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

From: Leonid Yegoshin
Date: Fri Oct 03 2014 - 23:17:46 EST

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@xxxxxxxxxx>
---
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);
+ } 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)
{

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