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Subject[GIT pull] x86/entry for 5.3-rc1
Linus,

please pull the latest x86-entry-for-linus git tree from:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git x86-entry-for-linus

up to: 7f0a5e075583: selftests/x86: Add a test for process_vm_readv() on the vsyscall page

Further hardening of the legacy vsyscall by providing support for execute
only mode and switching the default to it. This prevents a certain class of
attacks which rely on the vsyscall page being accessible at a fixed address
in the canonical kernel address space..

Thanks,

tglx

------------------>
Andy Lutomirski (8):
Documentation/admin: Remove the vsyscall=native documentation
x86/vsyscall: Add a new vsyscall=xonly mode
x86/vsyscall: Show something useful on a read fault
x86/vsyscall: Document odd SIGSEGV error code for vsyscalls
selftests/x86/vsyscall: Verify that vsyscall=none blocks execution
x86/vsyscall: Change the default vsyscall mode to xonly
x86/vsyscall: Add __ro_after_init to global variables
selftests/x86: Add a test for process_vm_readv() on the vsyscall page


Documentation/admin-guide/kernel-parameters.txt | 11 +--
arch/x86/Kconfig | 35 +++++--
arch/x86/entry/vsyscall/vsyscall_64.c | 37 +++++++-
arch/x86/include/asm/vsyscall.h | 6 +-
arch/x86/mm/fault.c | 18 ++--
tools/testing/selftests/x86/test_vsyscall.c | 120 +++++++++++++++++++-----
6 files changed, 174 insertions(+), 53 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 138f6664b2e2..be8c3a680afa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5100,13 +5100,12 @@
targets for exploits that can control RIP.

emulate [default] Vsyscalls turn into traps and are
- emulated reasonably safely.
+ emulated reasonably safely. The vsyscall
+ page is readable.

- native Vsyscalls are native syscall instructions.
- This is a little bit faster than trapping
- and makes a few dynamic recompilers work
- better than they would in emulation mode.
- It also makes exploits much easier to write.
+ xonly Vsyscalls turn into traps and are
+ emulated reasonably safely. The vsyscall
+ page is not readable.

none Vsyscalls don't work at all. This makes
them quite hard to use for exploits but
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 2bbbd4d1ba31..32028edc1b0e 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -2285,7 +2285,7 @@ config COMPAT_VDSO
choice
prompt "vsyscall table for legacy applications"
depends on X86_64
- default LEGACY_VSYSCALL_EMULATE
+ default LEGACY_VSYSCALL_XONLY
help
Legacy user code that does not know how to find the vDSO expects
to be able to issue three syscalls by calling fixed addresses in
@@ -2293,23 +2293,38 @@ choice
it can be used to assist security vulnerability exploitation.

This setting can be changed at boot time via the kernel command
- line parameter vsyscall=[emulate|none].
+ line parameter vsyscall=[emulate|xonly|none].

On a system with recent enough glibc (2.14 or newer) and no
static binaries, you can say None without a performance penalty
to improve security.

- If unsure, select "Emulate".
+ If unsure, select "Emulate execution only".

config LEGACY_VSYSCALL_EMULATE
- bool "Emulate"
+ bool "Full emulation"
help
- The kernel traps and emulates calls into the fixed
- vsyscall address mapping. This makes the mapping
- non-executable, but it still contains known contents,
- which could be used in certain rare security vulnerability
- exploits. This configuration is recommended when userspace
- still uses the vsyscall area.
+ The kernel traps and emulates calls into the fixed vsyscall
+ address mapping. This makes the mapping non-executable, but
+ it still contains readable known contents, which could be
+ used in certain rare security vulnerability exploits. This
+ configuration is recommended when using legacy userspace
+ that still uses vsyscalls along with legacy binary
+ instrumentation tools that require code to be readable.
+
+ An example of this type of legacy userspace is running
+ Pin on an old binary that still uses vsyscalls.
+
+ config LEGACY_VSYSCALL_XONLY
+ bool "Emulate execution only"
+ help
+ The kernel traps and emulates calls into the fixed vsyscall
+ address mapping and does not allow reads. This
+ configuration is recommended when userspace might use the
+ legacy vsyscall area but support for legacy binary
+ instrumentation of legacy code is not needed. It mitigates
+ certain uses of the vsyscall area as an ASLR-bypassing
+ buffer.

config LEGACY_VSYSCALL_NONE
bool "None"
diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c b/arch/x86/entry/vsyscall/vsyscall_64.c
index d9d81ad7a400..07003f3f1bfc 100644
--- a/arch/x86/entry/vsyscall/vsyscall_64.c
+++ b/arch/x86/entry/vsyscall/vsyscall_64.c
@@ -42,9 +42,11 @@
#define CREATE_TRACE_POINTS
#include "vsyscall_trace.h"

-static enum { EMULATE, NONE } vsyscall_mode =
+static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
#ifdef CONFIG_LEGACY_VSYSCALL_NONE
NONE;
+#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
+ XONLY;
#else
EMULATE;
#endif
@@ -54,6 +56,8 @@ static int __init vsyscall_setup(char *str)
if (str) {
if (!strcmp("emulate", str))
vsyscall_mode = EMULATE;
+ else if (!strcmp("xonly", str))
+ vsyscall_mode = XONLY;
else if (!strcmp("none", str))
vsyscall_mode = NONE;
else
@@ -113,7 +117,8 @@ static bool write_ok_or_segv(unsigned long ptr, size_t size)
}
}

-bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
+bool emulate_vsyscall(unsigned long error_code,
+ struct pt_regs *regs, unsigned long address)
{
struct task_struct *tsk;
unsigned long caller;
@@ -122,6 +127,22 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
long ret;
unsigned long orig_dx;

+ /* Write faults or kernel-privilege faults never get fixed up. */
+ if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
+ return false;
+
+ if (!(error_code & X86_PF_INSTR)) {
+ /* Failed vsyscall read */
+ if (vsyscall_mode == EMULATE)
+ return false;
+
+ /*
+ * User code tried and failed to read the vsyscall page.
+ */
+ warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
+ return false;
+ }
+
/*
* No point in checking CS -- the only way to get here is a user mode
* trap to a high address, which means that we're in 64-bit user code.
@@ -284,7 +305,7 @@ static const char *gate_vma_name(struct vm_area_struct *vma)
static const struct vm_operations_struct gate_vma_ops = {
.name = gate_vma_name,
};
-static struct vm_area_struct gate_vma = {
+static struct vm_area_struct gate_vma __ro_after_init = {
.vm_start = VSYSCALL_ADDR,
.vm_end = VSYSCALL_ADDR + PAGE_SIZE,
.vm_page_prot = PAGE_READONLY_EXEC,
@@ -357,12 +378,20 @@ void __init map_vsyscall(void)
extern char __vsyscall_page;
unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);

- if (vsyscall_mode != NONE) {
+ /*
+ * For full emulation, the page needs to exist for real. In
+ * execute-only mode, there is no PTE at all backing the vsyscall
+ * page.
+ */
+ if (vsyscall_mode == EMULATE) {
__set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
PAGE_KERNEL_VVAR);
set_vsyscall_pgtable_user_bits(swapper_pg_dir);
}

+ if (vsyscall_mode == XONLY)
+ gate_vma.vm_flags = VM_EXEC;
+
BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
(unsigned long)VSYSCALL_ADDR);
}
diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
index b986b2ca688a..ab60a71a8dcb 100644
--- a/arch/x86/include/asm/vsyscall.h
+++ b/arch/x86/include/asm/vsyscall.h
@@ -13,10 +13,12 @@ extern void set_vsyscall_pgtable_user_bits(pgd_t *root);
* Called on instruction fetch fault in vsyscall page.
* Returns true if handled.
*/
-extern bool emulate_vsyscall(struct pt_regs *regs, unsigned long address);
+extern bool emulate_vsyscall(unsigned long error_code,
+ struct pt_regs *regs, unsigned long address);
#else
static inline void map_vsyscall(void) {}
-static inline bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
+static inline bool emulate_vsyscall(unsigned long error_code,
+ struct pt_regs *regs, unsigned long address)
{
return false;
}
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 46df4c6aae46..58e4f1f00bbc 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -710,6 +710,10 @@ static void set_signal_archinfo(unsigned long address,
* To avoid leaking information about the kernel page
* table layout, pretend that user-mode accesses to
* kernel addresses are always protection faults.
+ *
+ * NB: This means that failed vsyscalls with vsyscall=none
+ * will have the PROT bit. This doesn't leak any
+ * information and does not appear to cause any problems.
*/
if (address >= TASK_SIZE_MAX)
error_code |= X86_PF_PROT;
@@ -1369,16 +1373,18 @@ void do_user_addr_fault(struct pt_regs *regs,

#ifdef CONFIG_X86_64
/*
- * Instruction fetch faults in the vsyscall page might need
- * emulation. The vsyscall page is at a high address
- * (>PAGE_OFFSET), but is considered to be part of the user
- * address space.
+ * Faults in the vsyscall page might need emulation. The
+ * vsyscall page is at a high address (>PAGE_OFFSET), but is
+ * considered to be part of the user address space.
*
* The vsyscall page does not have a "real" VMA, so do this
* emulation before we go searching for VMAs.
+ *
+ * PKRU never rejects instruction fetches, so we don't need
+ * to consider the PF_PK bit.
*/
- if ((hw_error_code & X86_PF_INSTR) && is_vsyscall_vaddr(address)) {
- if (emulate_vsyscall(regs, address))
+ if (is_vsyscall_vaddr(address)) {
+ if (emulate_vsyscall(hw_error_code, regs, address))
return;
}
#endif
diff --git a/tools/testing/selftests/x86/test_vsyscall.c b/tools/testing/selftests/x86/test_vsyscall.c
index 0b4f1cc2291c..4602326b8f5b 100644
--- a/tools/testing/selftests/x86/test_vsyscall.c
+++ b/tools/testing/selftests/x86/test_vsyscall.c
@@ -18,6 +18,7 @@
#include <sched.h>
#include <stdbool.h>
#include <setjmp.h>
+#include <sys/uio.h>

#ifdef __x86_64__
# define VSYS(x) (x)
@@ -49,21 +50,21 @@ static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
}

/* vsyscalls and vDSO */
-bool should_read_vsyscall = false;
+bool vsyscall_map_r = false, vsyscall_map_x = false;

typedef long (*gtod_t)(struct timeval *tv, struct timezone *tz);
-gtod_t vgtod = (gtod_t)VSYS(0xffffffffff600000);
+const gtod_t vgtod = (gtod_t)VSYS(0xffffffffff600000);
gtod_t vdso_gtod;

typedef int (*vgettime_t)(clockid_t, struct timespec *);
vgettime_t vdso_gettime;

typedef long (*time_func_t)(time_t *t);
-time_func_t vtime = (time_func_t)VSYS(0xffffffffff600400);
+const time_func_t vtime = (time_func_t)VSYS(0xffffffffff600400);
time_func_t vdso_time;

typedef long (*getcpu_t)(unsigned *, unsigned *, void *);
-getcpu_t vgetcpu = (getcpu_t)VSYS(0xffffffffff600800);
+const getcpu_t vgetcpu = (getcpu_t)VSYS(0xffffffffff600800);
getcpu_t vdso_getcpu;

static void init_vdso(void)
@@ -107,7 +108,7 @@ static int init_vsys(void)
maps = fopen("/proc/self/maps", "r");
if (!maps) {
printf("[WARN]\tCould not open /proc/self/maps -- assuming vsyscall is r-x\n");
- should_read_vsyscall = true;
+ vsyscall_map_r = true;
return 0;
}

@@ -133,12 +134,8 @@ static int init_vsys(void)
}

printf("\tvsyscall permissions are %c-%c\n", r, x);
- should_read_vsyscall = (r == 'r');
- if (x != 'x') {
- vgtod = NULL;
- vtime = NULL;
- vgetcpu = NULL;
- }
+ vsyscall_map_r = (r == 'r');
+ vsyscall_map_x = (x == 'x');

found = true;
break;
@@ -148,10 +145,8 @@ static int init_vsys(void)

if (!found) {
printf("\tno vsyscall map in /proc/self/maps\n");
- should_read_vsyscall = false;
- vgtod = NULL;
- vtime = NULL;
- vgetcpu = NULL;
+ vsyscall_map_r = false;
+ vsyscall_map_x = false;
}

return nerrs;
@@ -183,9 +178,13 @@ static inline long sys_getcpu(unsigned * cpu, unsigned * node,
}

static jmp_buf jmpbuf;
+static volatile unsigned long segv_err;

static void sigsegv(int sig, siginfo_t *info, void *ctx_void)
{
+ ucontext_t *ctx = (ucontext_t *)ctx_void;
+
+ segv_err = ctx->uc_mcontext.gregs[REG_ERR];
siglongjmp(jmpbuf, 1);
}

@@ -238,7 +237,7 @@ static int test_gtod(void)
err(1, "syscall gettimeofday");
if (vdso_gtod)
ret_vdso = vdso_gtod(&tv_vdso, &tz_vdso);
- if (vgtod)
+ if (vsyscall_map_x)
ret_vsys = vgtod(&tv_vsys, &tz_vsys);
if (sys_gtod(&tv_sys2, &tz_sys) != 0)
err(1, "syscall gettimeofday");
@@ -252,7 +251,7 @@ static int test_gtod(void)
}
}

- if (vgtod) {
+ if (vsyscall_map_x) {
if (ret_vsys == 0) {
nerrs += check_gtod(&tv_sys1, &tv_sys2, &tz_sys, "vsyscall", &tv_vsys, &tz_vsys);
} else {
@@ -273,7 +272,7 @@ static int test_time(void) {
t_sys1 = sys_time(&t2_sys1);
if (vdso_time)
t_vdso = vdso_time(&t2_vdso);
- if (vtime)
+ if (vsyscall_map_x)
t_vsys = vtime(&t2_vsys);
t_sys2 = sys_time(&t2_sys2);
if (t_sys1 < 0 || t_sys1 != t2_sys1 || t_sys2 < 0 || t_sys2 != t2_sys2) {
@@ -294,7 +293,7 @@ static int test_time(void) {
}
}

- if (vtime) {
+ if (vsyscall_map_x) {
if (t_vsys < 0 || t_vsys != t2_vsys) {
printf("[FAIL]\tvsyscall failed (ret:%ld output:%ld)\n", t_vsys, t2_vsys);
nerrs++;
@@ -330,7 +329,7 @@ static int test_getcpu(int cpu)
ret_sys = sys_getcpu(&cpu_sys, &node_sys, 0);
if (vdso_getcpu)
ret_vdso = vdso_getcpu(&cpu_vdso, &node_vdso, 0);
- if (vgetcpu)
+ if (vsyscall_map_x)
ret_vsys = vgetcpu(&cpu_vsys, &node_vsys, 0);

if (ret_sys == 0) {
@@ -369,7 +368,7 @@ static int test_getcpu(int cpu)
}
}

- if (vgetcpu) {
+ if (vsyscall_map_x) {
if (ret_vsys) {
printf("[FAIL]\tvsyscall getcpu() failed\n");
nerrs++;
@@ -410,20 +409,88 @@ static int test_vsys_r(void)
can_read = false;
}

- if (can_read && !should_read_vsyscall) {
+ if (can_read && !vsyscall_map_r) {
printf("[FAIL]\tWe have read access, but we shouldn't\n");
return 1;
- } else if (!can_read && should_read_vsyscall) {
+ } else if (!can_read && vsyscall_map_r) {
printf("[FAIL]\tWe don't have read access, but we should\n");
return 1;
+ } else if (can_read) {
+ printf("[OK]\tWe have read access\n");
} else {
- printf("[OK]\tgot expected result\n");
+ printf("[OK]\tWe do not have read access: #PF(0x%lx)\n",
+ segv_err);
}
#endif

return 0;
}

+static int test_vsys_x(void)
+{
+#ifdef __x86_64__
+ if (vsyscall_map_x) {
+ /* We already tested this adequately. */
+ return 0;
+ }
+
+ printf("[RUN]\tMake sure that vsyscalls really page fault\n");
+
+ bool can_exec;
+ if (sigsetjmp(jmpbuf, 1) == 0) {
+ vgtod(NULL, NULL);
+ can_exec = true;
+ } else {
+ can_exec = false;
+ }
+
+ if (can_exec) {
+ printf("[FAIL]\tExecuting the vsyscall did not page fault\n");
+ return 1;
+ } else if (segv_err & (1 << 4)) { /* INSTR */
+ printf("[OK]\tExecuting the vsyscall page failed: #PF(0x%lx)\n",
+ segv_err);
+ } else {
+ printf("[FAILT]\tExecution failed with the wrong error: #PF(0x%lx)\n",
+ segv_err);
+ return 1;
+ }
+#endif
+
+ return 0;
+}
+
+static int test_process_vm_readv(void)
+{
+#ifdef __x86_64__
+ char buf[4096];
+ struct iovec local, remote;
+ int ret;
+
+ printf("[RUN]\tprocess_vm_readv() from vsyscall page\n");
+
+ local.iov_base = buf;
+ local.iov_len = 4096;
+ remote.iov_base = (void *)0xffffffffff600000;
+ remote.iov_len = 4096;
+ ret = process_vm_readv(getpid(), &local, 1, &remote, 1, 0);
+ if (ret != 4096) {
+ printf("[OK]\tprocess_vm_readv() failed (ret = %d, errno = %d)\n", ret, errno);
+ return 0;
+ }
+
+ if (vsyscall_map_r) {
+ if (!memcmp(buf, (const void *)0xffffffffff600000, 4096)) {
+ printf("[OK]\tIt worked and read correct data\n");
+ } else {
+ printf("[FAIL]\tIt worked but returned incorrect data\n");
+ return 1;
+ }
+ }
+#endif
+
+ return 0;
+}

#ifdef __x86_64__
#define X86_EFLAGS_TF (1UL << 8)
@@ -455,7 +522,7 @@ static int test_emulation(void)
time_t tmp;
bool is_native;

- if (!vtime)
+ if (!vsyscall_map_x)
return 0;

printf("[RUN]\tchecking that vsyscalls are emulated\n");
@@ -497,6 +564,9 @@ int main(int argc, char **argv)

sethandler(SIGSEGV, sigsegv, 0);
nerrs += test_vsys_r();
+ nerrs += test_vsys_x();
+
+ nerrs += test_process_vm_readv();

#ifdef __x86_64__
nerrs += test_emulation();
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