CVE-2018-8897

From patchwork Tue May  8 17:28:35 2018
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Subject: selftests/x86: Add mov_to_ss
From: Andrew Lutomirski <luto@kernel.org>
X-Patchwork-Id: 10386677
Message-Id: <67e08b69817171da8026e0eb3af0214b06b4d74f.1525800455.git.luto@kernel.org>
To: x86@kernel.org, LKML <linux-kernel@vger.kernel.org>,
 Linus Torvalds <torvalds@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>
Date: Tue,  8 May 2018 10:28:35 -0700

This exercises a nasty corner case of the x86 ISA.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 tools/testing/selftests/x86/Makefile      |   2 +-
 tools/testing/selftests/x86/mov_ss_trap.c | 285 ++++++++++++++++++++++
 2 files changed, 286 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/x86/mov_ss_trap.c

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index d744991c0f4f..39f66bc29b82 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -11,7 +11,7 @@ CAN_BUILD_X86_64 := $(shell ./check_cc.sh $(CC) trivial_64bit_program.c)

 TARGETS_C_BOTHBITS := single_step_syscall sysret_ss_attrs syscall_nt test_mremap_vdso \
            check_initial_reg_state sigreturn iopl mpx-mini-test ioperm \
-           protection_keys test_vdso test_vsyscall
+           protection_keys test_vdso test_vsyscall mov_ss_trap
 TARGETS_C_32BIT_ONLY := entry_from_vm86 syscall_arg_fault test_syscall_vdso unwind_vdso \
            test_FCMOV test_FCOMI test_FISTTP \
            vdso_restorer
diff --git a/tools/testing/selftests/x86/mov_ss_trap.c b/tools/testing/selftests/x86/mov_ss_trap.c
new file mode 100644
index 000000000000..3c3a022654f3
--- /dev/null
+++ b/tools/testing/selftests/x86/mov_ss_trap.c
@@ -0,0 +1,285 @@
/* SPDX-License-Identifier: GPL-2.0 */
/*
 * mov_ss_trap.c: Exercise the bizarre side effects of a watchpoint on MOV SS
 *
 * This does MOV SS from a watchpointed address followed by various
 * types of kernel entries.  A MOV SS that hits a watchpoint will queue
 * up a #DB trap but will not actually deliver that trap.  The trap
 * will be delivered after the next instruction instead.  The CPU's logic
 * seems to be:
 *
 *  - Any fault: drop the pending #DB trap.
 *  - INT $N, INT3, INTO, SYSCALL, SYSENTER: enter the kernel and then
 *    deliver #DB.
 *  - ICEBP: enter the kernel but do not deliver the watchpoint trap
 *  - breakpoint: only one #DB is delivered (phew!)
 *
 * There are plenty of ways for a kernel to handle this incorrectly.  This
 * test tries to exercise all the cases.
 *
 * This should mostly cover CVE-2018-1087 and CVE-2018-8897.
 */
#define _GNU_SOURCE

#include <stdlib.h>
#include <sys/ptrace.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <sys/syscall.h>
#include <unistd.h>
#include <errno.h>
#include <stddef.h>
#include <stdio.h>
#include <err.h>
#include <string.h>
#include <setjmp.h>
#include <sys/prctl.h>

#define X86_EFLAGS_RF (1UL << 16)

#if __x86_64__
# define REG_IP REG_RIP
#else
# define REG_IP REG_EIP
#endif

unsigned short ss;
extern unsigned char breakpoint_insn[];
sigjmp_buf jmpbuf;
static unsigned char altstack_data[SIGSTKSZ];

static void enable_watchpoint(void)
{
    pid_t parent = getpid();
    int status;

    pid_t child = fork();
    if (child < 0)
        err(1, "fork");

    if (child) {
        if (waitpid(child, &status, 0) != child)
            err(1, "waitpid for child");
    } else {
        unsigned long dr0, dr1, dr7;

        dr0 = (unsigned long)&ss;
        dr1 = (unsigned long)breakpoint_insn;
        dr7 = ((1UL << 1) | /* G0 */
               (3UL << 16) |    /* RW0 = read or write */
               (1UL << 18) |    /* LEN0 = 2 bytes */
               (1UL << 3)); /* G1, RW1 = insn */

        if (ptrace(PTRACE_ATTACH, parent, NULL, NULL) != 0)
            err(1, "PTRACE_ATTACH");

        if (waitpid(parent, &status, 0) != parent)
            err(1, "waitpid for child");

        if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[0]), dr0) != 0)
            err(1, "PTRACE_POKEUSER DR0");

        if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[1]), dr1) != 0)
            err(1, "PTRACE_POKEUSER DR1");

        if (ptrace(PTRACE_POKEUSER, parent, (void *)offsetof(struct user, u_debugreg[7]), dr7) != 0)
            err(1, "PTRACE_POKEUSER DR7");

        printf("\tDR0 = %lx, DR1 = %lx, DR7 = %lx\n", dr0, dr1, dr7);

        if (ptrace(PTRACE_DETACH, parent, NULL, NULL) != 0)
            err(1, "PTRACE_DETACH");

        exit(0);
    }
}

static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
               int flags)
{
    struct sigaction sa;
    memset(&sa, 0, sizeof(sa));
    sa.sa_sigaction = handler;
    sa.sa_flags = SA_SIGINFO | flags;
    sigemptyset(&sa.sa_mask);
    if (sigaction(sig, &sa, 0))
        err(1, "sigaction");
}

static char const * const signames[] = {
    [SIGSEGV] = "SIGSEGV",
    [SIGBUS] = "SIBGUS",
    [SIGTRAP] = "SIGTRAP",
    [SIGILL] = "SIGILL",
};

static void sigtrap(int sig, siginfo_t *si, void *ctx_void)
{
    ucontext_t *ctx = ctx_void;

    printf("\tGot SIGTRAP with RIP=%lx, EFLAGS.RF=%d\n",
           (unsigned long)ctx->uc_mcontext.gregs[REG_IP],
           !!(ctx->uc_mcontext.gregs[REG_EFL] & X86_EFLAGS_RF));
}

static void handle_and_return(int sig, siginfo_t *si, void *ctx_void)
{
    ucontext_t *ctx = ctx_void;

    printf("\tGot %s with RIP=%lx\n", signames[sig],
           (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);
}

static void handle_and_longjmp(int sig, siginfo_t *si, void *ctx_void)
{
    ucontext_t *ctx = ctx_void;

    printf("\tGot %s with RIP=%lx\n", signames[sig],
           (unsigned long)ctx->uc_mcontext.gregs[REG_IP]);

    siglongjmp(jmpbuf, 1);
}

int main()
{
    unsigned long nr;

    asm volatile ("mov %%ss, %[ss]" : [ss] "=m" (ss));
    printf("\tSS = 0x%hx, &SS = 0x%p\n", ss, &ss);

    if (prctl(PR_SET_PTRACER, PR_SET_PTRACER_ANY, 0, 0, 0) == 0)
        printf("\tPR_SET_PTRACER_ANY succeeded\n");

    printf("\tSet up a watchpoint\n");
    sethandler(SIGTRAP, sigtrap, 0);
    enable_watchpoint();

    printf("[RUN]\tRead from watched memory (should get SIGTRAP)\n");
    asm volatile ("mov %[ss], %[tmp]" : [tmp] "=r" (nr) : [ss] "m" (ss));

    printf("[RUN]\tMOV SS; INT3\n");
    asm volatile ("mov %[ss], %%ss; int3" :: [ss] "m" (ss));

    printf("[RUN]\tMOV SS; INT 3\n");
    asm volatile ("mov %[ss], %%ss; .byte 0xcd, 0x3" :: [ss] "m" (ss));

    printf("[RUN]\tMOV SS; CS CS INT3\n");
    asm volatile ("mov %[ss], %%ss; .byte 0x2e, 0x2e; int3" :: [ss] "m" (ss));

    printf("[RUN]\tMOV SS; CSx14 INT3\n");
    asm volatile ("mov %[ss], %%ss; .fill 14,1,0x2e; int3" :: [ss] "m" (ss));

    printf("[RUN]\tMOV SS; INT 4\n");
    sethandler(SIGSEGV, handle_and_return, SA_RESETHAND);
    asm volatile ("mov %[ss], %%ss; int $4" :: [ss] "m" (ss));

#ifdef __i386__
    printf("[RUN]\tMOV SS; INTO\n");
    sethandler(SIGSEGV, handle_and_return, SA_RESETHAND);
    nr = -1;
    asm volatile ("add $1, %[tmp]; mov %[ss], %%ss; into"
              : [tmp] "+r" (nr) : [ss] "m" (ss));
#endif

    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; ICEBP\n");

        /* Some emulators (e.g. QEMU TCG) don't emulate ICEBP. */
        sethandler(SIGILL, handle_and_longjmp, SA_RESETHAND);

        asm volatile ("mov %[ss], %%ss; .byte 0xf1" :: [ss] "m" (ss));
    }

    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; CLI\n");
        sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
        asm volatile ("mov %[ss], %%ss; cli" :: [ss] "m" (ss));
    }

    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; #PF\n");
        sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
        asm volatile ("mov %[ss], %%ss; mov (-1), %[tmp]"
                  : [tmp] "=r" (nr) : [ss] "m" (ss));
    }

    /*
     * INT $1: if #DB has DPL=3 and there isn't special handling,
     * then the kernel will die.
     */
    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; INT 1\n");
        sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
        asm volatile ("mov %[ss], %%ss; int $1" :: [ss] "m" (ss));
    }

#ifdef __x86_64__
    /*
     * In principle, we should test 32-bit SYSCALL as well, but
     * the calling convention is so unpredictable that it's
     * not obviously worth the effort.
     */
    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; SYSCALL\n");
        sethandler(SIGILL, handle_and_longjmp, SA_RESETHAND);
        nr = SYS_getpid;
        /*
         * Toggle the high bit of RSP to make it noncanonical to
         * strengthen this test on non-SMAP systems.
         */
        asm volatile ("btc $63, %%rsp\n\t"
                  "mov %[ss], %%ss; syscall\n\t"
                  "btc $63, %%rsp"
                  : "+a" (nr) : [ss] "m" (ss)
                  : "rcx"
#ifdef __x86_64__
                , "r11"
#endif
            );
    }
#endif

    printf("[RUN]\tMOV SS; breakpointed NOP\n");
    asm volatile ("mov %[ss], %%ss; breakpoint_insn: nop" :: [ss] "m" (ss));

    /*
     * Invoking SYSENTER directly breaks all the rules.  Just handle
     * the SIGSEGV.
     */
    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; SYSENTER\n");
        stack_t stack = {
            .ss_sp = altstack_data,
            .ss_size = SIGSTKSZ,
        };
        if (sigaltstack(&stack, NULL) != 0)
            err(1, "sigaltstack");
        sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND | SA_ONSTACK);
        nr = SYS_getpid;
        asm volatile ("mov %[ss], %%ss; SYSENTER" : "+a" (nr)
                  : [ss] "m" (ss) : "flags", "rcx"
#ifdef __x86_64__
                , "r11"
#endif
            );

        /* We're unreachable here.  SYSENTER forgets RIP. */
    }

    if (sigsetjmp(jmpbuf, 1) == 0) {
        printf("[RUN]\tMOV SS; INT $0x80\n");
        sethandler(SIGSEGV, handle_and_longjmp, SA_RESETHAND);
        nr = 20;    /* compat getpid */
        asm volatile ("mov %[ss], %%ss; int $0x80"
                  : "+a" (nr) : [ss] "m" (ss)
                  : "flags"
#ifdef __x86_64__
                , "r8", "r9", "r10", "r11"
#endif
            );
    }

    printf("[OK]\tI aten't dead\n");
    return 0;
}