On Mon, Jun 12, 2023 at 03:08:40AM +0000, Huang, Kai wrote:
> On Fri, 2023-06-09 at 16:17 +0300, kirill.shutemov@xxxxxxxxxxxxxxx wrote:
> > On Mon, Jun 05, 2023 at 02:27:32AM +1200, Kai Huang wrote:
> > > The first few generations of TDX hardware have an erratum. Triggering
> > > it in Linux requires some kind of kernel bug involving relatively exotic
> > > memory writes to TDX private memory and will manifest via
> > > spurious-looking machine checks when reading the affected memory.
> > >
> > > == Background ==
> > >
> > > Virtually all kernel memory accesses operations happen in full
> > > cachelines. In practice, writing a "byte" of memory usually reads a 64
> > > byte cacheline of memory, modifies it, then writes the whole line back.
> > > Those operations do not trigger this problem.
> > >
> > > This problem is triggered by "partial" writes where a write transaction
> > > of less than cacheline lands at the memory controller. The CPU does
> > > these via non-temporal write instructions (like MOVNTI), or through
> > > UC/WC memory mappings. The issue can also be triggered away from the
> > > CPU by devices doing partial writes via DMA.
> > >
> > > == Problem ==
> > >
> > > A partial write to a TDX private memory cacheline will silently "poison"
> > > the line. Subsequent reads will consume the poison and generate a
> > > machine check. According to the TDX hardware spec, neither of these
> > > things should have happened.
> > >
> > > To add insult to injury, the Linux machine code will present these as a
> > > literal "Hardware error" when they were, in fact, a software-triggered
> > > issue.
> > >
> > > == Solution ==
> > >
> > > In the end, this issue is hard to trigger. Rather than do something
> > > rash (and incomplete) like unmap TDX private memory from the direct map,
> > > improve the machine check handler.
> > >
> > > Currently, the #MC handler doesn't distinguish whether the memory is
> > > TDX private memory or not but just dump, for instance, below message:
> > >
> > > [...] mce: [Hardware Error]: CPU 147: Machine Check Exception: f Bank 1: bd80000000100134
> > > [...] mce: [Hardware Error]: RIP 10:<ffffffffadb69870> {__tlb_remove_page_size+0x10/0xa0}
> > > ...
> > > [...] mce: [Hardware Error]: Run the above through 'mcelog --ascii'
> > > [...] mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel
> > > [...] Kernel panic - not syncing: Fatal local machine check
> > >
> > > Which says "Hardware Error" and "Data load in unrecoverable area of
> > > kernel".
> > >
> > > Ideally, it's better for the log to say "software bug around TDX private
> > > memory" instead of "Hardware Error". But in reality the real hardware
> > > memory error can happen, and sadly such software-triggered #MC cannot be
> > > distinguished from the real hardware error. Also, the error message is
> > > used by userspace tool 'mcelog' to parse, so changing the output may
> > > break userspace.
> > >
> > > So keep the "Hardware Error". The "Data load in unrecoverable area of
> > > kernel" is also helpful, so keep it too.
> > >
> > > Instead of modifying above error log, improve the error log by printing
> > > additional TDX related message to make the log like:
> > >
> > > ...
> > > [...] mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel
> > > [...] mce: [Hardware Error]: Machine Check: Memory error from TDX private memory. May be result of CPU erratum.
> >
> > The message mentions one part of issue -- CPU erratum -- but misses the
> > other required part -- kernel bug that makes kernel access the memory it
> > not suppose to.
> >
>
> How about below?
>
> "Memory error from TDX private memory. May be result of CPU erratum caused by
> kernel bug."
Fine, I guess.
--
Kiryl Shutsemau / Kirill A. Shutemov
