Re: [PATCH v2 1/3] /dev/mem: disallow access to explicitly excluded system RAM regions

[Dan Williams]

On Mon, Aug 16, 2021 at 7:25 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> virtio-mem dynamically exposes memory inside a device memory region as
> system RAM to Linux, coordinating with the hypervisor which parts are
> actually "plugged" and consequently usable/accessible. On the one hand, the
> virtio-mem driver adds/removes whole memory blocks, creating/removing busy
> IORESOURCE_SYSTEM_RAM resources, on the other hand, it logically (un)plugs
> memory inside added memory blocks, dynamically either exposing them to
> the buddy or hiding them from the buddy and marking them PG_offline.
>
> virtio-mem wants to make sure that in a sane environment, nobody
> "accidentially" accesses unplugged memory inside the device managed
> region. After /proc/kcore has been sanitized and /dev/kmem has been
> removed, /dev/mem is the remaining interface that still allows uncontrolled
> access to the device-managed region of virtio-mem devices from user
> space.
>
> There is no known sane use case for mapping virtio-mem device memory
> via /dev/mem while virtio-mem driver concurrently (un)plugs memory inside
> that region. So once the driver was loaded and detected the device
> along the device-managed region, we just want to disallow any access via
> /dev/mem to it.
>
> Let's add the basic infrastructure to exclude some physical memory
> regions completely from /dev/mem access, on any architecture and under
> any system configuration (independent of CONFIG_STRICT_DEVMEM and
> independent of "iomem=").

I'm certainly on team "/dev/mem considered harmful", but this approach
feels awkward. It feels wrong for being non-committal about whether
CONFIG_STRICT_DEVMEM is in wide enough use that the safety can be
turned on all the time, and the configuration option dropped, or there
are users clinging onto /dev/mem where they expect to be able to build
a debug kernel to turn all of these restrictions off, even the
virtio-mem ones. This splits the difference and says some /dev/mem
accesses are always disallowed for "reasons", but I could say the same
thing about pmem, there's no sane reason to allow /dev/mem which has
no idea about the responsibilities of properly touching pmem to get
access to it.


>
> Any range marked with "IORESOURCE_SYSTEM_RAM | IORESOURCE_EXCLUSIVE"
> will be excluded, even if not busy. For now, there are no applicable
> ranges and we'll modify virtio-mem next to properly set
> IORESOURCE_EXCLUSIVE on the parent resource.
>
> As next_resource() will iterate over children although we might want to
> skip a certain range completely, let's add and use
> next_range_skip_children() and for_each_resource(), to optimize that case,
> avoding having to traverse subtrees that are not of interest.
>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  drivers/char/mem.c     | 22 ++++++++------------
>  include/linux/ioport.h |  1 +
>  kernel/resource.c      | 47 ++++++++++++++++++++++++++++++++++++++++++
>  lib/Kconfig.debug      |  4 +++-
>  4 files changed, 60 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 1c596b5cdb27..1829dc6a1f29 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -60,13 +60,18 @@ 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);
> +#ifdef CONFIG_STRICT_DEVMEM
> +       if (!devmem_is_allowed(pfn))
> +               return 0;
> +#endif /* CONFIG_STRICT_DEVMEM */
> +       return !iomem_range_contains_excluded_devmem(PFN_PHYS(pfn), PAGE_SIZE);
>  }
> +
>  static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>  {
> +#ifdef CONFIG_STRICT_DEVMEM
>         u64 from = ((u64)pfn) << PAGE_SHIFT;
>         u64 to = from + size;
>         u64 cursor = from;
> @@ -77,18 +82,9 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
>                 cursor += PAGE_SIZE;
>                 pfn++;
>         }
> -       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;
> +#endif /* CONFIG_STRICT_DEVMEM */
> +       return !iomem_range_contains_excluded_devmem(PFN_PHYS(pfn), size);
>  }
> -#endif
>
>  #ifndef unxlate_dev_mem_ptr
>  #define unxlate_dev_mem_ptr unxlate_dev_mem_ptr
> diff --git a/include/linux/ioport.h b/include/linux/ioport.h
> index 8359c50f9988..d31f83281327 100644
> --- a/include/linux/ioport.h
> +++ b/include/linux/ioport.h
> @@ -308,6 +308,7 @@ extern struct resource * __devm_request_region(struct device *dev,
>  extern void __devm_release_region(struct device *dev, struct resource *parent,
>                                   resource_size_t start, resource_size_t n);
>  extern int iomem_map_sanity_check(resource_size_t addr, unsigned long size);
> +extern bool iomem_range_contains_excluded_devmem(u64 addr, u64 size);
>  extern bool iomem_is_exclusive(u64 addr);
>
>  extern int
> diff --git a/kernel/resource.c b/kernel/resource.c
> index ca9f5198a01f..f57a14617c49 100644
> --- a/kernel/resource.c
> +++ b/kernel/resource.c
> @@ -73,6 +73,18 @@ static struct resource *next_resource(struct resource *p)
>         return p->sibling;
>  }
>
> +static struct resource *next_resource_skip_children(struct resource *p)
> +{
> +       while (!p->sibling && p->parent)
> +               p = p->parent;
> +       return p->sibling;
> +}
> +
> +#define for_each_resource(_root, _p, _skip_children) \
> +       for ((_p) = (_root)->child; (_p); \
> +            (_p) = (_skip_children) ? next_resource_skip_children(_p) : \
> +                                      next_resource(_p))
> +
>  static void *r_next(struct seq_file *m, void *v, loff_t *pos)
>  {
>         struct resource *p = v;
> @@ -1700,6 +1712,41 @@ int iomem_map_sanity_check(resource_size_t addr, unsigned long size)
>         return err;
>  }
>
> +/*
> + * Check if a physical memory range is completely excluded from getting
> + * mapped/accessed via /dev/mem.
> + */
> +bool iomem_range_contains_excluded_devmem(u64 addr, u64 size)
> +{
> +       const unsigned int flags = IORESOURCE_SYSTEM_RAM | IORESOURCE_EXCLUSIVE;
> +       bool skip_children = false, excluded = false;
> +       struct resource *p;
> +
> +       read_lock(&resource_lock);
> +       for_each_resource(&iomem_resource, p, skip_children) {
> +               if (p->start >= addr + size)
> +                       break;
> +               if (p->end < addr) {
> +                       skip_children = true;
> +                       continue;
> +               }
> +               skip_children = false;
> +
> +               /*
> +                * A system RAM resource is excluded if IORESOURCE_EXCLUSIVE
> +                * is set, even if not busy and even if we don't have strict
> +                * checks enabled -- no ifs or buts.
> +                */
> +               if ((p->flags & flags) == flags) {
> +                       excluded = true;
> +                       break;
> +               }
> +       }
> +       read_unlock(&resource_lock);
> +
> +       return excluded;
> +}
> +
>  #ifdef CONFIG_STRICT_DEVMEM
>  static int strict_iomem_checks = 1;
>  #else
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 5ddd575159fb..d0ce6e23a6db 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1780,7 +1780,9 @@ config STRICT_DEVMEM
>           access to this is obviously disastrous, but specific access can
>           be used by people debugging the kernel. Note that with PAT support
>           enabled, even in this case there are restrictions on /dev/mem
> -         use due to the cache aliasing requirements.
> +         use due to the cache aliasing requirements. Further, some drivers
> +         will still restrict access to some physical memory regions either
> +         already used or to be used in the future as system RAM.
>
>           If this option is switched on, and IO_STRICT_DEVMEM=n, the /dev/mem
>           file only allows userspace access to PCI space and the BIOS code and
> --
> 2.31.1
>