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The feature User-Mode Instruction Prevention present in recent Intel processor prevents a group of instructions (sgdt, sidt, sldt, smsw, and str) from being executed with CPL > 0. Otherwise, a general protection fault is issued. Rather than relaying to the user space the general protection fault caused by the UMIP-protected instructions (in the form of a SIGSEGV signal), it can be trapped and the instruction emulated to provide a dummy result. This allows to both conserve the current kernel behavior and not reveal the system resources that UMIP intends to protect (i.e., the locations of the global descriptor and interrupt descriptor tables, the segment selectors of the local descriptor table, the value of the task state register and the contents of the CR0 register). This emulation is needed because certain applications (e.g., WineHQ and DOSEMU2) rely on this subset of instructions to function. Given that sldt and str are not commonly used in programs that run on WineHQ or DOSEMU2, they are not emulated. Also, emulation is provided only for 32-bit processes; 64-bit processes that attempt to use the instructions that UMIP protects will receive the SIGSEGV signal issued as a consequence of the general protection fault. The instructions protected by UMIP can be split in two groups. Those which return a kernel memory address (sgdt and sidt) and those which return a value (smsw, sldt and str; the last two not emulated). For the instructions that return a kernel memory address, applications such as WineHQ rely on the result being located in the kernel memory space, not the actual location of the table. The result is emulated as a hard-coded value that lies close to the top of the kernel memory. The limit for the GDT and the IDT are set to zero. The instruction smsw is emulated to return the value that the register CR0 has at boot time as set in the head_32. Care is taken to appropriately emulate the results when segmentation is used. That is, rather than relying on USER_DS and USER_CS, the function insn_get_addr_ref() inspects the segment descriptor pointed by the registers in pt_regs. This ensures that we correctly obtain the segment base address and the address and operand sizes even if the user space application uses a local descriptor table. Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Borislav Petkov <bp@suse.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Chen Yucong <slaoub@gmail.com> Cc: Chris Metcalf <cmetcalf@mellanox.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: Fenghua Yu <fenghua.yu@intel.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Huang Rui <ray.huang@amd.com> Cc: Jiri Slaby <jslaby@suse.cz> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Josh Poimboeuf <jpoimboe@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Michael S. Tsirkin <mst@redhat.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Cc: Paul Gortmaker <paul.gortmaker@windriver.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ravi V. Shankar <ravi.v.shankar@intel.com> Cc: Shuah Khan <shuah@kernel.org> Cc: Tony Luck <tony.luck@intel.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: ricardo.neri@intel.com Link: http://lkml.kernel.org/r/1509935277-22138-8-git-send-email-ricardo.neri-calderon@linux.intel.com Signed-off-by: Ingo Molnar <mingo@kernel.org> |
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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.