Re: [PATCH v3 2/2] UML: add support for KASAN under x86_64

[David Gow]

On Thu, Jun 30, 2022 at 3:48 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 ~16TB 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. 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.
>
> If another architecture chooses to go down the same path, we should
> replace the checks 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.
>
> 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>
> Reviewed-by: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
> ---
> This is v3 of the KASAN/UML port. It should be ready to go.
>
> Note that this will fail to build if UML is linked statically due to:
> https://lore.kernel.org/all/20220526185402.955870-1-davidgow@xxxxxxxxxx/
>
>
> Changes since v2:
> https://lore.kernel.org/lkml/20220527185600.1236769-2-davidgow@xxxxxxxxxx/
> - Don't define CONFIG_KASAN in USER_CFLAGS, given we dont' use it.
>   (Thanks Johannes)
> - Update patch descriptions and comments given we allocate shadow memory based
>   on the size of the virtual address space, not the "physical" memory
>   used by UML.
>   - This was changed between the original RFC and v1, with
>     KASAN_SHADOW_SIZE's definition being updated.
>   - References to UML using 18TB of space and the shadow memory taking
>     2.25TB were updated. (Thanks Johannes)
>   - A mention of physical memory in a comment was updated. (Thanks
>     Andrey)
> - Move some discussion of how the vmalloc() handling could be made more
>   generic from a comment to the commit description. (Thanks Andrey)
>
> Changes since RFC v3:
> https://lore.kernel.org/all/20220526010111.755166-1-davidgow@xxxxxxxxxx/
> - No longer print "KernelAddressSanitizer initialized" (Johannes)
> - Document the reason for the CONFIG_UML checks in shadow.c (Dmitry)
> - Support static builds via kasan_arch_is_ready() (Dmitry)
> - Get rid of a redundant call to kasam_mem_to_shadow() (Dmitry)
> - Use PAGE_ALIGN and the new PAGE_ALIGN_DOWN macros (Dmitry)
> - Reinstate missing arch/um/include/asm/kasan.h file (Johannes)
>
> Changes since v1:
> https://lore.kernel.org/all/20200226004608.8128-1-trishalfonso@xxxxxxxxxx/
> - Include several fixes from Vincent Whitchurch:
> https://lore.kernel.org/all/20220525111756.GA15955@xxxxxxxx/
> - Support for KASAN_VMALLOC, by changing the way
>   kasan_{populate,release}_vmalloc work to update existing shadow
>   memory, rather than allocating anything new.
> - A similar fix for modules' shadow memory.
> - Support for KASAN_STACK
>   - This requires the bugfix here:
> https://lore.kernel.org/lkml/20220523140403.2361040-1-vincent.whitchurch@xxxxxxxx/
>   - Plus a couple of files excluded from KASAN.
> - Revert the default shadow offset to 0x100000000000
>   - This was breaking when mem=1G for me, at least.
> - A few minor fixes to linker sections and scripts.
>   - I've added one to dyn.lds.S on top of the ones Vincent added.
>
> ---

<... snip ...>

> diff --git a/mm/kasan/shadow.c b/mm/kasan/shadow.c
> index a4f07de21771..7a7fc76e99a8 100644
> --- a/mm/kasan/shadow.c
> +++ b/mm/kasan/shadow.c
> @@ -295,9 +295,22 @@ 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 virtual memory
> +        * at boot, so doesn't need to allocate more on vmalloc, just clear it.
> +        *
> +         * The remaining CONFIG_UML checks in this file exist for the same
> +         * reason.
> +        */

Whoops: these lines had tabs converted to spaces when I reformatted
them. I've sent out v4 which actually passes checkpatch:
https://lore.kernel.org/lkml/20220630080834.2742777-2-davidgow@xxxxxxxxxx/

Sorry for the spam!

-- David