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226d9b8f16
Add full memory barriers in kvm_mmu_track_write() and account_shadowed()
to plug a (very, very theoretical) race where kvm_mmu_track_write() could
miss a 0->1 transition of indirect_shadow_pages and fail to zap relevant,
*stale* SPTEs.
Without the barriers, because modern x86 CPUs allow (per the SDM):
Reads may be reordered with older writes to different locations but not
with older writes to the same location.
it's possible that the following could happen (terms of values being
visible/resolved):
CPU0 CPU1
read memory[gfn] (=Y)
memory[gfn] Y=>X
read indirect_shadow_pages (=0)
indirect_shadow_pages 0=>1
or conversely:
CPU0 CPU1
indirect_shadow_pages 0=>1
read indirect_shadow_pages (=0)
read memory[gfn] (=Y)
memory[gfn] Y=>X
E.g. in the below scenario, CPU0 could fail to zap SPTEs, and CPU1 could
fail to retry the faulting instruction, resulting in a KVM entering the
guest with a stale SPTE (map PTE=X instead of PTE=Y).
PTE = X;
CPU0:
emulator_write_phys()
PTE = Y
kvm_page_track_write()
kvm_mmu_track_write()
// memory barrier missing here
if (indirect_shadow_pages)
zap();
CPU1:
FNAME(page_fault)
FNAME(walk_addr)
FNAME(walk_addr_generic)
gw->pte = PTE; // X
FNAME(fetch)
kvm_mmu_get_child_sp
kvm_mmu_get_shadow_page
__kvm_mmu_get_shadow_page
kvm_mmu_alloc_shadow_page
account_shadowed
indirect_shadow_pages++
// memory barrier missing here
if (FNAME(gpte_changed)) // if (PTE == X)
return RET_PF_RETRY;
In practice, this bug likely cannot be observed as both the 0=>1
transition and reordering of this scope are extremely rare occurrences.
Note, if the cost of the barrier (which is simply a locked ADD, see commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText 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.