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mm: Tighten x86 /dev/mem with zeroing reads

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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>
This commit is contained in:
Kees Cook
2017-04-05 09:39:08 -07:00
parent b9b3322f13
commit a4866aa812
2 changed files with 82 additions and 41 deletions
Download Patch File Download Diff File Expand all files Collapse all files

82
drivers/char/mem.c
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@@ -60,6 +60,10 @@ static inline int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
#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 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
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 @@ static ssize_t read_mem(struct file *file, char __user *buf,
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);
unxlate_dev_mem_ptr(p, ptr);
}
remaining = copy_to_user(buf, ptr, sz);
unxlate_dev_mem_ptr(p, ptr);
if (remaining)
return -EFAULT;
@@ -181,30 +197,36 @@ static ssize_t write_mem(struct file *file, const char __user *buf,
#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;