Re: [PATCH v2 2/2] UML: add support for KASAN under x86_64

[Andrey Konovalov]

On Fri, May 27, 2022 at 8:56 PM David Gow <davidgow@xxxxxxxxxx> wrote:
>
> From: Patricia Alfonso <trishalfonso@xxxxxxxxxx>
>
> Make KASAN run on User Mode Linux on x86_64.
>
> The UML-specific KASAN initializer uses mmap to map the roughly 2.25TB
> of shadow memory to the location defined by KASAN_SHADOW_OFFSET.
> kasan_init() utilizes constructors to initialize KASAN before main().
>
> The location of the KASAN shadow memory, starting at
> KASAN_SHADOW_OFFSET, can be configured using the KASAN_SHADOW_OFFSET
> option. UML uses roughly 18TB of address space, and KASAN requires 1/8th
> of this. The default location of this offset is 0x100000000000, which
> keeps it out-of-the-way even on UML setups with more "physical" memory.
>
> For low-memory setups, 0x7fff8000 can be used instead, which fits in an
> immediate and is therefore faster, as suggested by Dmitry Vyukov. There
> is usually enough free space at this location; however, it is a config
> option so that it can be easily changed if needed.
>
> Note that, unlike KASAN on other architectures, vmalloc allocations
> still use the shadow memory allocated upfront, rather than allocating
> and free-ing it per-vmalloc allocation.
>
> Also note that, while UML supports both KASAN in inline mode
> (CONFIG_KASAN_INLINE) and static linking (CONFIG_STATIC_LINK), it does
> not support both at the same time.
>
> Signed-off-by: Patricia Alfonso <trishalfonso@xxxxxxxxxx>
> Co-developed-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@xxxxxxxx>
> Signed-off-by: David Gow <davidgow@xxxxxxxxxx>

Hi David,

Thanks for working on this!

> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index a4f07de21771..c993d99116f2 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -295,9 +295,29 @@ int kasan_populate_vmalloc(unsigned long addr, unsigned long size)
>                 return 0;
>
>         shadow_start = (unsigned long)kasan_mem_to_shadow((void *)addr);
> -       shadow_start = ALIGN_DOWN(shadow_start, PAGE_SIZE);
>         shadow_end = (unsigned long)kasan_mem_to_shadow((void *)addr + size);
> -       shadow_end = ALIGN(shadow_end, PAGE_SIZE);
> +
> +       /*
> +        * User Mode Linux maps enough shadow memory for all of physical memory
> +        * at boot, so doesn't need to allocate more on vmalloc, just clear it.

Should this say "for all of _virtual_ memory"?

Otherwise, this is confusing. All KASAN-enabled architectures map
shadow for physical memory. And they still need map shadow for
vmalloc() separately. This is what kasan_populate_vmalloc() is for.

> +        *
> +        * If another architecture chooses to go down the same path, we should
> +        * replace this check for CONFIG_UML with something more generic, such
> +        * as:
> +        * - A CONFIG_KASAN_NO_SHADOW_ALLOC option, which architectures could set
> +        * - or, a way of having architecture-specific versions of these vmalloc
> +        *   and module shadow memory allocation options.

I think this part above and the first sentence below belong to the
commit changelog, not to a comment.

> +        *
> +        * For the time being, though, this check works. The remaining CONFIG_UML
> +        * checks in this file exist for the same reason.
> +        */
> +       if (IS_ENABLED(CONFIG_UML)) {
> +               __memset((void *)shadow_start, KASAN_VMALLOC_INVALID, shadow_end - shadow_start);
> +               return 0;
> +       }
> +
> +       shadow_start = PAGE_ALIGN_DOWN(shadow_start);
> +       shadow_end = PAGE_ALIGN(shadow_end);
>
>         ret = apply_to_page_range(&init_mm, shadow_start,
>                                   shadow_end - shadow_start,
> @@ -466,6 +486,10 @@ void kasan_release_vmalloc(unsigned long start, unsigned long end,
>
>         if (shadow_end > shadow_start) {
>                 size = shadow_end - shadow_start;
> +               if (IS_ENABLED(CONFIG_UML)) {
> +                       __memset(shadow_start, KASAN_SHADOW_INIT, shadow_end - shadow_start);
> +                       return;
> +               }
>                 apply_to_existing_page_range(&init_mm,
>                                              (unsigned long)shadow_start,
>                                              size, kasan_depopulate_vmalloc_pte,
> @@ -531,6 +555,11 @@ int kasan_alloc_module_shadow(void *addr, size_t size, gfp_t gfp_mask)
>         if (WARN_ON(!PAGE_ALIGNED(shadow_start)))
>                 return -EINVAL;
>
> +       if (IS_ENABLED(CONFIG_UML)) {
> +               __memset((void *)shadow_start, KASAN_SHADOW_INIT, shadow_size);
> +               return 0;
> +       }
> +
>         ret = __vmalloc_node_range(shadow_size, 1, shadow_start,
>                         shadow_start + shadow_size,
>                         GFP_KERNEL,
> @@ -554,6 +583,9 @@ int kasan_alloc_module_shadow(void *addr, size_t size, gfp_t gfp_mask)
>
>  void kasan_free_module_shadow(const struct vm_struct *vm)
>  {
> +       if (IS_ENABLED(CONFIG_UML))
> +               return;
> +
>         if (vm->flags & VM_KASAN)
>                 vfree(kasan_mem_to_shadow(vm->addr));
>  }
> --
> 2.36.1.124.g0e6072fb45-goog
>

Thanks!