vmalloc: fix remap_vmalloc_range() bounds checks

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.

Fixes: 833423143c3a ("[PATCH] mm: introduce remap_vmalloc_range()")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: stable@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Link: http://lkml.kernel.org/r/20200415222312.236431-1-jannh@google.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/mm/vmalloc.c b/mm/vmalloc.c
index 399f219544f7..9a8227afa073 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 @@ finished:
  * @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 @@ finished:
  * 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);