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Date: Thu, 21 Jul 2011 17:03:29 +0400
From: Vasiliy Kulikov <segoon@...nwall.com>
To: kernel-hardening@...ts.openwall.com
Subject: Re: GNU_STACK policy problem
On Mon, Jul 18, 2011 at 23:48 +0400, Vasiliy Kulikov wrote:
> I've written kernel part of PT_GNU_STACK NX enforcement for binaries and
> trampolines emulation (the latter is taken from PaX as-is). It Just
> Works for both 32 and 64 bits.
If someone wants to test it, the patch is as follows (it doesn't handle
DSOs as I've written before):
(Some pr_err() should be removed.)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 2dbf6bf..c6ddb02 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -559,6 +559,152 @@ static int is_f00f_bug(struct pt_regs *regs, unsigned long address)
return 0;
}
+/*
+ * These hex patterns (both for 32 and 64 bits) for trampolines code are
+ * extracted directly from gcc sources, so the match shouldn't lead to
+ * false positive and false negative result.
+ */
+static bool handle_trampolines_32(struct pt_regs *regs)
+{
+ int err;
+
+#ifdef CONFIG_X86_64
+ /* If we overflow maximum x86-32 address space, something goes
+ * really wrong */
+ if ((regs->ip + 11) >> 32)
+ return;
+#endif
+
+ do {
+ unsigned char mov1, mov2;
+ unsigned short jmp;
+ unsigned int addr1, addr2;
+
+ err = get_user(mov1, (unsigned char __user *)regs->ip);
+ err |= get_user(addr1, (unsigned int __user *)(regs->ip + 1));
+ err |= get_user(mov2, (unsigned char __user *)(regs->ip + 5));
+ err |= get_user(addr2, (unsigned int __user *)(regs->ip + 6));
+ err |= get_user(jmp, (unsigned short __user *)(regs->ip + 10));
+
+ if (err)
+ break;
+
+ if (mov1 == 0xB9 && mov2 == 0xB8 && jmp == 0xE0FF) {
+ regs->cx = addr1;
+ regs->ax = addr2;
+ regs->ip = addr2;
+ return true;
+ }
+ } while (0);
+
+ do {
+ unsigned char mov, jmp;
+ unsigned int addr1, addr2;
+
+ err = get_user(mov, (unsigned char __user *)regs->ip);
+ err |= get_user(addr1, (unsigned int __user *)(regs->ip + 1));
+ err |= get_user(jmp, (unsigned char __user *)(regs->ip + 5));
+ err |= get_user(addr2, (unsigned int __user *)(regs->ip + 6));
+
+ if (err)
+ break;
+
+ if (mov == 0xB9 && jmp == 0xE9) {
+ regs->cx = addr1;
+ regs->ip = (unsigned int)(regs->ip + addr2 + 10);
+ return true;
+ }
+ } while (0);
+
+ /* Not a trampoline, don't block SEGFAULT */
+ return false;
+}
+
+#ifdef CONFIG_X86_64
+static bool handle_trampolines_64(struct pt_regs *regs)
+{
+ int err;
+
+ do {
+ unsigned short mov1, mov2, jmp1;
+ unsigned char jmp2;
+ unsigned int addr1;
+ unsigned long addr2;
+
+ err = get_user(mov1, (unsigned short __user *)regs->ip);
+ err |= get_user(addr1, (unsigned int __user *)(regs->ip + 2));
+ err |= get_user(mov2, (unsigned short __user *)(regs->ip + 6));
+ err |= get_user(addr2, (unsigned long __user *)(regs->ip + 8));
+ err |= get_user(jmp1, (unsigned short __user *)(regs->ip + 16));
+ err |= get_user(jmp2, (unsigned char __user *)(regs->ip + 18));
+
+ if (err)
+ break;
+
+ if (mov1 == 0xBB41 && mov2 == 0xBA49 && jmp1 == 0xFF49 && jmp2 == 0xE3) {
+ regs->r11 = addr1;
+ regs->r10 = addr2;
+ regs->ip = addr1;
+ return true;
+ }
+ } while (0);
+
+ do {
+ unsigned short mov1, mov2, jmp1;
+ unsigned char jmp2;
+ unsigned long addr1, addr2;
+
+ err = get_user(mov1, (unsigned short __user *)regs->ip);
+ err |= get_user(addr1, (unsigned long __user *)(regs->ip + 2));
+ err |= get_user(mov2, (unsigned short __user *)(regs->ip + 10));
+ err |= get_user(addr2, (unsigned long __user *)(regs->ip + 12));
+ err |= get_user(jmp1, (unsigned short __user *)(regs->ip + 20));
+ err |= get_user(jmp2, (unsigned char __user *)(regs->ip + 22));
+
+ if (err)
+ break;
+
+ if (mov1 == 0xBB49 && mov2 == 0xBA49 && jmp1 == 0xFF49 && jmp2 == 0xE3) {
+ regs->r11 = addr1;
+ regs->r10 = addr2;
+ regs->ip = addr1;
+ return true;
+ }
+ } while (0);
+
+ /* Not a trampoline, don't block SEGFAULT */
+ return false;
+}
+#endif
+
+static bool handle_trampolines(struct pt_regs *regs, unsigned long error_code,
+ unsigned long address)
+{
+ if ((current->flags & PF_EMULTRAMP) == 0) {
+ pr_err("tramp: no EMULTRAMP\n");
+ return false;
+ }
+
+ if (v8086_mode(regs)) {
+ pr_err("tramp: v8086\n");
+ return false;
+ }
+ if ((error_code & PF_INSTR) == 0) {
+ pr_err("tramp: not PF_INSTR\n");
+ return false;
+ }
+
+#ifdef CONFIG_X86_32
+ return handle_trampolines_32(regs);
+#else
+ if (regs->cs == __USER32_CS || (regs->cs & SEGMENT_LDT))
+ return handle_trampolines_32(regs);
+ else
+ return handle_trampolines_64(regs);
+#endif
+}
+EXPORT_SYMBOL_GPL(handle_trampolines);
+
static const char nx_warning[] = KERN_CRIT
"kernel tried to execute NX-protected page - exploit attempt? (uid: %d)\n";
@@ -720,6 +867,9 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
if (is_errata100(regs, address))
return;
+ if (handle_trampolines(regs, error_code, address))
+ return;
+
if (unlikely(show_unhandled_signals))
show_signal_msg(regs, error_code, address, tsk);
diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 303983f..64330e5 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -32,6 +32,7 @@
#include <linux/elf.h>
#include <linux/utsname.h>
#include <linux/coredump.h>
+#include <linux/pid_namespace.h>
#include <asm/uaccess.h>
#include <asm/param.h>
#include <asm/page.h>
@@ -556,6 +557,53 @@ static unsigned long randomize_stack_top(unsigned long stack_top)
#endif
}
+static int get_execstack(bool gnu_stack_present, int p_flags)
+{
+ struct pid_namespace *ns;
+ int pf_x;
+
+ if (gnu_stack_present) {
+ pf_x = p_flags & PF_X;
+ /* GNU_STACK => NX */
+ if (!pf_x)
+ return EXSTACK_DISABLE_X;
+
+ /* GNU_STACK => X */
+ ns = current->nsproxy->pid_ns;
+ if (ns->execstack_mode & GNU_STACK_X_FORCE_NX)
+ return EXSTACK_DISABLE_X;
+
+ return EXSTACK_ENABLE_X;
+ } else {
+ /* GNU_STACK => ? */
+ if (ns->execstack_mode & NO_GNU_STACK_FORCE_NX)
+ return EXSTACK_DISABLE_X;
+
+ return EXSTACK_DEFAULT;
+ }
+}
+
+static bool need_emultramp(bool gnu_stack_present, int p_flags)
+{
+ struct pid_namespace *ns;
+ int pf_x;
+
+ if (gnu_stack_present) {
+ pf_x = p_flags & PF_X;
+ /* GNU_STACK => NX */
+ if (!pf_x)
+ return false;
+
+ /* GNU_STACK => X */
+ ns = current->nsproxy->pid_ns;
+ return ns->execstack_mode & GNU_STACK_X_EMULTRAMP;
+ } else {
+ /* GNU_STACK => ? */
+ return ns->execstack_mode & NO_GNU_STACK_EMULTRAMP;
+ }
+
+}
+
static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
{
struct file *interpreter = NULL; /* to shut gcc up */
@@ -571,12 +619,14 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
unsigned long interp_load_addr = 0;
unsigned long start_code, end_code, start_data, end_data;
unsigned long reloc_func_desc __maybe_unused = 0;
- int executable_stack = EXSTACK_DEFAULT;
+ int executable_stack;
unsigned long def_flags = 0;
struct {
struct elfhdr elf_ex;
struct elfhdr interp_elf_ex;
} *loc;
+ bool gnu_stack_present = false;
+ int gnu_stack_flags = 0; /* unused value */
loc = kmalloc(sizeof(*loc), GFP_KERNEL);
if (!loc) {
@@ -689,13 +739,16 @@ static int load_elf_binary(struct linux_binprm *bprm, struct pt_regs *regs)
elf_ppnt = elf_phdata;
for (i = 0; i < loc->elf_ex.e_phnum; i++, elf_ppnt++)
if (elf_ppnt->p_type == PT_GNU_STACK) {
- if (elf_ppnt->p_flags & PF_X)
- executable_stack = EXSTACK_ENABLE_X;
- else
- executable_stack = EXSTACK_DISABLE_X;
- break;
+ gnu_stack_present = true;
+ gnu_stack_flags = elf_ppnt->p_flags;
}
+ executable_stack = get_execstack(gnu_stack_present, gnu_stack_flags);
+ if (need_emultramp(gnu_stack_present, gnu_stack_flags)) {
+ pr_err("need emultramp\n");
+ current->flags |= PF_EMULTRAMP;
+ }
+
/* Some simple consistency checks for the interpreter */
if (elf_interpreter) {
retval = -ELIBBAD;
diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 38d1032..c54c43c 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -16,6 +16,13 @@ struct pidmap {
struct bsd_acct_struct;
+enum execstack_mode {
+ GNU_STACK_X_FORCE_NX = 1,
+ GNU_STACK_X_EMULTRAMP = 2,
+ NO_GNU_STACK_FORCE_NX = 4,
+ NO_GNU_STACK_EMULTRAMP = 8
+};
+
struct pid_namespace {
struct kref kref;
struct pidmap pidmap[PIDMAP_ENTRIES];
@@ -30,6 +37,7 @@ struct pid_namespace {
#ifdef CONFIG_BSD_PROCESS_ACCT
struct bsd_acct_struct *bacct;
#endif
+ enum execstack_mode execstack_mode;
};
extern struct pid_namespace init_pid_ns;
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 496770a..1b4a25e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1765,6 +1765,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
#define PF_FROZEN 0x00010000 /* frozen for system suspend */
#define PF_FSTRANS 0x00020000 /* inside a filesystem transaction */
#define PF_KSWAPD 0x00040000 /* I am kswapd */
+#define PF_EMULTRAMP 0x00080000 /* gcc tramplolines must be emulated */
#define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */
#define PF_KTHREAD 0x00200000 /* I am a kernel thread */
#define PF_RANDOMIZE 0x00400000 /* randomize virtual address space */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index f175d98..de049f9 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -175,6 +175,8 @@ static int proc_taint(struct ctl_table *table, int write,
static int proc_dmesg_restrict(struct ctl_table *table, int write,
void __user *buffer, size_t *lenp, loff_t *ppos);
#endif
+static int proc_execstack_mode(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos);
#ifdef CONFIG_MAGIC_SYSRQ
/* Note: sysrq code uses it's own private copy */
@@ -984,6 +986,12 @@ static struct ctl_table kern_table[] = {
.proc_handler = proc_dointvec,
},
#endif
+ {
+ .procname = "execstack_mode",
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_execstack_mode,
+ },
{ }
};
@@ -2426,6 +2434,15 @@ static int proc_dmesg_restrict(struct ctl_table *table, int write,
}
#endif
+static int proc_execstack_mode(struct ctl_table *table, int write,
+ void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+ struct ctl_table tbl = *table;
+ tbl.data = ¤t->nsproxy->pid_ns->execstack_mode;
+
+ return proc_dointvec_minmax(&tbl, write, buffer, lenp, ppos);
+}
+
struct do_proc_dointvec_minmax_conv_param {
int *min;
int *max;
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