Re: [PATCH 2/3] MIPS: Setup an instruction emulation in VDSO protected page instead of user stack

[Paul Burton <paul.burton@imgtec.com>]

On Fri, Oct 03, 2014 at 08:17:30PM -0700, Leonid Yegoshin wrote:
> Historically, during FPU emulation MIPS runs live BD-slot instruction in stack.
> This is needed because it was the only way to correctly handle branch
> exceptions with unknown COP2 instructions in BD-slot. Now there is
> an eXecuteInhibit feature and it is desirable to protect stack from execution
> for security reasons.
> This patch moves FPU emulation from stack area to VDSO-located page which is set
> write-protected for application access. VDSO page itself is now per-thread and
> it's addresses and offsets are stored in thread_info.
> Small stack of emulation blocks is supported because nested traps are possible
> in MIPS32/64 R6 emulation mix with FPU emulation.
> 
> Explanation of problem (current state before patch):
> 
> If I set eXecute-disabled stack in ELF binary initialisation then GLIBC ignores
> it and may change stack protection at library load. If this library has
> eXecute-disabled stack then anything is OK, but if this section (PT_GNU_STACK)
> is just missed as usual, then GLIBC applies it's own default == eXecute-enabled
> stack.
> So, ssh_keygen is built explicitly with eXecute-disabled stack. But GLIBC
> ignores it and set stack executable. And because of that - anything works,
> FPU emulation and hacker tools.
> However, if I use all *.SO libraries with eXecute-disabled stack in PT_GNU_STACK
> section then GLIBC keeps stack non-executable but things fails at FPU emulation
> later.
> 
> Here are two issues which are bind together and to solve an incorrect
> behaviour of GLIBC (ignoring X ssh-keygen intention) the splitting both issues
> is needed. So, I did a kernel emulation protected and out of stack.
> 
> Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>

Hi Leonid,

First some general questions: is there any reason to need the page used
to be at the same virtual address for each thread? I can't think of one,
and if that's the case then why not simply allocate a series of pages
per-thread via mmap_region or similar, on demand when a thread first
executes an FP branch instruction? That would of course consume some
more virtual address space, but would avoid the hassles of manually
prodding at the TLB, tracking ASIDs & flushing the caches. Perhaps the
shrinker interface could be used to allow freeing those pages if & when
it becomes necessary for long running threads.

Also VDSO is really a misnomer throughout, as I've pointed out to you
before. I'm aware it's an existing misnomer, but it would be nice if
we could clear that up rather than repeat it...

> ---
>  arch/mips/include/asm/mmu.h         |    2 +
>  arch/mips/include/asm/processor.h   |    2 -
>  arch/mips/include/asm/switch_to.h   |   12 ++++
>  arch/mips/include/asm/thread_info.h |    3 +
>  arch/mips/include/asm/tlbmisc.h     |    1 
>  arch/mips/include/asm/vdso.h        |    2 +
>  arch/mips/kernel/process.c          |    7 ++
>  arch/mips/kernel/vdso.c             |   41 ++++++++++++-
>  arch/mips/math-emu/dsemul.c         |  114 ++++++++++++++++++++++++++---------
>  arch/mips/mm/fault.c                |    5 ++
>  arch/mips/mm/tlb-r4k.c              |   39 ++++++++++++
>  11 files changed, 197 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/mips/include/asm/mmu.h b/arch/mips/include/asm/mmu.h
> index c436138..96266b8 100644
> --- a/arch/mips/include/asm/mmu.h
> +++ b/arch/mips/include/asm/mmu.h
> @@ -3,7 +3,9 @@
>  
>  typedef struct {
>  	unsigned long asid[NR_CPUS];
> +	unsigned long vdso_asid[NR_CPUS];
>  	void *vdso;
> +	struct vm_area_struct   *vdso_vma;
>  } mm_context_t;
>  
>  #endif /* __ASM_MMU_H */
> diff --git a/arch/mips/include/asm/processor.h b/arch/mips/include/asm/processor.h
> index 05f0843..c87b1da 100644
> --- a/arch/mips/include/asm/processor.h
> +++ b/arch/mips/include/asm/processor.h
> @@ -40,7 +40,7 @@ extern unsigned int vced_count, vcei_count;
>   * A special page (the vdso) is mapped into all processes at the very
>   * top of the virtual memory space.
>   */
> -#define SPECIAL_PAGES_SIZE PAGE_SIZE
> +#define SPECIAL_PAGES_SIZE (PAGE_SIZE * 2)
>  
>  #ifdef CONFIG_32BIT
>  #ifdef CONFIG_KVM_GUEST
> diff --git a/arch/mips/include/asm/switch_to.h b/arch/mips/include/asm/switch_to.h
> index b928b6f..0968f51 100644
> --- a/arch/mips/include/asm/switch_to.h
> +++ b/arch/mips/include/asm/switch_to.h
> @@ -17,6 +17,7 @@
>  #include <asm/dsp.h>
>  #include <asm/cop2.h>
>  #include <asm/msa.h>
> +#include <asm/mmu_context.h>
>  
>  struct task_struct;
>  
> @@ -104,6 +105,16 @@ do {									\
>  	disable_msa();							\
>  } while (0)
>  
> +static inline void flush_vdso_page(void)
> +{
> +	if (current->mm && cpu_context(raw_smp_processor_id(), current->mm) &&
> +	    (current->mm->context.vdso_asid[raw_smp_processor_id()] ==
> +	     cpu_asid(raw_smp_processor_id(), current->mm))) {
> +		local_flush_tlb_page(current->mm->mmap, (unsigned long)current->mm->context.vdso);
> +		current->mm->context.vdso_asid[raw_smp_processor_id()] = 0;
> +	}
> +}
> +
>  #define finish_arch_switch(prev)					\
>  do {									\
>  	u32 __c0_stat;							\
> @@ -118,6 +129,7 @@ do {									\
>  		__restore_dsp(current);					\
>  	if (cpu_has_userlocal)						\
>  		write_c0_userlocal(current_thread_info()->tp_value);	\
> +	flush_vdso_page();                                              \
>  	__restore_watch();						\
>  } while (0)
>  
> diff --git a/arch/mips/include/asm/thread_info.h b/arch/mips/include/asm/thread_info.h
> index 7de8658..8d16003 100644
> --- a/arch/mips/include/asm/thread_info.h
> +++ b/arch/mips/include/asm/thread_info.h
> @@ -34,6 +34,8 @@ struct thread_info {
>  						 * 0x7fffffff for user-thead
>  						 * 0xffffffff for kernel-thread
>  						 */
> +	unsigned long           vdso_offset;
> +	struct page             *vdso_page;
>  	struct restart_block	restart_block;
>  	struct pt_regs		*regs;
>  };
> @@ -49,6 +51,7 @@ struct thread_info {
>  	.cpu		= 0,			\
>  	.preempt_count	= INIT_PREEMPT_COUNT,	\
>  	.addr_limit	= KERNEL_DS,		\
> +	.vdso_page      = NULL,                 \
>  	.restart_block	= {			\
>  		.fn = do_no_restart_syscall,	\
>  	},					\
> diff --git a/arch/mips/include/asm/tlbmisc.h b/arch/mips/include/asm/tlbmisc.h
> index 3a45228..abd7bf6 100644
> --- a/arch/mips/include/asm/tlbmisc.h
> +++ b/arch/mips/include/asm/tlbmisc.h
> @@ -6,5 +6,6 @@
>   */
>  extern void add_wired_entry(unsigned long entrylo0, unsigned long entrylo1,
>  	unsigned long entryhi, unsigned long pagemask);
> +int install_vdso_tlb(void);
>  
>  #endif /* __ASM_TLBMISC_H */
> diff --git a/arch/mips/include/asm/vdso.h b/arch/mips/include/asm/vdso.h
> index cca56aa..baf3402 100644
> --- a/arch/mips/include/asm/vdso.h
> +++ b/arch/mips/include/asm/vdso.h
> @@ -11,6 +11,8 @@
>  
>  #include <linux/types.h>
>  
> +void mips_thread_vdso(struct thread_info *ti);
> +void arch_release_thread_info(struct thread_info *info);
>  
>  #ifdef CONFIG_32BIT
>  struct mips_vdso {
> diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> index 636b074..762738a 100644
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -42,6 +42,7 @@
>  #include <asm/isadep.h>
>  #include <asm/inst.h>
>  #include <asm/stacktrace.h>
> +#include <asm/vdso.h>
>  
>  #ifdef CONFIG_HOTPLUG_CPU
>  void arch_cpu_idle_dead(void)
> @@ -59,6 +60,8 @@ void start_thread(struct pt_regs * regs, unsigned long pc, unsigned long sp)
>  {
>  	unsigned long status;
>  
> +	mips_thread_vdso(current_thread_info());
> +
>  	/* New thread loses kernel privileges. */
>  	status = regs->cp0_status & ~(ST0_CU0|ST0_CU1|ST0_FR|KU_MASK);
>  	status |= KU_USER;
> @@ -75,6 +78,7 @@ void start_thread(struct pt_regs * regs, unsigned long pc, unsigned long sp)
>  
>  void exit_thread(void)
>  {
> +	arch_release_thread_info(current_thread_info());
>  }
>  
>  void flush_thread(void)
> @@ -103,6 +107,9 @@ int copy_thread(unsigned long clone_flags, unsigned long usp,
>  
>  	preempt_enable();
>  
> +	ti->vdso_page = NULL;
> +	mips_thread_vdso(ti);
> +
>  	/* set up new TSS. */
>  	childregs = (struct pt_regs *) childksp - 1;
>  	/*  Put the stack after the struct pt_regs.  */
> diff --git a/arch/mips/kernel/vdso.c b/arch/mips/kernel/vdso.c
> index 0f1af58..7b31bba 100644
> --- a/arch/mips/kernel/vdso.c
> +++ b/arch/mips/kernel/vdso.c
> @@ -19,6 +19,8 @@
>  
>  #include <asm/vdso.h>
>  #include <asm/uasm.h>
> +#include <asm/cacheflush.h>
> +#include <asm/tlbflush.h>
>  
>  /*
>   * Including <asm/unistd.h> would give use the 64-bit syscall numbers ...
> @@ -88,14 +90,18 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, int uses_interp)
>  
>  	ret = install_special_mapping(mm, addr, PAGE_SIZE,
>  				      VM_READ|VM_EXEC|
> -				      VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC,
> +				      VM_MAYREAD|VM_MAYEXEC,
>  				      &vdso_page);
>  
>  	if (ret)
>  		goto up_fail;
>  
>  	mm->context.vdso = (void *)addr;
> +	/* if cache aliasing - use a different cache flush later */
> +	if (cpu_has_rixi && cpu_has_dc_aliases)
> +		mm->context.vdso_vma = find_vma(mm,addr);
>  
> +	mips_thread_vdso(current_thread_info());
>  up_fail:
>  	up_write(&mm->mmap_sem);
>  	return ret;
> @@ -107,3 +113,36 @@ const char *arch_vma_name(struct vm_area_struct *vma)
>  		return "[vdso]";
>  	return NULL;
>  }
> +
> +void mips_thread_vdso(struct thread_info *ti)
> +{
> +	struct page *vdso;
> +	unsigned long addr;
> +
> +	if (cpu_has_rixi && ti->task->mm && !ti->vdso_page) {
> +		vdso = alloc_page(GFP_USER);
> +		if (!vdso)
> +			return;
> +		ti->vdso_page = vdso;
> +		ti->vdso_offset = PAGE_SIZE;
> +		addr = (unsigned long)page_address(vdso);
> +		copy_page((void *)addr, (void *)page_address(vdso_page));
> +		if (!cpu_has_ic_fills_f_dc)
> +			flush_data_cache_page(addr);
> +		/* any vma in mmap is used, just to get ASIDs back from mm */
> +		local_flush_tlb_page(ti->task->mm->mmap,addr);
> +	}
> +}
> +
> +void arch_release_thread_info(struct thread_info *info)
> +{
> +	if (info->vdso_page) {
> +		if (info->task->mm) {
> +			/* any vma in mmap is used, just to get ASIDs */
> +			local_flush_tlb_page(info->task->mm->mmap,(unsigned long)page_address(info->vdso_page));
> +			info->task->mm->context.vdso_asid[smp_processor_id()] = 0;
> +		}
> +		__free_page(info->vdso_page);
> +		info->vdso_page = NULL;
> +	}
> +}
> diff --git a/arch/mips/math-emu/dsemul.c b/arch/mips/math-emu/dsemul.c
> index 4f514f3..274f9b7 100644
> --- a/arch/mips/math-emu/dsemul.c
> +++ b/arch/mips/math-emu/dsemul.c
> @@ -3,6 +3,7 @@
>  #include <asm/fpu_emulator.h>
>  #include <asm/inst.h>
>  #include <asm/mipsregs.h>
> +#include <asm/vdso.h>
>  #include <asm/uaccess.h>
>  
>  #include "ieee754.h"
> @@ -30,12 +31,15 @@ struct emuframe {
>  	mips_instruction	cookie;
>  	unsigned long		epc;
>  };
> +/* round structure size to N*8 to force a fit two instructions in a single cache line */
> +#define EMULFRAME_ROUNDED_SIZE  ((sizeof(struct emuframe) + 0x7) & ~0x7)
>  
>  int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
>  {
>  	extern asmlinkage void handle_dsemulret(void);
>  	struct emuframe __user *fr;
>  	int err;
> +	unsigned char *pg_addr;
>  
>  	if ((get_isa16_mode(regs->cp0_epc) && ((ir >> 16) == MM_NOP16)) ||
>  		(ir == 0)) {
> @@ -48,7 +52,7 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
>  	pr_debug("dsemul %lx %lx\n", regs->cp0_epc, cpc);
>  
>  	/*
> -	 * The strategy is to push the instruction onto the user stack
> +	 * The strategy is to push the instruction onto the user stack/VDSO page
>  	 * and put a trap after it which we can catch and jump to
>  	 * the required address any alternative apart from full
>  	 * instruction emulation!!.
> @@ -65,37 +69,78 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
>  	 * handler (single entry point).
>  	 */
>  
> -	/* Ensure that the two instructions are in the same cache line */
> -	fr = (struct emuframe __user *)
> -		((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);
> -
> -	/* Verify that the stack pointer is not competely insane */
> -	if (unlikely(!access_ok(VERIFY_WRITE, fr, sizeof(struct emuframe))))
> -		return SIGBUS;
> -
> -	if (get_isa16_mode(regs->cp0_epc)) {
> -		err = __put_user(ir >> 16, (u16 __user *)(&fr->emul));
> -		err |= __put_user(ir & 0xffff, (u16 __user *)((long)(&fr->emul) + 2));
> -		err |= __put_user(BREAK_MATH >> 16, (u16 __user *)(&fr->badinst));
> -		err |= __put_user(BREAK_MATH & 0xffff, (u16 __user *)((long)(&fr->badinst) + 2));
> +	if (current_thread_info()->vdso_page) {
> +		/*
> +		 * Use VDSO page and fill structure via kernel VA,
> +		 * user write is disabled
> +		 */
> +		pg_addr = (unsigned char *)page_address(current_thread_info()->vdso_page);
> +		fr = (struct emuframe __user *)
> +			    (pg_addr + current_thread_info()->vdso_offset -
> +			     EMULFRAME_ROUNDED_SIZE);
> +
> +		/* verify that we don't overflow into trampoline areas */
> +		if ((unsigned char *)fr < (unsigned char *)(((struct mips_vdso *)pg_addr) + 1)) {
> +			MIPS_FPU_EMU_INC_STATS(errors);
> +			return SIGBUS;
> +		}
> +
> +		current_thread_info()->vdso_offset -= EMULFRAME_ROUNDED_SIZE;
> +
> +		if (get_isa16_mode(regs->cp0_epc)) {
> +			*(u16 *)&fr->emul = (u16)(ir >> 16);
> +			*((u16 *)(&fr->emul) + 1) = (u16)(ir & 0xffff);

This microMIPS case doesn't set badinst, as I pointed out internally. I
think it would be much cleaner if you were to do something along the
lines of:

  struct emuframe __user _fr, *fr, *user_fr;

  if (current_thread_info()->vdso_page) {
    fr = user_fr = (struct emuframe __user *)
         (pg_addr + current_thread_info()->vdso_offset -
         EMULFRAME_ROUNDED_SIZE);
  } else {
    fr = &_fr;
    user_fr = (struct emuframe __user *)
              ((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);
  }

  /* initialize frame */
  fr->inst = ...;
  fr->badinst = ...;

  if (fr != user_fr)
    copy_to_user(user_fr, fr, sizeof(*fr));

That way you don't duplicate the initialization of the struct emuframe
and leave it less likely you'll miss cases like above.

Or the on-stack case could be removed entirely.

Paul

> +		} else {
> +			fr->emul = ir;
> +			fr->badinst = (mips_instruction)BREAK_MATH;
> +		}
> +		fr->cookie = (mips_instruction)BD_COOKIE;
> +		fr->epc = cpc;
> +
> +		/* fill CP0_EPC with user VA */
> +		regs->cp0_epc = ((unsigned long)(current->mm->context.vdso +
> +				current_thread_info()->vdso_offset)) |
> +				get_isa16_mode(regs->cp0_epc);
> +		if (cpu_has_dc_aliases)
> +			mips_flush_data_cache_range(current->mm->context.vdso_vma,
> +				regs->cp0_epc, current_thread_info()->vdso_page,
> +				(unsigned long)fr, sizeof(struct emuframe));
> +		else
> +			/* it is a less expensive on CPU with correct SYNCI */
> +			flush_cache_sigtramp((unsigned long)fr);
>  	} else {
> -		err = __put_user(ir, &fr->emul);
> -		err |= __put_user((mips_instruction)BREAK_MATH, &fr->badinst);
> +		/* Ensure that the two instructions are in the same cache line */
> +		fr = (struct emuframe __user *)
> +			((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);
> +
> +		/* Verify that the stack pointer is not competely insane */
> +		if (unlikely(!access_ok(VERIFY_WRITE, fr, sizeof(struct emuframe))))
> +			return SIGBUS;
> +
> +		if (get_isa16_mode(regs->cp0_epc)) {
> +			err = __put_user(ir >> 16, (u16 __user *)(&fr->emul));
> +			err |= __put_user(ir & 0xffff, (u16 __user *)((long)(&fr->emul) + 2));
> +			err |= __put_user(BREAK_MATH >> 16, (u16 __user *)(&fr->badinst));
> +			err |= __put_user(BREAK_MATH & 0xffff, (u16 __user *)((long)(&fr->badinst) + 2));
> +		} else {
> +			err = __put_user(ir, &fr->emul);
> +			err |= __put_user((mips_instruction)BREAK_MATH, &fr->badinst);
> +		}
> +
> +		err |= __put_user((mips_instruction)BD_COOKIE, &fr->cookie);
> +		err |= __put_user(cpc, &fr->epc);
> +
> +		if (unlikely(err)) {
> +			MIPS_FPU_EMU_INC_STATS(errors);
> +			return SIGBUS;
> +		}
> +
> +		regs->cp0_epc = ((unsigned long) &fr->emul) |
> +			get_isa16_mode(regs->cp0_epc);
> +
> +		flush_cache_sigtramp((unsigned long)&fr->badinst);
>  	}
>  
> -	err |= __put_user((mips_instruction)BD_COOKIE, &fr->cookie);
> -	err |= __put_user(cpc, &fr->epc);
> -
> -	if (unlikely(err)) {
> -		MIPS_FPU_EMU_INC_STATS(errors);
> -		return SIGBUS;
> -	}
> -
> -	regs->cp0_epc = ((unsigned long) &fr->emul) |
> -		get_isa16_mode(regs->cp0_epc);
> -
> -	flush_cache_sigtramp((unsigned long)&fr->badinst);
> -
>  	return SIGILL;		/* force out of emulation loop */
>  }
>  
> @@ -156,6 +201,17 @@ int do_dsemulret(struct pt_regs *xcp)
>  		return 0;
>  	}
>  
> +	if (current_thread_info()->vdso_page) {
> +		/* restore VDSO stack level */
> +		current_thread_info()->vdso_offset += EMULFRAME_ROUNDED_SIZE;
> +		if (current_thread_info()->vdso_offset > PAGE_SIZE) {
> +			/* This is not a good situation to be in */
> +			current_thread_info()->vdso_offset -= EMULFRAME_ROUNDED_SIZE;
> +			force_sig(SIGBUS, current);
> +
> +			return 0;
> +		}
> +	}
>  	/* Set EPC to return to post-branch instruction */
>  	xcp->cp0_epc = epc;
>  
> diff --git a/arch/mips/mm/fault.c b/arch/mips/mm/fault.c
> index becc42b..516045a 100644
> --- a/arch/mips/mm/fault.c
> +++ b/arch/mips/mm/fault.c
> @@ -26,6 +26,7 @@
>  #include <asm/uaccess.h>
>  #include <asm/ptrace.h>
>  #include <asm/highmem.h>		/* For VMALLOC_END */
> +#include <asm/tlbmisc.h>
>  #include <linux/kdebug.h>
>  
>  /*
> @@ -138,6 +139,9 @@ good_area:
>  #endif
>  				goto bad_area;
>  			}
> +			if (((address & PAGE_MASK) == (unsigned long)(mm->context.vdso)) &&
> +			    install_vdso_tlb())
> +				goto up_return;
>  		} else {
>  			if (!(vma->vm_flags & (VM_READ | VM_WRITE | VM_EXEC)))
>  				goto bad_area;
> @@ -186,6 +190,7 @@ good_area:
>  		}
>  	}
>  
> +up_return:
>  	up_read(&mm->mmap_sem);
>  	return;
>  
> diff --git a/arch/mips/mm/tlb-r4k.c b/arch/mips/mm/tlb-r4k.c
> index fa6ebd4..a857e21 100644
> --- a/arch/mips/mm/tlb-r4k.c
> +++ b/arch/mips/mm/tlb-r4k.c
> @@ -350,6 +350,45 @@ void __update_tlb(struct vm_area_struct * vma, unsigned long address, pte_t pte)
>  	local_irq_restore(flags);
>  }
>  
> +int install_vdso_tlb(void)
> +{
> +	int tlbidx;
> +	unsigned long flags;
> +
> +	if (!current_thread_info()->vdso_page)
> +		return(0);
> +
> +	local_irq_save(flags);
> +	write_c0_entryhi(((unsigned long)current->mm->context.vdso & (PAGE_MASK << 1)) |
> +			 cpu_asid(smp_processor_id(), current->mm));
> +
> +	mtc0_tlbw_hazard();
> +	tlb_probe();
> +	tlb_probe_hazard();
> +	tlbidx = read_c0_index();
> +#if defined(CONFIG_64BIT_PHYS_ADDR) && defined(CONFIG_CPU_MIPS32)
> +		write_c0_entrylo0(pte_val(pfn_pte(
> +			page_to_pfn(current_thread_info()->vdso_page),
> +			__pgprot(_page_cachable_default|_PAGE_VALID)))>>32);
> +#else
> +		write_c0_entrylo0(pte_to_entrylo(pte_val(pfn_pte(
> +			page_to_pfn(current_thread_info()->vdso_page),
> +			__pgprot(_page_cachable_default|_PAGE_VALID)))));
> +#endif
> +	write_c0_entrylo1(0);
> +	mtc0_tlbw_hazard();
> +	if (tlbidx < 0)
> +		tlb_write_random();
> +	else
> +		tlb_write_indexed();
> +	tlbw_use_hazard();
> +
> +	current->mm->context.vdso_asid[smp_processor_id()] = cpu_asid(smp_processor_id(), current->mm);
> +	local_irq_restore(flags);
> +
> +	return(1);
> +}
> +
>  void add_wired_entry(unsigned long entrylo0, unsigned long entrylo1,
>  		     unsigned long entryhi, unsigned long pagemask)
>  {
>