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949f5c1244
There are multiple things broken about our storage key handling
functions:
1. We should not walk/touch page tables outside of VMA boundaries when
holding only the mmap sem in read mode. Evil user space can modify the
VMA layout just before this function runs and e.g., trigger races with
page table removal code since commit dd2283f2605e ("mm: mmap: zap pages
with read mmap_sem in munmap"). gfn_to_hva() will only translate using
KVM memory regions, but won't validate the VMA.
2. We should not allocate page tables outside of VMA boundaries: if
evil user space decides to map hugetlbfs to these ranges, bad things
will happen because we suddenly have PTE or PMD page tables where we
shouldn't have them.
3. We don't handle large PUDs that might suddenly appeared inside our page
table hierarchy.
Don't manually allocate page tables, properly validate that we have VMA and
bail out on pud_large().
All callers of page table handling functions, except
get_guest_storage_key(), call fixup_user_fault() in case they
receive an -EFAULT and retry; this will allocate the necessary page tables
if required.
To keep get_guest_storage_key() working as expected and not requiring
kvm_s390_get_skeys() to call fixup_user_fault() distinguish between
"there is simply no page table or huge page yet and the key is assumed
to be 0" and "this is a fault to be reported".
Although commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key handling")
introduced most of the affected code, it was actually already broken
before when using get_locked_pte() without any VMA checks.
Note: Ever since commit 637ff9efe5ea ("s390/mm: Add huge pmd storage key
handling") we can no longer set a guest storage key (for example from
QEMU during VM live migration) without actually resolving a fault.
Although we would have created most page tables, we would choke on the
!pmd_present(), requiring a call to fixup_user_fault(). I would
have thought that this is problematic in combination with postcopy life
migration ... but nobody noticed and this patch doesn't change the
situation. So maybe it's just fine.
Fixes: 9fcf93b5de06 ("KVM: S390: Create helper function get_guest_storage_key")
Fixes: 24d5dd0208ed ("s390/kvm: Provide function for setting the guest storage key")
Fixes: a7e19ab55ffd ("KVM: s390: handle missing storage-key facility")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lore.kernel.org/r/20210909162248.14969-5-david@redhat.com
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.