This is v7 of this series. The six previous submissions can be found here [1], here [2], here[3], here[4], here[5] and here[6]. This version addresses the comments received in v6 plus improvements of the handling of exceptions unrelated to UMIP as well as corner cases in virtual-8086 mode. Please see details in the change log.
=== What is UMIP? User-Mode Instruction Prevention (UMIP) is a security feature present in new Intel Processors. If enabled, it prevents the execution of certain instructions if the Current Privilege Level (CPL) is greater than 0. If these instructions were executed while in CPL > 0, user space applications could have access to system-wide settings such as the global and local descriptor tables, the segment selectors to the current task state and the local descriptor table. Hiding these system resources reduces the tools available to craft privilege escalation attacks such as [7]. These are the instructions covered by UMIP: * SGDT - Store Global Descriptor Table * SIDT - Store Interrupt Descriptor Table * SLDT - Store Local Descriptor Table * SMSW - Store Machine Status Word * STR - Store Task Register If any of these instructions is executed with CPL > 0, a general protection exception is issued when UMIP is enabled. === How does it impact applications? We want to have UMIP enabled by default. However, UMIP will change the behavior that certain applications expect from the operating system. For instance, programs running on WineHQ and DOSEMU2 rely on some of these instructions to function. Stas Sergeev found that Microsoft Windows 3.1 and dos4gw use the instruction SMSW when running in virtual-8086 mode[8]. SGDT and SIDT can also run on virtual-8086 mode. In order to not change the behavior of the system. This patchset emulates the SGDT, SIDT and SMSW. This should be sufficient to not break the applications mentioned above. Regarding the two remaining instructions, STR and SLDT, the WineHQ team has shown interest catching the general protection fault and use it as a vehicle to fix broken applications[9]. Furthermore, STR and SLDT can only run in protected and long modes. DOSEMU2 emulates virtual-8086 mode via KVM. No applications will be broken unless DOSEMU2 decides to enable the CR4.UMIP bit in platforms that support it. Also, this should not pose a security risk as no system resouces would be revealed. Instead, code running inside the KVM would only see the KVM's GDT, IDT and MSW. Please note that UMIP is always enabled for both 64-bit and 32-bit Linux builds. However, emulation of the UMIP-protected instructions is not done for 64-bit processes. 64-bit user space applications will receive the SIGSEGV signal when UMIP instructions causes a general protection fault. === How are UMIP-protected instructions emulated? This version keeps UMIP enabled at all times and by default. If a general protection fault caused by the instructions protected by UMIP is detected, such fault will be fixed-up by returning dummy values as follows: * SGDT and SIDT return hard-coded dummy values as the base of the global descriptor and interrupt descriptor tables. These hard-coded values correspond to memory addresses that are near the end of the kernel memory map. This is also the case for virtual-8086 mode tasks. In all my experiments in x86_32, the base of GDT and IDT was always a 4-byte address, even for 16-bit operands. Thus, my emulation code does the same. In all cases, the limit of the table is set to 0. * SMSW returns the value with which the CR0 register is programmed in head_32/64.S at boot time. This is, the following bits are enabled: CR0.0 for Protection Enable, CR.1 for Monitor Coprocessor, CR.4 for Extension Type, which will always be 1 in recent processors with UMIP; CR.5 for Numeric Error, CR0.16 for Write Protect, CR0.18 for Alignment Mask. As per the Intel 64 and IA-32 Architectures Software Developer's Manual, SMSW returns a 16-bit results for memory operands. However, when the operand is a register, the results can be up to CR0[63:0]. Since the emulation code only kicks-in in x86_32, we return up to CR[31:0]. * The proposed emulation code is handles faults that happens in both protected and virtual-8086 mode. * Again, STR and SLDT are not emulated. === How is this series laid out? ++ Preparatory work As per suggestions from Andy Lutormirsky and Borislav Petkov, I moved the x86 page fault error codes to a header. Also, I made user_64bit_mode available to x86_32 builds. This helps to reuse code and reduce the number of #ifdef's in these patches. ++ Fix bugs in MPX address evaluator I found very useful the code for Intel MPX (Memory Protection Extensions) used to parse opcodes and the memory locations contained in the general purpose registers when used as operands. I put some of this code in a separate library file that both MPX and UMIP can access and avoid code duplication. Before creating the new library, I fixed a couple of bugs that I found in how MPX determines the address contained in the instruction and operands. ++ Provide a new x86 instruction evaluating library With bugs fixed, the MPX evaluating code is relocated in a new insn-eval.c library. The basic functionality of this library is extended to obtain the segment descriptor selected by either segment override prefixes or the default segment by the involved registers in the calculation of the effective address. It was also extended to obtain the default address and operand sizes as well as the segment base address. Also, support to process 16-bit address encodings. Armed with this arsenal, it is now possible to determine the linear address onto which the emulated results shall be copied. This code supports Normal 32-bit and 64-bit (i.e., __USER32_CS and/or __USER_CS) protected mode, virtual-8086 mode, 16-bit protected mode with 32-bit base address. ++ Emulate UMIP instructions A new fixup_umip_exception functions inspect the instruction at the instruction pointer. If it is an UMIP-protected instruction, it executes the emulation code. This uses all the address-computing code of the previous section. ++ Add self-tests Lastly, self-tests are added to entry_from_v86.c to exercise the most typical use cases of UMIP-protected instructions in a virtual-8086 mode. ++ Extensive tests Extensive tests were performed to test all the combinations of ModRM, SiB and displacements for 16-bit and 32-bit encodings for the ss, ds, es, fs and gs segments. Tests also include a 64-bit program that uses segmentation via fs and gs. For this purpose, I temporarily enabled UMIP support for 64-bit process. This change is not part of this patchset. The intention is to test the computations of linear addresses in 64-bit mode, including the extra R8-R15 registers. Extensive test is also implemented for virtual-8086 tasks. Code of these tests can be found here [10] and here [11]. ++ Merging this series? Am I any close to see these patches merged? :) [1]. https://lwn.net/Articles/705877/ [2]. https://lkml.org/lkml/2016/12/23/265 [3]. https://lkml.org/lkml/2017/1/25/622 [4]. https://lkml.org/lkml/2017/2/23/40 [5]. https://lkml.org/lkml/2017/3/3/678 [6]. https://lkml.org/lkml/2017/3/7/866 [7]. http://timetobleed.com/a-closer-look-at-a-recent-privilege-escalation-bug-in-linux-cve-2013-2094/ [8]. https://www.winehq.org/pipermail/wine-devel/2017-April/117159.html [10]. https://github.com/01org/luv-yocto/tree/rneri/umip/meta-luv/recipes-core/umip/files [11]. https://github.com/01org/luv-yocto/commit/a72a7fe7d68693c0f4100ad86de6ecabde57334f#diff-3860c136a63add269bce4ea50222c248R1 Thanks and BR, Ricardo Changes since V6: *Reworded and addded more details on the special cases of ModRM and SIB bytes. To avoid confusion, I ommited mentioning the involved registers (EBP and ESP). *Replaced BUG() with printk_ratelimited in function get_reg_offset of insn-eval.c *Removed unused utility functions that obtain a register value from pt_regs given a SIB base and index. *Clarified nomenclature to call CS, DS, ES, FS, GS and SS segment registers and their values segment selectors. *Reworked function resolve_seg_register to issue an error when more than one segment overrides prefixes are used in the instruction. *Added logic in resolve_seg_register to ignore segment register when in long mode and not using FS or GS. *Added logic to ensure the effective address is within the limits of the segment in protected mode. *Added logic to ensure segment override prefixes are ignored when resolving the segment of EIP and EDI with string instructions. *Added code to make user_64bit_mode() available in CONFIG_X86_32... and make it return false, of course. *Merged the two functions that obtain the default address and operand size of a code segment into one as they are always used together. *Corrected logic of displacement-only addressing in long mode to make the displacement relative to the RIP of the next instruction. *Reworked logic to sign-extend 32-bit memory offsets into 64-bit signed memory offsets. This include more checks and putting all together in an utility function. *Removed the 'unlikely' of conditional statements as we are not in a critical path. *In virtual-8086 mode, ensure that effective addresses are always less than 0x10000, even when address override prefixes are used. Also, ensure that linear addresses have a size of 20-bits. Changes since V5: * Relocate the page fault error code enumerations to traps.h Changes since V4: * Audited patches to use braces in all the branches of conditional. statements, except those in which the conditional action only takes one line. * Implemented support in 64-builds for both 32-bit and 64-bit tasks in the instruction evaluating library. * Split segment selector function in the instruction evaluating library into two functions to resolve the segment type by instruction override or default and a separate function to actually read the segment selector. * Fixed a bug when evaluating 32-bit effective addresses with 64-bit kernels. * Split patches further for for easier review. * Use signed variables for computation of effective address. * Fixed issue with a spurious static modifier in function insn_get_addr_ref found by kbuild test bot. * Removed comparison between true and fixup_umip_exception. * Reworked check logic when identifying erroneous vs invalid values of the SiB base and index. Changes since V3: * Limited emulation to 32-bit and 16-bit modes. For 64-bit mode, a general protection fault is still issued when UMIP-protected instructions are executed with CPL > 0. * Expanded instruction-evaluating code to obtain segment descriptor along with their attributes such as base address and default address and operand sizes. Also, support for 16-bit encodings in protected mode was implemented. * When getting a segment descriptor, this include support to obtain those of a local descriptor table. * Now the instruction-evaluating code returns -EDOM when the value of registers should not be used in calculating the effective address. The value -EINVAL is left for errors. * Incorporate the value of the segment base address in the computation of linear addresses. * Renamed new instruction evaluation library from insn-kernel.c to insn-eval.c * Exported functions insn_get_reg_offset_* to obtain the register offset by ModRM r/m, SiB base and SiB index. * Improved documentation of functions. * Split patches further for easier review. Changes since V2: * Added new utility functions to decode the memory addresses contained in registers when the 16-bit addressing encodings are used. This includes code to obtain and compute memory addresses using segment selectors for real-mode address translation. * Added support to emulate UMIP-protected instructions for virtual-8086 tasks. * Added self-tests for virtual-8086 mode that contains representative use cases: address represented as a displacement, address in registers and registers as operands. * Instead of maintaining a static variable for the dummy base addresses of the IDT and GDT, a hard-coded value is used. * The emulated SMSW instructions now return the value with which the CR0 register is programmed in head_32/64.S This is: PE | MP | ET | NE | WP | AM. For x86_64, PG is also enabled. * The new file arch/x86/lib/insn-utils.c is now renamed as arch/x86/lib/ insn-kernel.c. It also has its own header. This helps keep in sync the the kernel and objtool instruction decoders. Also, the new insn-kernel.c contains utility functions that are only relevant in a kernel context. * Removed printed warnings for errors that occur when decoding instructions with invalid operands. * Added more comments on fixes in the instruction-decoding MPX functions. * Now user_64bit_mode(regs) is used instead of test_thread_flag(TIF_IA32) to determine if the task is 32-bit or 64-bit. * Found and fixed a bug in insn-decoder in which X86_MODRM_RM was incorrectly used to obtain the mod part of the ModRM byte. * Added more explanatory code in emulation and instruction decoding code. This includes a comment regarding that copy_from_user could fail if there exists a memory protection key in place. * Tested code with CONFIG_X86_DECODER_SELFTEST=y and everything passes now. * Prefixed get_reg_offset_rm with insn_ as this function is exposed via a header file. For clarity, this function was added in a separate patch. Changes since V1: * Virtual-8086 mode tasks are not treated in a special manner. All code for this purpose was removed. * Instead of attempting to disable UMIP during a context switch or when entering virtual-8086 mode, UMIP remains enabled all the time. General protection faults that occur are fixed-up by returning dummy values as detailed above. * Removed umip= kernel parameter in favor of using clearcpuid=514 to disable UMIP. * Removed selftests designed to detect the absence of SIGSEGV signals when running in virtual-8086 mode. * Reused code from MPX to decode instructions operands. For this purpose code was put in a common location. * Fixed two bugs in MPX code that decodes operands. Ricardo Neri (26): ptrace,x86: Make user_64bit_mode() available to 32-bit builds x86/mm: Relocate page fault error codes to traps.h x86/mpx: Use signed variables to compute effective addresses x86/mpx: Do not use SIB.index if its value is 100b and ModRM.mod is not 11b x86/mpx: Do not use SIB.base if its value is 101b and ModRM.mod = 0 x86/mpx, x86/insn: Relocate insn util functions to a new insn-eval file x86/insn-eval: Do not BUG on invalid register type x86/insn-eval: Add a utility function to get register offsets x86/insn-eval: Add utility function to identify string instructions x86/insn-eval: Add utility functions to get segment selector x86/insn-eval: Add utility function to get segment descriptor x86/insn-eval: Add utility functions to get segment descriptor base address and limit x86/insn-eval: Add function to get default params of code segment x86/insn-eval: Indicate a 32-bit displacement if ModRM.mod is 0 and ModRM.rm is 5 x86/insn-eval: Incorporate segment base and limit in linear address computation x86/insn-eval: Support both signed 32-bit and 64-bit effective addresses x86/insn-eval: Handle 32-bit address encodings in virtual-8086 mode x86/insn-eval: Add support to resolve 16-bit addressing encodings x86/insn-eval: Add wrapper function for 16-bit and 32-bit address encodings x86/cpufeature: Add User-Mode Instruction Prevention definitions x86: Add emulation code for UMIP instructions x86/umip: Force a page fault when unable to copy emulated result to user x86/traps: Fixup general protection faults caused by UMIP x86: Enable User-Mode Instruction Prevention selftests/x86: Add tests for User-Mode Instruction Prevention selftests/x86: Add tests for instruction str and sldt arch/x86/Kconfig | 10 + arch/x86/include/asm/cpufeatures.h | 1 + arch/x86/include/asm/disabled-features.h | 8 +- arch/x86/include/asm/insn-eval.h | 25 + arch/x86/include/asm/ptrace.h | 6 +- arch/x86/include/asm/traps.h | 18 + arch/x86/include/asm/umip.h | 15 + arch/x86/include/uapi/asm/processor-flags.h | 2 + arch/x86/kernel/Makefile | 1 + arch/x86/kernel/cpu/common.c | 16 +- arch/x86/kernel/traps.c | 4 + arch/x86/kernel/umip.c | 286 +++++++ arch/x86/lib/Makefile | 2 +- arch/x86/lib/insn-eval.c | 1066 +++++++++++++++++++++++++ arch/x86/mm/fault.c | 88 +- arch/x86/mm/mpx.c | 120 +-- tools/testing/selftests/x86/entry_from_vm86.c | 89 ++- 17 files changed, 1580 insertions(+), 177 deletions(-) create mode 100644 arch/x86/include/asm/insn-eval.h create mode 100644 arch/x86/include/asm/umip.h create mode 100644 arch/x86/kernel/umip.c create mode 100644 arch/x86/lib/insn-eval.c -- 2.9.3 -- To unsubscribe from this list: send the line "unsubscribe linux-msdos" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
