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3c0696076a
It is currently possible for a userspace application to enter an
infinite page fault loop when using HugeTLB pages implemented with
contiguous PTEs when HAFDBS is not available. This happens because:
1. The kernel may sometimes write PTEs that are sw-dirty but hw-clean
(PTE_DIRTY | PTE_RDONLY | PTE_WRITE).
2. If, during a write, the CPU uses a sw-dirty, hw-clean PTE in handling
the memory access on a system without HAFDBS, we will get a page
fault.
3. HugeTLB will check if it needs to update the dirty bits on the PTE.
For contiguous PTEs, it will check to see if the pgprot bits need
updating. In this case, HugeTLB wants to write a sequence of
sw-dirty, hw-dirty PTEs, but it finds that all the PTEs it is about
to overwrite are all pte_dirty() (pte_sw_dirty() => pte_dirty()),
so it thinks no update is necessary.
We can get the kernel to write a sw-dirty, hw-clean PTE with the
following steps (showing the relevant VMA flags and pgprot bits):
i. Create a valid, writable contiguous PTE.
VMA vmflags: VM_SHARED | VM_READ | VM_WRITE
VMA pgprot bits: PTE_RDONLY | PTE_WRITE
PTE pgprot bits: PTE_DIRTY | PTE_WRITE
ii. mprotect the VMA to PROT_NONE.
VMA vmflags: VM_SHARED
VMA pgprot bits: PTE_RDONLY
PTE pgprot bits: PTE_DIRTY | PTE_RDONLY
iii. mprotect the VMA back to PROT_READ | PROT_WRITE.
VMA vmflags: VM_SHARED | VM_READ | VM_WRITE
VMA pgprot bits: PTE_RDONLY | PTE_WRITE
PTE pgprot bits: PTE_DIRTY | PTE_WRITE | PTE_RDONLY
Make it impossible to create a writeable sw-dirty, hw-clean PTE with
pte_modify(). Such a PTE should be impossible to create, and there may
be places that assume that pte_dirty() implies pte_hw_dirty().
Signed-off-by: James Houghton <jthoughton@google.com>
Fixes: 031e6e6b4e12 ("arm64: hugetlb: Avoid unnecessary clearing in huge_ptep_set_access_flags")
Cc: <stable@vger.kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20231204172646.2541916-3-jthoughton@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.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.