Re: [PATCH v2 08/10] x86-64: Emulate vsyscalls

[Borislav Petkov]

On Sun, May 29, 2011 at 11:48:45PM -0400, Andy Lutomirski wrote:
> There's a fair amount of code in the vsyscall page, and who knows
> what will happen if an exploit jumps into the middle of it.  Reduce
> the risk by replacing most of it with short magic incantations that
> are useless if entered in the middle.  This change can be disabled
> by CONFIG_UNSAFE_VSYSCALLS (default y).
> 
> This causes vsyscalls to be a little more expensive than real
> syscalls.  Fortunately sensible programs don't use them.
> 
> Some care is taken to make sure that tools like valgrind and
> ThreadSpotter still work.
> 
> This patch is not perfect: the vread_tsc and vread_hpet functions
> are still at a fixed address.  Fixing that might involve making
> alternative patching work in the vDSO.
> 
> Signed-off-by: Andy Lutomirski <luto@xxxxxxx>
> ---
>  arch/x86/Kconfig                  |   17 +++++
>  arch/x86/kernel/Makefile          |    3 +
>  arch/x86/kernel/vsyscall_64.c     |  121 +++++++++++++++++++++++++++++++++++--
>  arch/x86/kernel/vsyscall_emu_64.S |   40 ++++++++++++
>  4 files changed, 176 insertions(+), 5 deletions(-)
>  create mode 100644 arch/x86/kernel/vsyscall_emu_64.S
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index cc6c53a..186018b 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1650,6 +1650,23 @@ config COMPAT_VDSO
>  
>  	  If unsure, say Y.
>  
> +config UNSAFE_VSYSCALLS
> +	def_bool y
> +	prompt "Unsafe fast legacy vsyscalls"
> +	depends on X86_64
> +	---help---
> +	  Legacy user code expects to be able to issue three syscalls
> +	  by calling fixed addresses in kernel space.  If you say N,
> +	  then the kernel traps and emulates these calls.  If you say
> +	  Y, then there is actual executable code at a fixed address
> +	  to implement these calls efficiently.
> +
> +	  On a system with recent enough glibc (probably 2.14 or
> +	  newer) and no static binaries, you can say N without a
> +	  performance penalty to improve security
> +
> +	  If unsure, say Y.
> +
>  config CMDLINE_BOOL
>  	bool "Built-in kernel command line"
>  	---help---
> diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> index a24521b..b901781 100644
> --- a/arch/x86/kernel/Makefile
> +++ b/arch/x86/kernel/Makefile
> @@ -42,6 +42,9 @@ obj-$(CONFIG_X86_32)	+= probe_roms_32.o
>  obj-$(CONFIG_X86_32)	+= sys_i386_32.o i386_ksyms_32.o
>  obj-$(CONFIG_X86_64)	+= sys_x86_64.o x8664_ksyms_64.o
>  obj-$(CONFIG_X86_64)	+= syscall_64.o vsyscall_64.o vread_tsc_64.o
> +ifndef CONFIG_UNSAFE_VSYSCALLS
> +	obj-$(CONFIG_X86_64)	+= vsyscall_emu_64.o
> +endif
>  obj-y			+= bootflag.o e820.o
>  obj-y			+= pci-dma.o quirks.o topology.o kdebugfs.o
>  obj-y			+= alternative.o i8253.o pci-nommu.o hw_breakpoint.o
> diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
> index 71fa506..5b3d62a 100644
> --- a/arch/x86/kernel/vsyscall_64.c
> +++ b/arch/x86/kernel/vsyscall_64.c
> @@ -48,9 +48,6 @@
>  #include <asm/vgtod.h>
>  #include <asm/traps.h>
>  
> -#define __vsyscall(nr) \
> -		__attribute__ ((unused, __section__(".vsyscall_" #nr))) notrace
> -
>  DEFINE_VVAR(int, vgetcpu_mode);
>  DEFINE_VVAR(struct vsyscall_gtod_data, vsyscall_gtod_data) =
>  {
> @@ -96,6 +93,7 @@ static void warn_bad_vsyscall(struct pt_regs *regs, bool is_warning,
>  		return;
>  
>  	tsk = current;
> +
>  	printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx",
>  	       is_warning ? KERN_WARNING : KERN_INFO,
>  	       tsk->comm, task_pid_nr(tsk),
> @@ -106,6 +104,12 @@ static void warn_bad_vsyscall(struct pt_regs *regs, bool is_warning,
>  	printk("\n");
>  }
>  
> +#ifdef CONFIG_UNSAFE_VSYSCALLS
> +
> +#define __vsyscall(nr)							\
> +	__attribute__ ((unused, __section__(".vsyscall_" #nr))) notrace
> +
> +
>  /* RED-PEN may want to readd seq locking, but then the variable should be
>   * write-once.
>   */
> @@ -117,8 +121,11 @@ static __always_inline void do_get_tz(struct timezone * tz)
>  static __always_inline int fallback_gettimeofday(struct timeval *tv)
>  {
>  	int ret;
> -	/* Invoke do_emulate_vsyscall. */
> -	asm volatile("movb $0xce, %%al;\n\t"
> +	/*
> +	 * Invoke do_emulate_vsyscall.  Intentionally incompatible with
> +	 * the CONFIG_UNSAFE_VSYSCALLS=n case.
> +	 */
> +	asm volatile("mov $0xce, %%al;\n\t"
>  		     "int %[vec]"
>  		     : "=a" (ret)
>  		     : "D" (tv), [vec] "i" (VSYSCALL_EMU_VECTOR));
> @@ -237,6 +244,8 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
>  	long ret;
>  
>  	/* Kernel code must never get here. */
> +	if (!user_mode(regs))
> +		early_printk("oh crap!\n");
>  	BUG_ON(!user_mode(regs));
>  
>  	local_irq_enable();
> @@ -278,6 +287,106 @@ out:
>  	local_irq_disable();
>  }
>  
> +#else /* CONFIG_UNSAFE_VSYSCALLS=n below */
> +
> +static inline unsigned long vsyscall_intcc_addr(int vsyscall_nr)
> +{
> +	return VSYSCALL_START + 1024*vsyscall_nr + 2;
> +}
> +
> +void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
> +{
> +	u8 vsyscall_nr, al;
> +	long ret;
> +
> +	/* Kernel code must never get here. */
> +	BUG_ON(!user_mode(regs));
> +
> +	local_irq_enable();
> +
> +	/*
> +	 * Decode the vsyscall number.
> +	 * 0xcc -> 0, 0xce -> 1, 0xf0 -> 2; see vsyscall_emu_64.S for why.
> +	 */
> +	al = regs->ax & 0xff;
> +	vsyscall_nr = (al - 0xcc) >> 1;

Ok, but

	(0xf0 - 0xcc) >> 1 == 0x12

Don't you mean 0xd0 here? Although 0xd0 is opcode for all those
rotate/shift insns. What am I missing?

> +	if (vsyscall_nr > 2 || al != (vsyscall_nr << 1) + 0xcc) {
> +		warn_bad_vsyscall(regs, false, "illegal int 0xcc "
> +				  "(exploit attempt?)");
> +		force_sig(SIGSEGV, current);
> +		goto out;
> +	}
> +
> +	if (regs->ip - 2 != vsyscall_intcc_addr(vsyscall_nr)) {
> +		if (in_vsyscall_page(regs->ip - 2)) {
> +			/* This should not be possible. */
> +			warn_bad_vsyscall(regs, true, "int 0xcc severe badness"
> +					  " (exploit attempt?)");
> +			force_sig(SIGSEGV, current);
> +			goto out;
> +		} else {
> +			/*
> +			 * We allow the call because tools like ThreadSpotter
> +			 * might copy the int 0xcc instruction to user memory.
> +			 * We make it annoying, though, to try to persuade
> +			 * the authors to stop doing that...
> +			 */
> +			warn_bad_vsyscall(regs, true, "int 0xcc in user code "
> +					  "(exploit attempt? legacy "
> +					  "instrumented code?)");
> +		}
> +	}
> +
> +	if (current->seccomp.mode) {
> +		do_exit(SIGKILL);
> +		goto out;
> +	}
> +
> +	switch(vsyscall_nr)
> +	{
> +	case 0:
> +		ret = sys_gettimeofday(
> +			(struct timeval __user *)regs->di,
> +			(struct timezone __user *)regs->si);
> +		break;
> +
> +	case 1:
> +		ret = sys_time((time_t __user *)regs->di);
> +		break;
> +
> +	case 2:
> +		ret = sys_getcpu((unsigned __user *)regs->di,
> +				 (unsigned __user *)regs->si,
> +				 0);
> +		break;
> +
> +	default:
> +		unreachable();
> +	}
> +
> +	if (ret == -EFAULT) {
> +		/*
> +		 * Bad news -- userspace fed a bad pointer to a vsyscall.
> +		 *
> +		 * With a real vsyscall, that would have caused SIGSEGV.
> +		 * To make writing reliable exploits using the emulated
> +		 * vsyscalls harder, generate SIGSEGV here as well.
> +		 */
> +		warn_bad_vsyscall(regs, false,
> +				  "vsyscall fault (exploit attempt?)");
> +		force_sig(SIGSEGV, current);
> +		goto out;
> +	}
> +
> +	regs->ax = ret;
> +
> +out:
> +	local_irq_disable();
> +	return;
> +}
> +
> +#endif /* CONFIG_UNSAFE_VSYSCALLS */
> +
>  /* Assume __initcall executes before all user space. Hopefully kmod
>     doesn't violate that. We'll find out if it does. */
>  static void __cpuinit vsyscall_set_cpu(int cpu)
> @@ -331,11 +440,13 @@ void __init map_vsyscall(void)
>  
>  static int __init vsyscall_init(void)
>  {
> +#ifdef CONFIG_UNSAFE_VSYSCALLS
>  	BUG_ON(((unsigned long) &vgettimeofday !=
>  			VSYSCALL_ADDR(__NR_vgettimeofday)));
>  	BUG_ON((unsigned long) &vtime != VSYSCALL_ADDR(__NR_vtime));
>  	BUG_ON((VSYSCALL_ADDR(0) != __fix_to_virt(VSYSCALL_FIRST_PAGE)));
>  	BUG_ON((unsigned long) &vgetcpu != VSYSCALL_ADDR(__NR_vgetcpu));
> +#endif
>  	on_each_cpu(cpu_vsyscall_init, NULL, 1);
>  	/* notifier priority > KVM */
>  	hotcpu_notifier(cpu_vsyscall_notifier, 30);
> diff --git a/arch/x86/kernel/vsyscall_emu_64.S b/arch/x86/kernel/vsyscall_emu_64.S
> new file mode 100644
> index 0000000..3e1cad2
> --- /dev/null
> +++ b/arch/x86/kernel/vsyscall_emu_64.S
> @@ -0,0 +1,40 @@
> +/*
> + * vsyscall_emu_64.S: Vsyscall emulation page
> + * Copyright (c) 2011 Andy Lutomirski
> + * Subject to the GNU General Public License, version 2
> +*/
> +
> +#include <linux/linkage.h>
> +#include <asm/irq_vectors.h>
> +
> +/*
> + * These magic incantations are chosen so that they fault if entered anywhere
> + * other than an instruction boundary.  The movb instruction is two bytes, and
> + * the int imm8 instruction is also two bytes, so the only misaligned places
> + * to enter are the immediate values for the two instructions.  0xcc is int3
> + * (always faults), 0xce is into (faults on x64-64, and 32-bit code can't get
> + * here), and 0xf0 is lock (lock int is invalid).
> + *
> + * The unused parts of the page are filled with 0xcc by the linker script.
> + */
> +
> +.section .vsyscall_0, "a"
> +ENTRY(vsyscall_0)
> +	movb $0xcc, %al
> +	int $VSYSCALL_EMU_VECTOR
> +	ret
> +END(vsyscall_0)
> +
> +.section .vsyscall_1, "a"
> +ENTRY(vsyscall_1)
> +	movb $0xce, %al
> +	int $VSYSCALL_EMU_VECTOR
> +	ret
> +END(vsyscall_1)
> +
> +.section .vsyscall_2, "a"
> +ENTRY(vsyscall_2)
> +	movb $0xf0, %al
> +	int $VSYSCALL_EMU_VECTOR
> +	ret
> +END(vsyscall_2)
> -- 
> 1.7.5.1
> 
> 

-- 
Regards/Gruss,
    Boris.
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