CVE-2024-50022: device-dax: correct pgoff align in dax_set_mapping()
In the Linux kernel, the following vulnerability has been resolved:

device-dax: correct pgoff align in dax_set_mapping()

pgoff should be aligned using ALIGN_DOWN() instead of ALIGN().  Otherwise,
vmf->address not aligned to fault_size will be aligned to the next
alignment, that can result in memory failure getting the wrong address.

It's a subtle situation that only can be observed in
page_mapped_in_vma() after the page is page fault handled by
dev_dax_huge_fault.  Generally, there is little chance to perform
page_mapped_in_vma in dev-dax's page unless in specific error injection
to the dax device to trigger an MCE - memory-failure.  In that case,
page_mapped_in_vma() will be triggered to determine which task is
accessing the failure address and kill that task in the end.


We used self-developed dax device (which is 2M aligned mapping) , to
perform error injection to random address.  It turned out that error
injected to non-2M-aligned address was causing endless MCE until panic.
Because page_mapped_in_vma() kept resulting wrong address and the task
accessing the failure address was never killed properly:


[ 3783.719419] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3784.049006] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3784.049190] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3784.448042] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3784.448186] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3784.792026] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3784.792179] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3785.162502] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3785.162633] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3785.461116] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3785.461247] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3785.764730] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3785.764859] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3786.042128] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3786.042259] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3786.464293] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3786.464423] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3786.818090] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3786.818217] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered
[ 3787.085297] mce: Uncorrected hardware memory error in user-access at 
200c9742380
[ 3787.085424] Memory failure: 0x200c9742: recovery action for dax page: 
Recovered

It took us several weeks to pinpoint this problem,  but we eventually
used bpftrace to trace the page fault and mce address and successfully
identified the issue.


Joao added:

; Likely we never reproduce in production because we always pin
: device-dax regions in the region align they provide (Qemu does
: similarly with prealloc in hugetlb/file backed memory).  I think this
: bug requires that we touch *unpinned* device-dax regions unaligned to
: the device-dax selected alignment (page size i.e.  4K/2M/1G)
CVSS v3.1: 5.5