X-Git-Url: https://err.no/cgi-bin/gitweb.cgi?a=blobdiff_plain;f=arch%2Fx86%2Fmm%2Ffault_32.c;h=f4f8c324715f5abdf835fdf80c444f024fdedaf8;hb=3c1df68b848b39270752ff8d4b956cc4a4dce0f6;hp=c686ae20fd6bbd802b18048ba9e7914c50053ac4;hpb=dcca2bde4f86a14d3291660bede8f1844fe2b3df;p=linux-2.6 diff --git a/arch/x86/mm/fault_32.c b/arch/x86/mm/fault_32.c index c686ae20fd..f4f8c32471 100644 --- a/arch/x86/mm/fault_32.c +++ b/arch/x86/mm/fault_32.c @@ -1,6 +1,4 @@ /* - * linux/arch/i386/mm/fault.c - * * Copyright (C) 1995 Linus Torvalds */ @@ -25,17 +23,28 @@ #include #include #include -#include #include #include #include -extern void die(const char *,struct pt_regs *,long); +/* + * Page fault error code bits + * bit 0 == 0 means no page found, 1 means protection fault + * bit 1 == 0 means read, 1 means write + * bit 2 == 0 means kernel, 1 means user-mode + * bit 3 == 1 means use of reserved bit detected + * bit 4 == 1 means fault was an instruction fetch + */ +#define PF_PROT (1<<0) +#define PF_WRITE (1<<1) +#define PF_USER (1<<2) +#define PF_RSVD (1<<3) +#define PF_INSTR (1<<4) -#ifdef CONFIG_KPROBES static inline int notify_page_fault(struct pt_regs *regs) { +#ifdef CONFIG_KPROBES int ret = 0; /* kprobe_running() needs smp_processor_id() */ @@ -47,162 +56,112 @@ static inline int notify_page_fault(struct pt_regs *regs) } return ret; -} #else -static inline int notify_page_fault(struct pt_regs *regs) -{ return 0; -} #endif +} /* - * Return EIP plus the CS segment base. The segment limit is also - * adjusted, clamped to the kernel/user address space (whichever is - * appropriate), and returned in *eip_limit. + * X86_32 + * Sometimes AMD Athlon/Opteron CPUs report invalid exceptions on prefetch. + * Check that here and ignore it. * - * The segment is checked, because it might have been changed by another - * task between the original faulting instruction and here. + * X86_64 + * Sometimes the CPU reports invalid exceptions on prefetch. + * Check that here and ignore it. * - * If CS is no longer a valid code segment, or if EIP is beyond the - * limit, or if it is a kernel address when CS is not a kernel segment, - * then the returned value will be greater than *eip_limit. - * - * This is slow, but is very rarely executed. + * Opcode checker based on code by Richard Brunner */ -static inline unsigned long get_segment_eip(struct pt_regs *regs, - unsigned long *eip_limit) +static int is_prefetch(struct pt_regs *regs, unsigned long addr, + unsigned long error_code) { - unsigned long eip = regs->eip; - unsigned seg = regs->xcs & 0xffff; - u32 seg_ar, seg_limit, base, *desc; - - /* Unlikely, but must come before segment checks. */ - if (unlikely(regs->eflags & VM_MASK)) { - base = seg << 4; - *eip_limit = base + 0xffff; - return base + (eip & 0xffff); - } - - /* The standard kernel/user address space limit. */ - *eip_limit = user_mode(regs) ? USER_DS.seg : KERNEL_DS.seg; - - /* By far the most common cases. */ - if (likely(SEGMENT_IS_FLAT_CODE(seg))) - return eip; - - /* Check the segment exists, is within the current LDT/GDT size, - that kernel/user (ring 0..3) has the appropriate privilege, - that it's a code segment, and get the limit. */ - __asm__ ("larl %3,%0; lsll %3,%1" - : "=&r" (seg_ar), "=r" (seg_limit) : "0" (0), "rm" (seg)); - if ((~seg_ar & 0x9800) || eip > seg_limit) { - *eip_limit = 0; - return 1; /* So that returned eip > *eip_limit. */ - } + unsigned char *instr; + int scan_more = 1; + int prefetch = 0; + unsigned char *max_instr; - /* Get the GDT/LDT descriptor base. - When you look for races in this code remember that - LDT and other horrors are only used in user space. */ - if (seg & (1<<2)) { - /* Must lock the LDT while reading it. */ - down(¤t->mm->context.sem); - desc = current->mm->context.ldt; - desc = (void *)desc + (seg & ~7); +#ifdef CONFIG_X86_32 + if (unlikely(boot_cpu_data.x86_vendor == X86_VENDOR_AMD && + boot_cpu_data.x86 >= 6)) { + /* Catch an obscure case of prefetch inside an NX page. */ + if (nx_enabled && (error_code & PF_INSTR)) + return 0; } else { - /* Must disable preemption while reading the GDT. */ - desc = (u32 *)get_cpu_gdt_table(get_cpu()); - desc = (void *)desc + (seg & ~7); + return 0; } +#else + /* If it was a exec fault ignore */ + if (error_code & PF_INSTR) + return 0; +#endif - /* Decode the code segment base from the descriptor */ - base = get_desc_base((unsigned long *)desc); - - if (seg & (1<<2)) { - up(¤t->mm->context.sem); - } else - put_cpu(); - - /* Adjust EIP and segment limit, and clamp at the kernel limit. - It's legitimate for segments to wrap at 0xffffffff. */ - seg_limit += base; - if (seg_limit < *eip_limit && seg_limit >= base) - *eip_limit = seg_limit; - return eip + base; -} + instr = (unsigned char *)convert_ip_to_linear(current, regs); + max_instr = instr + 15; -/* - * Sometimes AMD Athlon/Opteron CPUs report invalid exceptions on prefetch. - * Check that here and ignore it. - */ -static int __is_prefetch(struct pt_regs *regs, unsigned long addr) -{ - unsigned long limit; - unsigned char *instr = (unsigned char *)get_segment_eip (regs, &limit); - int scan_more = 1; - int prefetch = 0; - int i; + if (user_mode(regs) && instr >= (unsigned char *)TASK_SIZE) + return 0; - for (i = 0; scan_more && i < 15; i++) { + while (scan_more && instr < max_instr) { unsigned char opcode; unsigned char instr_hi; unsigned char instr_lo; - if (instr > (unsigned char *)limit) - break; if (probe_kernel_address(instr, opcode)) - break; + break; - instr_hi = opcode & 0xf0; - instr_lo = opcode & 0x0f; + instr_hi = opcode & 0xf0; + instr_lo = opcode & 0x0f; instr++; - switch (instr_hi) { + switch (instr_hi) { case 0x20: case 0x30: - /* Values 0x26,0x2E,0x36,0x3E are valid x86 prefixes. */ + /* + * Values 0x26,0x2E,0x36,0x3E are valid x86 prefixes. + * In X86_64 long mode, the CPU will signal invalid + * opcode if some of these prefixes are present so + * X86_64 will never get here anyway + */ scan_more = ((instr_lo & 7) == 0x6); break; - +#ifdef CONFIG_X86_64 + case 0x40: + /* + * In AMD64 long mode 0x40..0x4F are valid REX prefixes + * Need to figure out under what instruction mode the + * instruction was issued. Could check the LDT for lm, + * but for now it's good enough to assume that long + * mode only uses well known segments or kernel. + */ + scan_more = (!user_mode(regs)) || (regs->cs == __USER_CS); + break; +#endif case 0x60: /* 0x64 thru 0x67 are valid prefixes in all modes. */ scan_more = (instr_lo & 0xC) == 0x4; - break; + break; case 0xF0: - /* 0xF0, 0xF2, and 0xF3 are valid prefixes */ + /* 0xF0, 0xF2, 0xF3 are valid prefixes in all modes. */ scan_more = !instr_lo || (instr_lo>>1) == 1; - break; + break; case 0x00: /* Prefetch instruction is 0x0F0D or 0x0F18 */ scan_more = 0; - if (instr > (unsigned char *)limit) - break; + if (probe_kernel_address(instr, opcode)) break; prefetch = (instr_lo == 0xF) && (opcode == 0x0D || opcode == 0x18); - break; + break; default: scan_more = 0; break; - } + } } return prefetch; } -static inline int is_prefetch(struct pt_regs *regs, unsigned long addr, - unsigned long error_code) -{ - if (unlikely(boot_cpu_data.x86_vendor == X86_VENDOR_AMD && - boot_cpu_data.x86 >= 6)) { - /* Catch an obscure case of prefetch inside an NX page. */ - if (nx_enabled && (error_code & 16)) - return 0; - return __is_prefetch(regs, addr); - } - return 0; -} - -static noinline void force_sig_info_fault(int si_signo, int si_code, +static void force_sig_info_fault(int si_signo, int si_code, unsigned long address, struct task_struct *tsk) { siginfo_t info; @@ -214,7 +173,45 @@ static noinline void force_sig_info_fault(int si_signo, int si_code, force_sig_info(si_signo, &info, tsk); } -fastcall void do_invalid_op(struct pt_regs *, unsigned long); +void dump_pagetable(unsigned long address) +{ + __typeof__(pte_val(__pte(0))) page; + + page = read_cr3(); + page = ((__typeof__(page) *) __va(page))[address >> PGDIR_SHIFT]; +#ifdef CONFIG_X86_PAE + printk("*pdpt = %016Lx ", page); + if ((page >> PAGE_SHIFT) < max_low_pfn + && page & _PAGE_PRESENT) { + page &= PAGE_MASK; + page = ((__typeof__(page) *) __va(page))[(address >> PMD_SHIFT) + & (PTRS_PER_PMD - 1)]; + printk(KERN_CONT "*pde = %016Lx ", page); + page &= ~_PAGE_NX; + } +#else + printk("*pde = %08lx ", page); +#endif + + /* + * We must not directly access the pte in the highpte + * case if the page table is located in highmem. + * And let's rather not kmap-atomic the pte, just in case + * it's allocated already. + */ + if ((page >> PAGE_SHIFT) < max_low_pfn + && (page & _PAGE_PRESENT) + && !(page & _PAGE_PSE)) { + page &= PAGE_MASK; + page = ((__typeof__(page) *) __va(page))[(address >> PAGE_SHIFT) + & (PTRS_PER_PTE - 1)]; + printk("*pte = %0*Lx ", sizeof(page)*2, (u64)page); + } + + printk("\n"); +} + +void do_invalid_op(struct pt_regs *, unsigned long); static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address) { @@ -252,6 +249,46 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address) return pmd_k; } +#ifdef CONFIG_X86_64 +static const char errata93_warning[] = +KERN_ERR "******* Your BIOS seems to not contain a fix for K8 errata #93\n" +KERN_ERR "******* Working around it, but it may cause SEGVs or burn power.\n" +KERN_ERR "******* Please consider a BIOS update.\n" +KERN_ERR "******* Disabling USB legacy in the BIOS may also help.\n"; +#endif + +/* Workaround for K8 erratum #93 & buggy BIOS. + BIOS SMM functions are required to use a specific workaround + to avoid corruption of the 64bit RIP register on C stepping K8. + A lot of BIOS that didn't get tested properly miss this. + The OS sees this as a page fault with the upper 32bits of RIP cleared. + Try to work around it here. + Note we only handle faults in kernel here. + Does nothing for X86_32 + */ +static int is_errata93(struct pt_regs *regs, unsigned long address) +{ +#ifdef CONFIG_X86_64 + static int warned; + if (address != regs->ip) + return 0; + if ((address >> 32) != 0) + return 0; + address |= 0xffffffffUL << 32; + if ((address >= (u64)_stext && address <= (u64)_etext) || + (address >= MODULES_VADDR && address <= MODULES_END)) { + if (!warned) { + printk(errata93_warning); + warned = 1; + } + regs->ip = address; + return 1; + } +#endif + return 0; +} + + /* * Handle a fault on the vmalloc or module mapping area * @@ -259,6 +296,7 @@ static inline pmd_t *vmalloc_sync_one(pgd_t *pgd, unsigned long address) */ static inline int vmalloc_fault(unsigned long address) { +#ifdef CONFIG_X86_32 unsigned long pgd_paddr; pmd_t *pmd_k; pte_t *pte_k; @@ -277,6 +315,51 @@ static inline int vmalloc_fault(unsigned long address) if (!pte_present(*pte_k)) return -1; return 0; +#else + pgd_t *pgd, *pgd_ref; + pud_t *pud, *pud_ref; + pmd_t *pmd, *pmd_ref; + pte_t *pte, *pte_ref; + + /* Copy kernel mappings over when needed. This can also + happen within a race in page table update. In the later + case just flush. */ + + pgd = pgd_offset(current->mm ?: &init_mm, address); + pgd_ref = pgd_offset_k(address); + if (pgd_none(*pgd_ref)) + return -1; + if (pgd_none(*pgd)) + set_pgd(pgd, *pgd_ref); + else + BUG_ON(pgd_page_vaddr(*pgd) != pgd_page_vaddr(*pgd_ref)); + + /* Below here mismatches are bugs because these lower tables + are shared */ + + pud = pud_offset(pgd, address); + pud_ref = pud_offset(pgd_ref, address); + if (pud_none(*pud_ref)) + return -1; + if (pud_none(*pud) || pud_page_vaddr(*pud) != pud_page_vaddr(*pud_ref)) + BUG(); + pmd = pmd_offset(pud, address); + pmd_ref = pmd_offset(pud_ref, address); + if (pmd_none(*pmd_ref)) + return -1; + if (pmd_none(*pmd) || pmd_page(*pmd) != pmd_page(*pmd_ref)) + BUG(); + pte_ref = pte_offset_kernel(pmd_ref, address); + if (!pte_present(*pte_ref)) + return -1; + pte = pte_offset_kernel(pmd, address); + /* Don't use pte_page here, because the mappings can point + outside mem_map, and the NUMA hash lookup cannot handle + that. */ + if (!pte_present(*pte) || pte_pfn(*pte) != pte_pfn(*pte_ref)) + BUG(); + return 0; +#endif } int show_unhandled_signals = 1; @@ -285,31 +368,33 @@ int show_unhandled_signals = 1; * This routine handles page faults. It determines the address, * and the problem, and then passes it off to one of the appropriate * routines. - * - * error_code: - * bit 0 == 0 means no page found, 1 means protection fault - * bit 1 == 0 means read, 1 means write - * bit 2 == 0 means kernel, 1 means user-mode - * bit 3 == 1 means use of reserved bit detected - * bit 4 == 1 means fault was an instruction fetch */ -fastcall void __kprobes do_page_fault(struct pt_regs *regs, - unsigned long error_code) +void __kprobes do_page_fault(struct pt_regs *regs, unsigned long error_code) { struct task_struct *tsk; struct mm_struct *mm; - struct vm_area_struct * vma; + struct vm_area_struct *vma; unsigned long address; int write, si_code; int fault; - /* get the address */ - address = read_cr2(); + /* + * We can fault from pretty much anywhere, with unknown IRQ state. + */ + trace_hardirqs_fixup(); tsk = current; + mm = tsk->mm; + prefetchw(&mm->mmap_sem); + + /* get the address */ + address = read_cr2(); si_code = SEGV_MAPERR; + if (notify_page_fault(regs)) + return; + /* * We fault-in kernel-space virtual memory on-demand. The * 'reference' page table is init_mm.pgd. @@ -324,9 +409,8 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs, * protection error (error_code & 9) == 0. */ if (unlikely(address >= TASK_SIZE)) { - if (!(error_code & 0x0000000d) && vmalloc_fault(address) >= 0) - return; - if (notify_page_fault(regs)) + if (!(error_code & (PF_RSVD|PF_USER|PF_PROT)) && + vmalloc_fault(address) >= 0) return; /* * Don't take the mm semaphore here. If we fixup a prefetch @@ -335,26 +419,21 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs, goto bad_area_nosemaphore; } - if (notify_page_fault(regs)) - return; - /* It's safe to allow irq's after cr2 has been saved and the vmalloc fault has been handled. */ - if (regs->eflags & (X86_EFLAGS_IF|VM_MASK)) + if (regs->flags & (X86_EFLAGS_IF|VM_MASK)) local_irq_enable(); - mm = tsk->mm; - /* * If we're in an interrupt, have no user context or are running in an - * atomic region then we must not take the fault.. + * atomic region then we must not take the fault. */ if (in_atomic() || !mm) goto bad_area_nosemaphore; /* When running in the kernel we expect faults to occur only to * addresses in user space. All other faults represent errors in the - * kernel and should generate an OOPS. Unfortunatly, in the case of an + * kernel and should generate an OOPS. Unfortunately, in the case of an * erroneous fault occurring in a code path which already holds mmap_sem * we will deadlock attempting to validate the fault against the * address space. Luckily the kernel only validly references user @@ -362,14 +441,14 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs, * exceptions table. * * As the vast majority of faults will be valid we will only perform - * the source reference check when there is a possibilty of a deadlock. + * the source reference check when there is a possibility of a deadlock. * Attempt to lock the address space, if we cannot we then validate the * source. If this is invalid we can skip the address space check, * thus avoiding the deadlock. */ if (!down_read_trylock(&mm->mmap_sem)) { - if ((error_code & 4) == 0 && - !search_exception_tables(regs->eip)) + if ((error_code & PF_USER) == 0 && + !search_exception_tables(regs->ip)) goto bad_area_nosemaphore; down_read(&mm->mmap_sem); } @@ -381,14 +460,14 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs, goto good_area; if (!(vma->vm_flags & VM_GROWSDOWN)) goto bad_area; - if (error_code & 4) { + if (error_code & PF_USER) { /* - * Accessing the stack below %esp is always a bug. + * Accessing the stack below %sp is always a bug. * The large cushion allows instructions like enter * and pusha to work. ("enter $65535,$31" pushes - * 32 pointers and then decrements %esp by 65535.) + * 32 pointers and then decrements %sp by 65535.) */ - if (address + 65536 + 32 * sizeof(unsigned long) < regs->esp) + if (address + 65536 + 32 * sizeof(unsigned long) < regs->sp) goto bad_area; } if (expand_stack(vma, address)) @@ -400,19 +479,19 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs, good_area: si_code = SEGV_ACCERR; write = 0; - switch (error_code & 3) { - default: /* 3: write, present */ - /* fall through */ - case 2: /* write, not present */ - if (!(vma->vm_flags & VM_WRITE)) - goto bad_area; - write++; - break; - case 1: /* read, present */ + switch (error_code & (PF_PROT|PF_WRITE)) { + default: /* 3: write, present */ + /* fall through */ + case PF_WRITE: /* write, not present */ + if (!(vma->vm_flags & VM_WRITE)) + goto bad_area; + write++; + break; + case PF_PROT: /* read, present */ + goto bad_area; + case 0: /* read, not present */ + if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))) goto bad_area; - case 0: /* read, not present */ - if (!(vma->vm_flags & (VM_READ | VM_EXEC | VM_WRITE))) - goto bad_area; } survive: @@ -434,14 +513,16 @@ good_area: else tsk->min_flt++; +#ifdef CONFIG_X86_32 /* * Did it hit the DOS screen memory VA from vm86 mode? */ - if (regs->eflags & VM_MASK) { + if (v8086_mode(regs)) { unsigned long bit = (address - 0xA0000) >> PAGE_SHIFT; if (bit < 32) tsk->thread.screen_bitmap |= 1 << bit; } +#endif up_read(&mm->mmap_sem); return; @@ -454,14 +535,14 @@ bad_area: bad_area_nosemaphore: /* User mode accesses just cause a SIGSEGV */ - if (error_code & 4) { + if (error_code & PF_USER) { /* * It's possible to have interrupts off here. */ local_irq_enable(); - /* - * Valid to do another page fault here because this one came + /* + * Valid to do another page fault here because this one came * from user space. */ if (is_prefetch(regs, address, error_code)) @@ -469,11 +550,17 @@ bad_area_nosemaphore: if (show_unhandled_signals && unhandled_signal(tsk, SIGSEGV) && printk_ratelimit()) { - printk("%s%s[%d]: segfault at %08lx eip %08lx " - "esp %08lx error %lx\n", - tsk->pid > 1 ? KERN_INFO : KERN_EMERG, - tsk->comm, tsk->pid, address, regs->eip, - regs->esp, error_code); + printk( +#ifdef CONFIG_X86_32 + "%s%s[%d]: segfault at %lx ip %08lx sp %08lx error %lx", +#else + "%s%s[%d]: segfault at %lx ip %lx sp %lx error %lx", +#endif + task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG, + tsk->comm, task_pid_nr(tsk), address, regs->ip, + regs->sp, error_code); + print_vma_addr(" in ", regs->ip); + printk("\n"); } tsk->thread.cr2 = address; /* Kernel addresses are always protection faults */ @@ -489,7 +576,7 @@ bad_area_nosemaphore: */ if (boot_cpu_data.f00f_bug) { unsigned long nr; - + nr = (address - idt_descr.address) >> 3; if (nr == 6) { @@ -504,13 +591,16 @@ no_context: if (fixup_exception(regs)) return; - /* + /* * Valid to do another page fault here, because if this fault - * had been triggered by is_prefetch fixup_exception would have + * had been triggered by is_prefetch fixup_exception would have * handled it. */ - if (is_prefetch(regs, address, error_code)) - return; + if (is_prefetch(regs, address, error_code)) + return; + + if (is_errata93(regs, address)) + return; /* * Oops. The kernel tried to access some bad page. We'll have to @@ -520,13 +610,13 @@ no_context: bust_spinlocks(1); if (oops_may_print()) { - __typeof__(pte_val(__pte(0))) page; #ifdef CONFIG_X86_PAE - if (error_code & 16) { - pte_t *pte = lookup_address(address); + if (error_code & PF_INSTR) { + int level; + pte_t *pte = lookup_address(address, &level); - if (pte && pte_present(*pte) && !pte_exec_kernel(*pte)) + if (pte && pte_present(*pte) && !pte_exec(*pte)) printk(KERN_CRIT "kernel tried to execute " "NX-protected page - exploit attempt? " "(uid: %d)\n", current->uid); @@ -538,39 +628,10 @@ no_context: else printk(KERN_ALERT "BUG: unable to handle kernel paging" " request"); - printk(" at virtual address %08lx\n",address); - printk(KERN_ALERT " printing eip:\n"); - printk("%08lx\n", regs->eip); + printk(" at virtual address %08lx\n", address); + printk(KERN_ALERT "printing ip: %08lx ", regs->ip); - page = read_cr3(); - page = ((__typeof__(page) *) __va(page))[address >> PGDIR_SHIFT]; -#ifdef CONFIG_X86_PAE - printk(KERN_ALERT "*pdpt = %016Lx\n", page); - if ((page >> PAGE_SHIFT) < max_low_pfn - && page & _PAGE_PRESENT) { - page &= PAGE_MASK; - page = ((__typeof__(page) *) __va(page))[(address >> PMD_SHIFT) - & (PTRS_PER_PMD - 1)]; - printk(KERN_ALERT "*pde = %016Lx\n", page); - page &= ~_PAGE_NX; - } -#else - printk(KERN_ALERT "*pde = %08lx\n", page); -#endif - - /* - * We must not directly access the pte in the highpte - * case if the page table is located in highmem. - * And let's rather not kmap-atomic the pte, just in case - * it's allocated already. - */ - if ((page >> PAGE_SHIFT) < max_low_pfn - && (page & _PAGE_PRESENT)) { - page &= PAGE_MASK; - page = ((__typeof__(page) *) __va(page))[(address >> PAGE_SHIFT) - & (PTRS_PER_PTE - 1)]; - printk(KERN_ALERT "*pte = %0*Lx\n", sizeof(page)*2, (u64)page); - } + dump_pagetable(address); } tsk->thread.cr2 = address; @@ -586,13 +647,13 @@ no_context: */ out_of_memory: up_read(&mm->mmap_sem); - if (is_init(tsk)) { + if (is_global_init(tsk)) { yield(); down_read(&mm->mmap_sem); goto survive; } printk("VM: killing process %s\n", tsk->comm); - if (error_code & 4) + if (error_code & PF_USER) do_group_exit(SIGKILL); goto no_context; @@ -600,7 +661,7 @@ do_sigbus: up_read(&mm->mmap_sem); /* Kernel mode? Handle exceptions or die */ - if (!(error_code & 4)) + if (!(error_code & PF_USER)) goto no_context; /* User space => ok to do another page fault */