Re: [PATCH V6 2/2] xen/arm: Harden the P2M code in p2m_remove_mapping()

[Julien Grall <julien@xxxxxxx>]

Hi Oleksandr,

On 11/05/2022 19:47, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
>
> Borrow the x86's check from p2m_remove_page() which was added
> by the following commit: c65ea16dbcafbe4fe21693b18f8c2a3c5d14600e
> "x86/p2m: don't assert that the passed in MFN matches for a remove"
> and adjust it to the Arm code base.
>
> Basically, this check is strictly needed for the xenheap pages only
> since there are several non-protected read accesses to our simplified
> xenheap based M2P approach on Arm (most calls to page_get_xenheap_gfn()
> are not protected by the P2M lock).
To me, this read as you introduced a bug in patch #1 and now you are 
fixing it. So this patch should have been first.
> But, it will be a good opportunity to harden the P2M code for *every*
> RAM pages since it is possible to remove any GFN - MFN mapping
> currently on Arm (even with the wrong helpers).
> This can result in
> a few issues when mapping is overridden silently (in particular when
> building dom0).
Hmmm... AFAIU, in such situation p2m_remove_mapping() wouldn't be 
called. Instead, we would call the mapping helper twice and the override 
would still happen.
> Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
> You can find the corresponding discussion at:
> https://lore.kernel.org/xen-devel/82d8bfe0-cb46-d303-6a60-2324dd76a1f7@xxxxxxx/
>
> Changes V5 -> V6:
>   - new patch
> ---
>   xen/arch/arm/p2m.c | 21 +++++++++++++++++++++
>   1 file changed, 21 insertions(+)
>
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index f87b48e..635e474 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1311,11 +1311,32 @@ static inline int p2m_remove_mapping(struct domain *d,
>                                        mfn_t mfn)
>   {
>       struct p2m_domain *p2m = p2m_get_hostp2m(d);
> +    unsigned long i;
>       int rc;
>   
>       p2m_write_lock(p2m);
> +    for ( i = 0; i < nr; )
One bit I really hate in the x86 code is the lack of in-code 
documentation. It makes really difficult to understand the logic.
I know this code was taken from x86, but I would like to avoid making 
same mistake (this code is definitely not trivial). So can we document 
the logic?
The code itself looks good to me.

> +    {
> +        unsigned int cur_order;
> +        p2m_type_t t;
> +        mfn_t mfn_return = p2m_get_entry(p2m, gfn_add(start_gfn, i), &t, NULL,
> +                                         &cur_order, NULL);
> +
> +        if ( p2m_is_any_ram(t) &&
> +             (!mfn_valid(mfn) || !mfn_eq(mfn_add(mfn, i), mfn_return)) )
> +        {
> +            rc = -EILSEQ;
> +            goto out;
> +        }
> +
> +        i += (1UL << cur_order) -
> +             ((gfn_x(start_gfn) + i) & ((1UL << cur_order) - 1));
> +    }
> +
>       rc = p2m_set_entry(p2m, start_gfn, nr, INVALID_MFN,
>                          p2m_invalid, p2m_access_rwx);
> +
> +out:
>       p2m_write_unlock(p2m);
>   
>       return rc;
Cheers,

--
Julien Grall