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[ Upstream commit ad546940b5991d3e141238cd80a6d1894b767184 ] The first GDT descriptor is reserved as 'NULL descriptor'. As bits 0 and 1 of a segment selector, i.e., the RPL bits, are NOT used to index GDT, selector values 0~3 all point to the NULL descriptor, thus values 0, 1, 2 and 3 are all valid NULL selector values. When a NULL selector value is to be loaded into a segment register, reload_segments() sets its RPL bits. Later IRET zeros ES, FS, GS, and DS segment registers if any of them is found to have any nonzero NULL selector value. The two operations offset each other to actually effect a nop. Besides, zeroing of RPL in NULL selector values is an information leak in pre-FRED systems as userspace can spot any interrupt/exception by loading a nonzero NULL selector, and waiting for it to become zero. But there is nothing software can do to prevent it before FRED. ERETU, the only legit instruction to return to userspace from kernel under FRED, by design does NOT zero any segment register to avoid this problem behavior. As such, leave NULL selector values 0~3 unchanged and close the leak. Do the same on 32-bit kernel as well. Signed-off-by: Xin Li (Intel) <xin@zytor.com> Signed-off-by: Ingo Molnar <mingo@kernel.org> Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Brian Gerst <brgerst@gmail.com> Cc: Peter Zijlstra <peterz@infradead.org> Link: https://lore.kernel.org/r/20241126184529.1607334-1-xin@zytor.com Signed-off-by: Sasha Levin <sashal@kernel.org>
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.