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SS is initialized to NULL during boot time and not explicitly set to __KERNEL_DS. With FRED enabled, if a kernel event is delivered before a CPU goes to user level for the first time, its SS is NULL thus NULL is pushed into the SS field of the FRED stack frame. But before ERETS is executed, the CPU may context switch to another task and go to user level. Then when the CPU comes back to kernel mode, SS is changed to __KERNEL_DS. Later when ERETS is executed to return from the kernel event handler, a #GP fault is generated because SS doesn't match the SS saved in the FRED stack frame. Initialize SS to __KERNEL_DS when enabling FRED to prevent that. Note, IRET doesn't check if SS matches the SS saved in its stack frame, thus IDT doesn't have this problem. For IDT it doesn't matter whether SS is set to __KERNEL_DS or not, because it's set to NULL upon interrupt or exception delivery and __KERNEL_DS upon SYSCALL. Thus it's pointless to initialize SS for IDT. Signed-off-by: Xin Li (Intel) <xin@zytor.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/all/20240816104316.2276968-1-xin@zytor.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 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.