[RFC PATCH] drivers/char: kmem: disable read/write if VMALLOC_START < PAGE_OFFSET

[Ard Biesheuvel]

On 9 June 2017 at 16:32, Ard Biesheuvel <ard.biesheuvel at linaro.org> wrote:
> As it turns out, arm64 deviates from other architectures in the way it
> maps the VMALLOC region: on most (all?) other architectures, it resides
> strictly above the kernel's direct mapping of DRAM, but on arm64, this
> is the other way around. For instance, for a 48-bit VA configuration,
> we have
>
>   modules : 0xffff000000000000 - 0xffff000008000000   (   128 MB)
>   vmalloc : 0xffff000008000000 - 0xffff7dffbfff0000   (129022 GB)
>   ...
>   vmemmap : 0xffff7e0000000000 - 0xffff800000000000   (  2048 GB maximum)
>             0xffff7e0000000000 - 0xffff7e0003ff0000   (    63 MB actual)
>   memory  : 0xffff800000000000 - 0xffff8000ffc00000   (  4092 MB)
>
> This has mostly gone unnoticed until now, but it does appear that it
> breaks an assumption in the kcore read/write code, which does something

kmem not kcore

> like
>
>   if (p < (unsigned long) high_memory) {
>     ... use straight copy_[to|from]_user() using p as virtual address ...
>   }
>   ...
>   if (count > 0) {
>     ... use vread/vwrite for accesses past high_memory ...
>   }
>
> The first condition will inadvertently hold for the VMALLOC region if
> VMALLOC_START < PAGE_OFFSET, but the read/write will subsequently fail
> the virt_addr_valid() check, resulting in a -ENXIO return value.
>
> Given how kmem seems to be living in borrowed time anyway, and given
> the fact that nobody noticed that the read/write interface is broken
> on arm64 in the first place, let's not bother trying to fix it, but
> simply fail such calls with a warning if VMALLOC_START < PAGE_OFFSET.
> (Note that kmem's mmap() interface is not affected by this)
>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>
> This is just an RFC. There may be better ways to deal with this, including
> disabling /dev/kmem altogether on arm64.
>
>  drivers/char/mem.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/drivers/char/mem.c b/drivers/char/mem.c
> index 6e0cbe092220..c90ca6703dd5 100644
> --- a/drivers/char/mem.c
> +++ b/drivers/char/mem.c
> @@ -408,6 +408,10 @@ static ssize_t read_kmem(struct file *file, char __user *buf,
>         char *kbuf; /* k-addr because vread() takes vmlist_lock rwlock */
>         int err = 0;
>
> +       /* the code below assumes VMALLOC_START > PAGE_OFFSET */
> +       if (WARN_ON_ONCE(VMALLOC_START < PAGE_OFFSET))
> +               return -ENXIO;
> +
>         read = 0;
>         if (p < (unsigned long) high_memory) {
>                 low_count = count;
> @@ -484,6 +488,10 @@ static ssize_t do_write_kmem(unsigned long p, const char __user *buf,
>         ssize_t written, sz;
>         unsigned long copied;
>
> +       /* the code below assumes VMALLOC_START > PAGE_OFFSET */
> +       if (WARN_ON_ONCE(VMALLOC_START < PAGE_OFFSET))
> +               return -ENXIO;
> +
>         written = 0;
>  #ifdef __ARCH_HAS_NO_PAGE_ZERO_MAPPED
>         /* we don't have page 0 mapped on sparc and m68k.. */
> --
> 2.9.3
>