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In con_font_set(), when we need to guess font height (for compat reasons?), the current approach uses multiple userspace fetches, i.e., get_user(tmp, &charmap[32*i+h-1]), to derive the height. This has two drawbacks: 1. performance: accessing userspace memory is less efficient than directly de-reference the byte 2. security: a more critical problem is that the height derived might not match with the actual font.data. This is because a user thread might race condition to change the memory of op->data after the op->height guessing but before the second fetch: font.data = memdup_user(op->data, size). Leaving font.height = 32 while the actual height is 1 or vice-versa. This patch tries to resolve both issues by re-locating the height guessing part after the font.data is fetched in. In this way, the userspace data is fetched in one shot and we directly dereference the font.data in kernel space to probe for the height. Signed-off-by: Meng Xu <mengxu.gatech@gmail.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.