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Zizhi Wo reports: "During the execution of hash_name()->load_unaligned_zeropad(), a potential memory access beyond the PAGE boundary may occur. For example, when the filename length is near the PAGE_SIZE boundary. This triggers a page fault, which leads to a call to do_page_fault()->mmap_read_trylock(). If we can't acquire the lock, we have to fall back to the mmap_read_lock() path, which calls might_sleep(). This breaks RCU semantics because path lookup occurs under an RCU read-side critical section." This is seen with CONFIG_DEBUG_ATOMIC_SLEEP=y and CONFIG_KFENCE=y. Kernel addresses (with the exception of the vectors/kuser helper page) do not have VMAs associated with them. If the vectors/kuser helper page faults, then there are two possibilities: 1. if the fault happened while in kernel mode, then we're basically dead, because the CPU won't be able to vector through this page to handle the fault. 2. if the fault happened while in user mode, that means the page was protected from user access, and we want to fault anyway. Thus, we can handle kernel addresses from any context entirely separately without going anywhere near the mmap lock. This gives us an entirely non-sleeping path for all kernel mode kernel address faults. As we handle the kernel address faults before interrupts are enabled, this change has the side effect of improving the branch predictor hardening, but does not completely solve the issue. Reported-by: Zizhi Wo <wozizhi@huaweicloud.com> Reported-by: Xie Yuanbin <xieyuanbin1@huawei.com> Link: https://lore.kernel.org/r/20251126090505.3057219-1-wozizhi@huaweicloud.com Reviewed-by: Xie Yuanbin <xieyuanbin1@huawei.com> Tested-by: Xie Yuanbin <xieyuanbin1@huawei.com> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
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.