[PATCH] vmalloc: Fix remap_vmalloc_range() bounds checks

[PATCH] vmalloc: Fix remap_vmalloc_range() bounds checks

[Jann Horn]

remap_vmalloc_range() has had various issues with the bounds checks it
promises to perform ("This function checks that addr is a valid vmalloc'ed
area, and that it is big enough to cover the vma") over time, e.g.:

 - not detecting pgoff<<PAGE_SHIFT overflow
 - not detecting (pgoff<<PAGE_SHIFT)+usize overflow
 - not checking whether addr and addr+(pgoff<<PAGE_SHIFT) are the same
   vmalloc allocation
 - comparing a potentially wildly out-of-bounds pointer with the end of the
   vmalloc region

In particular, since commit fc9702273e2e ("bpf: Add mmap() support for
BPF_MAP_TYPE_ARRAY"), unprivileged users can cause kernel null pointer
dereferences by calling mmap() on a BPF map with a size that is bigger than
the distance from the start of the BPF map to the end of the address space.
This could theoretically be used as a kernel ASLR bypass, by using whether
mmap() with a given offset oopses or returns an error code to perform a
binary search over the possible address range.

To allow remap_vmalloc_range_partial() to verify that addr and
addr+(pgoff<<PAGE_SHIFT) are in the same vmalloc region, pass the offset
to remap_vmalloc_range_partial() instead of adding it to the pointer in
remap_vmalloc_range().

In remap_vmalloc_range_partial(), fix the check against get_vm_area_size()
by using size comparisons instead of pointer comparisons, and add checks
for pgoff.

Cc: stable@vger.kernel.org
Fixes: 833423143c3a ("[PATCH] mm: introduce remap_vmalloc_range()")
Signed-off-by: Jann Horn <jannh@google.com>
---
I'm just sending this on the public list, since the worst-case impact for
non-root users is leaking kernel pointers to userspace. In a context where
you can reach BPF (no sandboxing), I don't think that kernel ASLR is very
effective at the moment anyway.

 fs/proc/vmcore.c         |  5 +++--
 include/linux/vmalloc.h  |  2 +-
 mm/vmalloc.c             | 16 +++++++++++++---
 samples/vfio-mdev/mdpy.c |  2 +-
 4 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/fs/proc/vmcore.c b/fs/proc/vmcore.c
index 7dc800cce3543..c663202da8de7 100644
--- a/fs/proc/vmcore.c
+++ b/fs/proc/vmcore.c
@@ -266,7 +266,8 @@ static int vmcoredd_mmap_dumps(struct vm_area_struct *vma, unsigned long dst,
 		if (start < offset + dump->size) {
 			tsz = min(offset + (u64)dump->size - start, (u64)size);
 			buf = dump->buf + start - offset;
-			if (remap_vmalloc_range_partial(vma, dst, buf, tsz)) {
+			if (remap_vmalloc_range_partial(vma, dst, buf, 0,
+							tsz)) {
 				ret = -EFAULT;
 				goto out_unlock;
 			}
@@ -624,7 +625,7 @@ static int mmap_vmcore(struct file *file, struct vm_area_struct *vma)
 		tsz = min(elfcorebuf_sz + elfnotes_sz - (size_t)start, size);
 		kaddr = elfnotes_buf + start - elfcorebuf_sz - vmcoredd_orig_sz;
 		if (remap_vmalloc_range_partial(vma, vma->vm_start + len,
-						kaddr, tsz))
+						kaddr, 0, tsz))
 			goto fail;
 
 		size -= tsz;
diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
index 0507a162ccd0e..a95d3cc74d79b 100644
--- a/include/linux/vmalloc.h
+++ b/include/linux/vmalloc.h
@@ -137,7 +137,7 @@ extern void vunmap(const void *addr);
 
 extern int remap_vmalloc_range_partial(struct vm_area_struct *vma,
 				       unsigned long uaddr, void *kaddr,
-				       unsigned long size);
+				       unsigned long pgoff, unsigned long size);
 
 extern int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
 							unsigned long pgoff);
diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 399f219544f74..9a8227afa0738 100644
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -34,6 +34,7 @@
 #include <linux/llist.h>
 #include <linux/bitops.h>
 #include <linux/rbtree_augmented.h>
+#include <linux/overflow.h>
 
 #include <linux/uaccess.h>
 #include <asm/tlbflush.h>
@@ -3054,6 +3055,7 @@ long vwrite(char *buf, char *addr, unsigned long count)
  * @vma:		vma to cover
  * @uaddr:		target user address to start at
  * @kaddr:		virtual address of vmalloc kernel memory
+ * @pgoff:		offset from @kaddr to start at
  * @size:		size of map area
  *
  * Returns:	0 for success, -Exxx on failure
@@ -3066,9 +3068,15 @@ long vwrite(char *buf, char *addr, unsigned long count)
  * Similar to remap_pfn_range() (see mm/memory.c)
  */
 int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr,
-				void *kaddr, unsigned long size)
+				void *kaddr, unsigned long pgoff,
+				unsigned long size)
 {
 	struct vm_struct *area;
+	unsigned long off;
+	unsigned long end_index;
+
+	if (check_shl_overflow(pgoff, PAGE_SHIFT, &off))
+		return -EINVAL;
 
 	size = PAGE_ALIGN(size);
 
@@ -3082,8 +3090,10 @@ int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr,
 	if (!(area->flags & (VM_USERMAP | VM_DMA_COHERENT)))
 		return -EINVAL;
 
-	if (kaddr + size > area->addr + get_vm_area_size(area))
+	if (check_add_overflow(size, off, &end_index) ||
+	    end_index > get_vm_area_size(area))
 		return -EINVAL;
+	kaddr += off;
 
 	do {
 		struct page *page = vmalloc_to_page(kaddr);
@@ -3122,7 +3132,7 @@ int remap_vmalloc_range(struct vm_area_struct *vma, void *addr,
 						unsigned long pgoff)
 {
 	return remap_vmalloc_range_partial(vma, vma->vm_start,
-					   addr + (pgoff << PAGE_SHIFT),
+					   addr, pgoff,
 					   vma->vm_end - vma->vm_start);
 }
 EXPORT_SYMBOL(remap_vmalloc_range);
diff --git a/samples/vfio-mdev/mdpy.c b/samples/vfio-mdev/mdpy.c
index cc86bf6566e42..9894693f3be17 100644
--- a/samples/vfio-mdev/mdpy.c
+++ b/samples/vfio-mdev/mdpy.c
@@ -418,7 +418,7 @@ static int mdpy_mmap(struct mdev_device *mdev, struct vm_area_struct *vma)
 		return -EINVAL;
 
 	return remap_vmalloc_range_partial(vma, vma->vm_start,
-					   mdev_state->memblk,
+					   mdev_state->memblk, 0,
 					   vma->vm_end - vma->vm_start);
 }
 

base-commit: 8632e9b5645bbc2331d21d892b0d6961c1a08429
-- 
2.26.0.110.g2183baf09c-goog

Re: [PATCH] vmalloc: Fix remap_vmalloc_range() bounds checks

[Jann Horn]

On Thu, Apr 16, 2020 at 7:02 AM Hillf Danton <hdanton@sina.com> wrote:
> On Thu, 16 Apr 2020 00:23:12 +0200 Jann Horn wrote:
> > remap_vmalloc_range() has had various issues with the bounds checks it
> > promises to perform ("This function checks that addr is a valid vmalloc'ed
> > area, and that it is big enough to cover the vma") over time, e.g.:
[...]
> > @@ -3082,8 +3090,10 @@ int remap_vmalloc_range_partial(struct vm_area_struct *vma, unsigned long uaddr,
> >       if (!(area->flags & (VM_USERMAP | VM_DMA_COHERENT)))
> >               return -EINVAL;
> >
> The current kaddr is checked valid by finding area with it despite
> there is room for adding change in checking its boundary in a valid
> area.

I have no idea what you're trying to say. Could you rephrase, please?