CVE-2024-40918

In the Linux kernel, the following vulnerability has been resolved:

parisc: Try to fix random segmentation faults in package builds

PA-RISC systems with PA8800 and PA8900 processors have had problems

with random segmentation faults for many years. Systems with earlier

processors are much more stable.

Systems with PA8800 and PA8900 processors have a large L2 cache which

needs per page flushing for decent performance when a large range is

flushed. The combined cache in these systems is also more sensitive to

non-equivalent aliases than the caches in earlier systems.

The majority of random segmentation faults that I have looked at

appear to be memory corruption in memory allocated using mmap and

malloc.

My first attempt at fixing the random faults didn't work. On

reviewing the cache code, I realized that there were two issues

which the existing code didn't handle correctly. Both relate

to cache move-in. Another issue is that the present bit in PTEs

is racy.

1) PA-RISC caches have a mind of their own and they can speculatively

load data and instructions for a page as long as there is a entry in

the TLB for the page which allows move-in. TLBs are local to each

CPU. Thus, the TLB entry for a page must be purged before flushing

the page. This is particularly important on SMP systems.

In some of the flush routines, the flush routine would be called

and then the TLB entry would be purged. This was because the flush

routine needed the TLB entry to do the flush.

2) My initial approach to trying the fix the random faults was to

try and use flush_cache_page_if_present for all flush operations.

This actually made things worse and led to a couple of hardware

lockups. It finally dawned on me that some lines weren't being

flushed because the pte check code was racy. This resulted in

random inequivalent mappings to physical pages.

The __flush_cache_page tmpalias flush sets up its own TLB entry

and it doesn't need the existing TLB entry. As long as we can find

the pte pointer for the vm page, we can get the pfn and physical

address of the page. We can also purge the TLB entry for the page

before doing the flush. Further, __flush_cache_page uses a special

TLB entry that inhibits cache move-in.

When switching page mappings, we need to ensure that lines are

removed from the cache. It is not sufficient to just flush the

lines to memory as they may come back.

This made it clear that we needed to implement all the required

flush operations using tmpalias routines. This includes flushes

for user and kernel pages.

After modifying the code to use tmpalias flushes, it became clear

that the random segmentation faults were not fully resolved. The

frequency of faults was worse on systems with a 64 MB L2 (PA8900)

and systems with more CPUs (rp4440).

The warning that I added to flush_cache_page_if_present to detect

pages that couldn't be flushed triggered frequently on some systems.

Helge and I looked at the pages that couldn't be flushed and found

that the PTE was either cleared or for a swap page. Ignoring pages

that were swapped out seemed okay but pages with cleared PTEs seemed

problematic.

I looked at routines related to pte_clear and noticed ptep_clear_flush.

The default implementation just flushes the TLB entry. However, it was

obvious that on parisc we need to flush the cache page as well. If

we don't flush the cache page, stale lines will be left in the cache

and cause random corruption. Once a PTE is cleared, there is no way

to find the physical address associated with the PTE and flush the

associated page at a later time.

I implemented an updated change with a parisc specific version of

ptep_clear_flush. It fixed the random data corruption on Helge's rp4440

and rp3440, as well as on my c8000.

At this point, I realized that I could restore the code where we only

flush in flush_cache_page_if_present if the page has been accessed.

However, for this, we also need to flush the cache when the accessed

bit is cleared in

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