Re: [RFC PATCH 1/5] cxl/core: correct length of DPA field masks

[Dan Williams <dan.j.williams@intel.com>]

[ add Ira and Davidlohr ]

Shiyang Ruan wrote:
> 
> 
> 在 2024/2/10 14:34, Dan Williams 写道:
> > Shiyang Ruan wrote:
> >> The length of Physical Address in General Media Event Record/DRAM Event
> >> Record is 64-bit, so the field mask should be defined as such length.
> > 
> > Can you include this user visible side-effect of this change. Looks like
> > this could cause usages of CXL_DPA_FLAGS_MASK to return an incorrect
> > result?
> 
> Ok.  Will change it to this:
> 
> The length of Physical Address in General Media Event Record/DRAM Event 
> Record is 64bit, per CXL Spec r3.0 - 8.2.9.2.1.1, Table 8-43.  Currently 
> CXL_DPA_FLAGS_MASK is defined as int (32bit), then CXL_DPA_MASK is a int 
> too, it will be 0x00000000FFFFFFC0 while using "->dpa & CXL_DPA_MASK" to 
> obtain real physical address (to drop flags in lower bits), in this case 
> the higher 32bit of ->dpa will be lost.
> 
> To avoid this, define CXL_DPA_FLAGS_MASK as 64bit: 0x3FULL.

That part was clear, what is missing is the impact. For example, this
bug only impacts the output of the dpa field in the cxl_general_media
and cxl_dram tracepoints, but no other part of the CXL code.

So in this case the impact of the bug is low, but in the worst case an
wrong size mask could compromise the security / integrity of the system.

So I would expect a changelog like this to make the importance of the
fix clear and to notify the original stakeholders where the bug was
introduced.

---
The length of Physical Address in General Media Event Record/DRAM Event
Record is 64-bit, so the field mask should be defined as such length.
Otherwise, this causes cxl_general_media and cxl_dram tracepoints to
mask off the upper-32-bits of DPA addresses. The cxl_poison event is
unaffected.

If userspace was doing its own DPA-to-HPA translation this could lead to
incorrect page retirement decisions, but there is no known consumer
(like rasdaemon) of this event today.

Fixes: d54a531a430b ("cxl/mem: Trace General Media Event Record")
Cc: <stable@vger.kernel.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Ira Weiny <ira.weiny@intel.com>
---