From: Andy Lutomirski The TSS is a fairly juicy target for exploits, and, now that the TSS is in the cpu_entry_area, it's no longer protected by kASLR. Make it read-only on x86_64. On x86_32, it can't be RO because it's written by the CPU during task switches, and we use a task gate for double faults. I'd also be nervous about errata if we tried to make it RO even on configurations without double fault handling. [ tglx: AMD confirmed that there is no problem on 64bit with TSS RO. So it's probably safe to assume that it's a non issue, though Intel might have been creative in that area. Still waiting for confirmation. ] Signed-off-by: Andy Lutomirski Signed-off-by: Thomas Gleixner Cc: Kees Cook Cc: Peter Zijlstra Cc: Brian Gerst Cc: David Laight Cc: Borislav Petkov Link: https://lkml.kernel.org/r/7d2f65f86a46e3489ba996932554485c3d345632.1512109321.git.luto@kernel.org --- arch/x86/entry/entry_32.S | 4 ++-- arch/x86/entry/entry_64.S | 8 ++++---- arch/x86/include/asm/fixmap.h | 13 +++++++++---- arch/x86/include/asm/processor.h | 17 ++++++++--------- arch/x86/include/asm/switch_to.h | 4 ++-- arch/x86/include/asm/thread_info.h | 2 +- arch/x86/kernel/asm-offsets.c | 5 ++--- arch/x86/kernel/asm-offsets_32.c | 4 ++-- arch/x86/kernel/cpu/common.c | 29 +++++++++++++++++++---------- arch/x86/kernel/ioport.c | 2 +- arch/x86/kernel/process.c | 6 +++--- arch/x86/kernel/process_32.c | 2 +- arch/x86/kernel/process_64.c | 2 +- arch/x86/kernel/traps.c | 4 ++-- arch/x86/lib/delay.c | 4 ++-- arch/x86/xen/enlighten_pv.c | 2 +- 16 files changed, 60 insertions(+), 48 deletions(-) --- a/arch/x86/entry/entry_32.S +++ b/arch/x86/entry/entry_32.S @@ -942,7 +942,7 @@ ENTRY(debug) /* Are we currently on the SYSENTER stack? */ movl PER_CPU_VAR(cpu_entry_area), %ecx - addl $CPU_ENTRY_AREA_tss + TSS_STRUCT_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx + addl $CPU_ENTRY_AREA_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx subl %eax, %ecx /* ecx = (end of SYSENTER_stack) - esp */ cmpl $SIZEOF_SYSENTER_stack, %ecx jb .Ldebug_from_sysenter_stack @@ -986,7 +986,7 @@ ENTRY(nmi) /* Are we currently on the SYSENTER stack? */ movl PER_CPU_VAR(cpu_entry_area), %ecx - addl $CPU_ENTRY_AREA_tss + TSS_STRUCT_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx + addl $CPU_ENTRY_AREA_SYSENTER_stack + SIZEOF_SYSENTER_stack, %ecx subl %eax, %ecx /* ecx = (end of SYSENTER_stack) - esp */ cmpl $SIZEOF_SYSENTER_stack, %ecx jb .Lnmi_from_sysenter_stack --- a/arch/x86/entry/entry_64.S +++ b/arch/x86/entry/entry_64.S @@ -158,7 +158,7 @@ END(native_usergs_sysret64) _entry_trampoline - CPU_ENTRY_AREA_entry_trampoline(%rip) /* The top word of the SYSENTER stack is hot and is usable as scratch space. */ -#define RSP_SCRATCH CPU_ENTRY_AREA_tss + TSS_STRUCT_SYSENTER_stack + \ +#define RSP_SCRATCH CPU_ENTRY_AREA_SYSENTER_stack + \ SIZEOF_SYSENTER_stack - 8 + CPU_ENTRY_AREA ENTRY(entry_SYSCALL_64_trampoline) @@ -394,7 +394,7 @@ GLOBAL(entry_SYSCALL_64_after_hwframe) * Save old stack pointer and switch to trampoline stack. */ movq %rsp, %rdi - movq PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp + movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp pushq RSP-RDI(%rdi) /* RSP */ pushq (%rdi) /* RDI */ @@ -723,7 +723,7 @@ GLOBAL(swapgs_restore_regs_and_return_to * Save old stack pointer and switch to trampoline stack. */ movq %rsp, %rdi - movq PER_CPU_VAR(cpu_tss + TSS_sp0), %rsp + movq PER_CPU_VAR(cpu_tss_rw + TSS_sp0), %rsp /* Copy the IRET frame to the trampoline stack. */ pushq 6*8(%rdi) /* SS */ @@ -938,7 +938,7 @@ apicinterrupt IRQ_WORK_VECTOR irq_work /* * Exception entry points. */ -#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss) + (TSS_ist + ((x) - 1) * 8) +#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw) + (TSS_ist + ((x) - 1) * 8) /* * Switch to the thread stack. This is called with the IRET frame and --- a/arch/x86/include/asm/fixmap.h +++ b/arch/x86/include/asm/fixmap.h @@ -56,9 +56,14 @@ struct cpu_entry_area { char gdt[PAGE_SIZE]; /* - * The GDT is just below cpu_tss and thus serves (on x86_64) as a - * a read-only guard page for the SYSENTER stack at the bottom - * of the TSS region. + * The GDT is just below SYSENTER_stack and thus serves (on x86_64) as + * a a read-only guard page. + */ + struct SYSENTER_stack_page SYSENTER_stack_page; + + /* + * On x86_64, the TSS is mapped RO. On x86_32, it's mapped RW because + * we need task switches to work, and task switches write to the TSS. */ struct tss_struct tss; @@ -241,7 +246,7 @@ static inline struct cpu_entry_area *get static inline struct SYSENTER_stack *cpu_SYSENTER_stack(int cpu) { - return &get_cpu_entry_area(cpu)->tss.SYSENTER_stack; + return &get_cpu_entry_area(cpu)->SYSENTER_stack_page.stack; } #endif /* !__ASSEMBLY__ */ --- a/arch/x86/include/asm/processor.h +++ b/arch/x86/include/asm/processor.h @@ -341,13 +341,11 @@ struct SYSENTER_stack { unsigned long words[64]; }; -struct tss_struct { - /* - * Space for the temporary SYSENTER stack, used for SYSENTER - * and the entry trampoline as well. - */ - struct SYSENTER_stack SYSENTER_stack; +struct SYSENTER_stack_page { + struct SYSENTER_stack stack; +} __aligned(PAGE_SIZE); +struct tss_struct { /* * The fixed hardware portion. This must not cross a page boundary * at risk of violating the SDM's advice and potentially triggering @@ -364,7 +362,7 @@ struct tss_struct { unsigned long io_bitmap[IO_BITMAP_LONGS + 1]; } __aligned(PAGE_SIZE); -DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss); +DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_struct, cpu_tss_rw); /* * sizeof(unsigned long) coming from an extra "long" at the end @@ -379,7 +377,8 @@ DECLARE_PER_CPU_PAGE_ALIGNED(struct tss_ #ifdef CONFIG_X86_32 DECLARE_PER_CPU(unsigned long, cpu_current_top_of_stack); #else -#define cpu_current_top_of_stack cpu_tss.x86_tss.sp1 +/* The RO copy can't be accessed with this_cpu_xyz(), so use the RW copy. */ +#define cpu_current_top_of_stack cpu_tss_rw.x86_tss.sp1 #endif /* @@ -539,7 +538,7 @@ static inline void native_set_iopl_mask( static inline void native_load_sp0(unsigned long sp0) { - this_cpu_write(cpu_tss.x86_tss.sp0, sp0); + this_cpu_write(cpu_tss_rw.x86_tss.sp0, sp0); } static inline void native_swapgs(void) --- a/arch/x86/include/asm/switch_to.h +++ b/arch/x86/include/asm/switch_to.h @@ -79,10 +79,10 @@ do { \ static inline void refresh_sysenter_cs(struct thread_struct *thread) { /* Only happens when SEP is enabled, no need to test "SEP"arately: */ - if (unlikely(this_cpu_read(cpu_tss.x86_tss.ss1) == thread->sysenter_cs)) + if (unlikely(this_cpu_read(cpu_tss_rw.x86_tss.ss1) == thread->sysenter_cs)) return; - this_cpu_write(cpu_tss.x86_tss.ss1, thread->sysenter_cs); + this_cpu_write(cpu_tss_rw.x86_tss.ss1, thread->sysenter_cs); wrmsr(MSR_IA32_SYSENTER_CS, thread->sysenter_cs, 0); } #endif --- a/arch/x86/include/asm/thread_info.h +++ b/arch/x86/include/asm/thread_info.h @@ -207,7 +207,7 @@ static inline int arch_within_stack_fram #else /* !__ASSEMBLY__ */ #ifdef CONFIG_X86_64 -# define cpu_current_top_of_stack (cpu_tss + TSS_sp1) +# define cpu_current_top_of_stack (cpu_tss_rw + TSS_sp1) #endif #endif --- a/arch/x86/kernel/asm-offsets.c +++ b/arch/x86/kernel/asm-offsets.c @@ -94,10 +94,9 @@ void common(void) { BLANK(); DEFINE(PTREGS_SIZE, sizeof(struct pt_regs)); - OFFSET(TSS_STRUCT_SYSENTER_stack, tss_struct, SYSENTER_stack); - DEFINE(SIZEOF_SYSENTER_stack, sizeof(struct SYSENTER_stack)); - /* Layout info for cpu_entry_area */ OFFSET(CPU_ENTRY_AREA_tss, cpu_entry_area, tss); OFFSET(CPU_ENTRY_AREA_entry_trampoline, cpu_entry_area, entry_trampoline); + OFFSET(CPU_ENTRY_AREA_SYSENTER_stack, cpu_entry_area, SYSENTER_stack_page); + DEFINE(SIZEOF_SYSENTER_stack, sizeof(struct SYSENTER_stack)); } --- a/arch/x86/kernel/asm-offsets_32.c +++ b/arch/x86/kernel/asm-offsets_32.c @@ -47,8 +47,8 @@ void foo(void) BLANK(); /* Offset from the sysenter stack to tss.sp0 */ - DEFINE(TSS_sysenter_sp0, offsetof(struct tss_struct, x86_tss.sp0) - - offsetofend(struct tss_struct, SYSENTER_stack)); + DEFINE(TSS_sysenter_sp0, offsetof(struct cpu_entry_area, tss.x86_tss.sp0) - + offsetofend(struct cpu_entry_area, SYSENTER_stack_page.stack)); #ifdef CONFIG_CC_STACKPROTECTOR BLANK(); --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -511,6 +511,9 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(char, [(N_EXCEPTION_STACKS - 1) * EXCEPTION_STKSZ + DEBUG_STKSZ]); #endif +static DEFINE_PER_CPU_PAGE_ALIGNED(struct SYSENTER_stack_page, + SYSENTER_stack_storage); + static void __init set_percpu_fixmap_pages(int idx, void *ptr, int pages, pgprot_t prot) { @@ -524,23 +527,29 @@ static void __init setup_cpu_entry_area( #ifdef CONFIG_X86_64 extern char _entry_trampoline[]; - /* On 64-bit systems, we use a read-only fixmap GDT. */ + /* On 64-bit systems, we use a read-only fixmap GDT and TSS. */ pgprot_t gdt_prot = PAGE_KERNEL_RO; + pgprot_t tss_prot = PAGE_KERNEL_RO; #else /* * On native 32-bit systems, the GDT cannot be read-only because * our double fault handler uses a task gate, and entering through - * a task gate needs to change an available TSS to busy. If the GDT - * is read-only, that will triple fault. + * a task gate needs to change an available TSS to busy. If the + * GDT is read-only, that will triple fault. The TSS cannot be + * read-only because the CPU writes to it on task switches. * - * On Xen PV, the GDT must be read-only because the hypervisor requires - * it. + * On Xen PV, the GDT must be read-only because the hypervisor + * requires it. */ pgprot_t gdt_prot = boot_cpu_has(X86_FEATURE_XENPV) ? PAGE_KERNEL_RO : PAGE_KERNEL; + pgprot_t tss_prot = PAGE_KERNEL; #endif __set_fixmap(get_cpu_entry_area_index(cpu, gdt), get_cpu_gdt_paddr(cpu), gdt_prot); + set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, SYSENTER_stack_page), + per_cpu_ptr(&SYSENTER_stack_storage, cpu), 1, + PAGE_KERNEL); /* * The Intel SDM says (Volume 3, 7.2.1): @@ -563,9 +572,9 @@ static void __init setup_cpu_entry_area( offsetofend(struct tss_struct, x86_tss)) & PAGE_MASK); BUILD_BUG_ON(sizeof(struct tss_struct) % PAGE_SIZE != 0); set_percpu_fixmap_pages(get_cpu_entry_area_index(cpu, tss), - &per_cpu(cpu_tss, cpu), + &per_cpu(cpu_tss_rw, cpu), sizeof(struct tss_struct) / PAGE_SIZE, - PAGE_KERNEL); + tss_prot); #ifdef CONFIG_X86_32 per_cpu(cpu_entry_area, cpu) = get_cpu_entry_area(cpu); @@ -1330,7 +1339,7 @@ void enable_sep_cpu(void) return; cpu = get_cpu(); - tss = &per_cpu(cpu_tss, cpu); + tss = &per_cpu(cpu_tss_rw, cpu); /* * We cache MSR_IA32_SYSENTER_CS's value in the TSS's ss1 field -- @@ -1600,7 +1609,7 @@ void cpu_init(void) if (cpu) load_ucode_ap(); - t = &per_cpu(cpu_tss, cpu); + t = &per_cpu(cpu_tss_rw, cpu); oist = &per_cpu(orig_ist, cpu); #ifdef CONFIG_NUMA @@ -1692,7 +1701,7 @@ void cpu_init(void) { int cpu = smp_processor_id(); struct task_struct *curr = current; - struct tss_struct *t = &per_cpu(cpu_tss, cpu); + struct tss_struct *t = &per_cpu(cpu_tss_rw, cpu); wait_for_master_cpu(cpu); --- a/arch/x86/kernel/ioport.c +++ b/arch/x86/kernel/ioport.c @@ -67,7 +67,7 @@ asmlinkage long sys_ioperm(unsigned long * because the ->io_bitmap_max value must match the bitmap * contents: */ - tss = &per_cpu(cpu_tss, get_cpu()); + tss = &per_cpu(cpu_tss_rw, get_cpu()); if (turn_on) bitmap_clear(t->io_bitmap_ptr, from, num); --- a/arch/x86/kernel/process.c +++ b/arch/x86/kernel/process.c @@ -47,7 +47,7 @@ * section. Since TSS's are completely CPU-local, we want them * on exact cacheline boundaries, to eliminate cacheline ping-pong. */ -__visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss) = { +__visible DEFINE_PER_CPU_SHARED_ALIGNED(struct tss_struct, cpu_tss_rw) = { .x86_tss = { /* * .sp0 is only used when entering ring 0 from a lower @@ -82,7 +82,7 @@ .io_bitmap = { [0 ... IO_BITMAP_LONGS] = ~0 }, #endif }; -EXPORT_PER_CPU_SYMBOL(cpu_tss); +EXPORT_PER_CPU_SYMBOL(cpu_tss_rw); DEFINE_PER_CPU(bool, __tss_limit_invalid); EXPORT_PER_CPU_SYMBOL_GPL(__tss_limit_invalid); @@ -111,7 +111,7 @@ void exit_thread(struct task_struct *tsk struct fpu *fpu = &t->fpu; if (bp) { - struct tss_struct *tss = &per_cpu(cpu_tss, get_cpu()); + struct tss_struct *tss = &per_cpu(cpu_tss_rw, get_cpu()); t->io_bitmap_ptr = NULL; clear_thread_flag(TIF_IO_BITMAP); --- a/arch/x86/kernel/process_32.c +++ b/arch/x86/kernel/process_32.c @@ -234,7 +234,7 @@ EXPORT_SYMBOL_GPL(start_thread); struct fpu *prev_fpu = &prev->fpu; struct fpu *next_fpu = &next->fpu; int cpu = smp_processor_id(); - struct tss_struct *tss = &per_cpu(cpu_tss, cpu); + struct tss_struct *tss = &per_cpu(cpu_tss_rw, cpu); /* never put a printk in __switch_to... printk() calls wake_up*() indirectly */ --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -399,7 +399,7 @@ void compat_start_thread(struct pt_regs struct fpu *prev_fpu = &prev->fpu; struct fpu *next_fpu = &next->fpu; int cpu = smp_processor_id(); - struct tss_struct *tss = &per_cpu(cpu_tss, cpu); + struct tss_struct *tss = &per_cpu(cpu_tss_rw, cpu); WARN_ON_ONCE(IS_ENABLED(CONFIG_DEBUG_ENTRY) && this_cpu_read(irq_count) != -1); --- a/arch/x86/kernel/traps.c +++ b/arch/x86/kernel/traps.c @@ -365,7 +365,7 @@ dotraplinkage void do_double_fault(struc regs->cs == __KERNEL_CS && regs->ip == (unsigned long)native_irq_return_iret) { - struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1; + struct pt_regs *gpregs = (struct pt_regs *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1; /* * regs->sp points to the failing IRET frame on the @@ -654,7 +654,7 @@ struct bad_iret_stack *fixup_bad_iret(st * exception came from the IRET target. */ struct bad_iret_stack *new_stack = - (struct bad_iret_stack *)this_cpu_read(cpu_tss.x86_tss.sp0) - 1; + (struct bad_iret_stack *)this_cpu_read(cpu_tss_rw.x86_tss.sp0) - 1; /* Copy the IRET target to the new stack. */ memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8); --- a/arch/x86/lib/delay.c +++ b/arch/x86/lib/delay.c @@ -107,10 +107,10 @@ static void delay_mwaitx(unsigned long _ delay = min_t(u64, MWAITX_MAX_LOOPS, loops); /* - * Use cpu_tss as a cacheline-aligned, seldomly + * Use cpu_tss_rw as a cacheline-aligned, seldomly * accessed per-cpu variable as the monitor target. */ - __monitorx(raw_cpu_ptr(&cpu_tss), 0, 0); + __monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0); /* * AMD, like Intel, supports the EAX hint and EAX=0xf --- a/arch/x86/xen/enlighten_pv.c +++ b/arch/x86/xen/enlighten_pv.c @@ -826,7 +826,7 @@ static void xen_load_sp0(unsigned long s mcs = xen_mc_entry(0); MULTI_stack_switch(mcs.mc, __KERNEL_DS, sp0); xen_mc_issue(PARAVIRT_LAZY_CPU); - this_cpu_write(cpu_tss.x86_tss.sp0, sp0); + this_cpu_write(cpu_tss_rw.x86_tss.sp0, sp0); } void xen_set_iopl_mask(unsigned mask)