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f587661f21
It is possible to take a stage-2 permission fault on a page larger than PAGE_SIZE. For example, when running a guest backed by 2M HugeTLB, KVM eagerly maps at the largest possible block size. When dirty logging is enabled on a memslot, KVM does *not* eagerly split these 2M stage-2 mappings and instead clears the write bit on the pte. Since dirty logging is always performed at PAGE_SIZE granularity, KVM lazily splits these 2M block mappings down to PAGE_SIZE in the stage-2 fault handler. This operation must be done under the write lock. Since commitf783ef1c0e("KVM: arm64: Add fast path to handle permission relaxation during dirty logging"), the stage-2 fault handler conditionally takes the read lock on permission faults with dirty logging enabled. To that end, it is possible to split a 2M block mapping while only holding the read lock. The problem is demonstrated by running kvm_page_table_test with 2M anonymous HugeTLB, which splats like so: WARNING: CPU: 5 PID: 15276 at arch/arm64/kvm/hyp/pgtable.c:153 stage2_map_walk_leaf+0x124/0x158 [...] Call trace: stage2_map_walk_leaf+0x124/0x158 stage2_map_walker+0x5c/0xf0 __kvm_pgtable_walk+0x100/0x1d4 __kvm_pgtable_walk+0x140/0x1d4 __kvm_pgtable_walk+0x140/0x1d4 kvm_pgtable_walk+0xa0/0xf8 kvm_pgtable_stage2_map+0x15c/0x198 user_mem_abort+0x56c/0x838 kvm_handle_guest_abort+0x1fc/0x2a4 handle_exit+0xa4/0x120 kvm_arch_vcpu_ioctl_run+0x200/0x448 kvm_vcpu_ioctl+0x588/0x664 __arm64_sys_ioctl+0x9c/0xd4 invoke_syscall+0x4c/0x144 el0_svc_common+0xc4/0x190 do_el0_svc+0x30/0x8c el0_svc+0x28/0xcc el0t_64_sync_handler+0x84/0xe4 el0t_64_sync+0x1a4/0x1a8 Fix the issue by only acquiring the read lock if the guest faulted on a PAGE_SIZE granule w/ dirty logging enabled. Add a WARN to catch locking bugs in future changes. Fixes:f783ef1c0e("KVM: arm64: Add fast path to handle permission relaxation during dirty logging") Cc: Jing Zhang <jingzhangos@google.com> Signed-off-by: Oliver Upton <oupton@google.com> Reviewed-by: Reiji Watanabe <reijiw@google.com> Signed-off-by: Marc Zyngier <maz@kernel.org> Link: https://lore.kernel.org/r/20220401194652.950240-1-oupton@google.com
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 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.