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.:
>
> - 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@xxxxxxxxxxxxxxx
> Fixes: 833423143c3a ("[PATCH] mm: introduce remap_vmalloc_range()")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> 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;
>
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.
> - 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
--- a/mm/vmalloc.c
+++ b/mm/vmalloc.c
@@ -3074,7 +3074,7 @@ int remap_vmalloc_range_partial(struct v
if (!(area->flags & (VM_USERMAP | VM_DMA_COHERENT)))
return -EINVAL;
- if (kaddr + size > area->addr + get_vm_area_size(area))
+ if (size > area->addr + get_vm_area_size(area) - kaddr)
return -EINVAL;
do {
