Re: [PATCH v2] x86/mm: Short circuit damage from "fishy" ref/typecount failure

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 19/01/2021 12:45, Paul Durrant wrote:
>> -----Original Message-----
>> From: Andrew Cooper <andrew.cooper3@citrix.com>
>> Sent: 19 January 2021 12:28
>> To: Xen-devel <xen-devel@lists.xenproject.org>
>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>; Jan Beulich <JBeulich@suse.com>; Roger Pau Monné
>> <roger.pau@citrix.com>; Wei Liu <wl@xen.org>; Paul Durrant <paul@xen.org>; Tamas K Lengyel
>> <tamas@tklengyel.com>
>> Subject: [PATCH v2] x86/mm: Short circuit damage from "fishy" ref/typecount failure
>>
>> This code has been copied in 3 places, but it is problematic.
>>
>> All cases will hit a BUG() later in domain teardown, when a the missing
>> type/count reference is underflowed.
>>
>> Don't complicated the logic by leaving a totally unqualified domain crash, and
>> a timebomb which will be triggered by the toolstack at a slightly later, and
>> seemingly unrelated, point.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>> ---
>> CC: Jan Beulich <JBeulich@suse.com>
>> CC: Roger Pau Monné <roger.pau@citrix.com>
>> CC: Wei Liu <wl@xen.org>
>> CC: Paul Durrant <paul@xen.org>
>> CC: Tamas K Lengyel <tamas@tklengyel.com>
>>
>> v2:
>>  * Reword the commit message.
>>  * Switch BUG() to BUG_ON() to further reduce code volume.
>> ---
>>  xen/arch/x86/hvm/ioreq.c     | 11 ++---------
>>  xen/arch/x86/hvm/vmx/vmx.c   | 11 ++---------
>>  xen/arch/x86/mm/mem_paging.c | 17 ++++-------------
>>  3 files changed, 8 insertions(+), 31 deletions(-)
>>
>> diff --git a/xen/arch/x86/hvm/ioreq.c b/xen/arch/x86/hvm/ioreq.c
>> index 1cc27df87f..0c38cfa151 100644
>> --- a/xen/arch/x86/hvm/ioreq.c
>> +++ b/xen/arch/x86/hvm/ioreq.c
>> @@ -366,15 +366,8 @@ static int hvm_alloc_ioreq_mfn(struct hvm_ioreq_server *s, bool buf)
>>      if ( !page )
>>          return -ENOMEM;
>>
>> -    if ( !get_page_and_type(page, s->target, PGT_writable_page) )
>> -    {
>> -        /*
>> -         * The domain can't possibly know about this page yet, so failure
>> -         * here is a clear indication of something fishy going on.
>> -         */
>> -        domain_crash(s->emulator);
>> -        return -ENODATA;
>> -    }
>> +    /* Domain can't know about this page yet - something fishy going on. */
>> +    BUG_ON(!get_page_and_type(page, s->target, PGT_writable_page));
>>
>>      iorp->va = __map_domain_page_global(page);
>>      if ( !iorp->va )
>> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
>> index 2d4475ee3d..8e438cb781 100644
>> --- a/xen/arch/x86/hvm/vmx/vmx.c
>> +++ b/xen/arch/x86/hvm/vmx/vmx.c
>> @@ -3042,15 +3042,8 @@ static int vmx_alloc_vlapic_mapping(struct domain *d)
>>      if ( !pg )
>>          return -ENOMEM;
>>
>> -    if ( !get_page_and_type(pg, d, PGT_writable_page) )
>> -    {
>> -        /*
>> -         * The domain can't possibly know about this page yet, so failure
>> -         * here is a clear indication of something fishy going on.
>> -         */
>> -        domain_crash(d);
>> -        return -ENODATA;
>> -    }
>> +    /* Domain can't know about this page yet - something fishy going on. */
>> +    BUG_ON(!get_page_and_type(page, s->target, PGT_writable_page));
> Does this compile?
>
> s/page/pg
> s/s->target/d
>
> ...and similar below is needed AFAICT.

Urgh no - missing the delta in my working tree.  Fixed both.

~Andrew