+ /* Show zeros for restricted memory. */
+ remaining = clear_user(buf, sz);
+ } else {
+ /*
+ * On ia64 if a page has been mapped somewhere as
+ * uncached, then it must also be accessed uncached
+ * by the kernel or data corruption may occur.
+ */
+ ptr = xlate_dev_mem_ptr(p);
+ if (!ptr)
+ return -EFAULT;
- /*
- * On ia64 if a page has been mapped somewhere as uncached, then
- * it must also be accessed uncached by the kernel or data
- * corruption may occur.
- */
- ptr = xlate_dev_mem_ptr(p);
- if (!ptr)
- return -EFAULT;
+ remaining = copy_to_user(buf, ptr, sz);
- remaining = copy_to_user(buf, ptr, sz);
- unxlate_dev_mem_ptr(p, ptr);
+ unxlate_dev_mem_ptr(p, ptr);
+ }
+
if (remaining)
return -EFAULT;
@@ -181,30 +197,36 @@
#endif
while (count > 0) {
+ int allowed;
+
sz = size_inside_page(p, count);
- if (!range_is_allowed(p >> PAGE_SHIFT, sz))
+ allowed = page_is_allowed(p >> PAGE_SHIFT);
+ if (!allowed)
return -EPERM;
- /*
- * On ia64 if a page has been mapped somewhere as uncached, then
- * it must also be accessed uncached by the kernel or data
- * corruption may occur.
- */
- ptr = xlate_dev_mem_ptr(p);
- if (!ptr) {
- if (written)
- break;
- return -EFAULT;
- }
+ /* Skip actual writing when a page is marked as restricted. */
+ if (allowed == 1) {
+ /*
+ * On ia64 if a page has been mapped somewhere as
+ * uncached, then it must also be accessed uncached
+ * by the kernel or data corruption may occur.
+ */
+ ptr = xlate_dev_mem_ptr(p);
+ if (!ptr) {
+ if (written)
+ break;
+ return -EFAULT;
+ }
- copied = copy_from_user(ptr, buf, sz);
- unxlate_dev_mem_ptr(p, ptr);
- if (copied) {
- written += sz - copied;
- if (written)
- break;
- return -EFAULT;
+ copied = copy_from_user(ptr, buf, sz);
+ unxlate_dev_mem_ptr(p, ptr);
+ if (copied) {
+ written += sz - copied;
+ if (written)
+ break;
+ return -EFAULT;
+ }
}
buf += sz;