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3f2c5085ed
This reinstates the following commit:2c7577a758("sched/x86_64: Don't save flags on context switch") which was reverted in:512255a2ad("Revert 'sched/x86_64: Don't save flags on context switch'") Historically, Linux has always saved and restored EFLAGS across context switches. As far as I know, the only reason to do this is because of the NT flag. In particular, if something calls switch_to() with the NT flag set, then we don't want to leak the NT flag into a different task that might try to IRET and fail because NT is set. Before this commit:8c7aa698ba("x86_64, entry: Filter RFLAGS.NT on entry from userspace") we could run system call bodies with NT set. This would be a DoS or possibly privilege escalation hole if scheduling in such a system call would leak NT into a different task. Importantly, we don't need to worry about NT being set while preemptible or across page faults. The only way we can schedule due to preemption or a page fault is in an interrupt entry that nests inside the SYSENTER prologue. The CPU will clear NT when entering through an interrupt gate, so we won't schedule with NT set. The only other interesting flags are IOPL and AC. Allowing switch_to() to change IOPL has no effect, as the value loaded during kernel execution doesn't matter at all except between a SYSENTER entry and the subsequent PUSHF, and anythign that interrupts in that window will restore IOPL on return. If we call __switch_to() with AC set, we have bigger problems. Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Borislav Petkov <bp@alien8.de> Cc: Brian Gerst <brgerst@gmail.com> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/d4440fdc2a89247bffb7c003d2a9a2952bd46827.1441146105.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
134 lines
4.7 KiB
C
134 lines
4.7 KiB
C
#ifndef _ASM_X86_SWITCH_TO_H
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#define _ASM_X86_SWITCH_TO_H
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struct task_struct; /* one of the stranger aspects of C forward declarations */
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__visible struct task_struct *__switch_to(struct task_struct *prev,
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struct task_struct *next);
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struct tss_struct;
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void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
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struct tss_struct *tss);
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#ifdef CONFIG_X86_32
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#ifdef CONFIG_CC_STACKPROTECTOR
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#define __switch_canary \
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"movl %P[task_canary](%[next]), %%ebx\n\t" \
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"movl %%ebx, "__percpu_arg([stack_canary])"\n\t"
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#define __switch_canary_oparam \
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, [stack_canary] "=m" (stack_canary.canary)
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#define __switch_canary_iparam \
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, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
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#else /* CC_STACKPROTECTOR */
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#define __switch_canary
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#define __switch_canary_oparam
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#define __switch_canary_iparam
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#endif /* CC_STACKPROTECTOR */
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/*
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* Saving eflags is important. It switches not only IOPL between tasks,
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* it also protects other tasks from NT leaking through sysenter etc.
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*/
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#define switch_to(prev, next, last) \
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do { \
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/* \
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* Context-switching clobbers all registers, so we clobber \
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* them explicitly, via unused output variables. \
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* (EAX and EBP is not listed because EBP is saved/restored \
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* explicitly for wchan access and EAX is the return value of \
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* __switch_to()) \
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*/ \
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unsigned long ebx, ecx, edx, esi, edi; \
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\
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asm volatile("pushfl\n\t" /* save flags */ \
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"pushl %%ebp\n\t" /* save EBP */ \
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"movl %%esp,%[prev_sp]\n\t" /* save ESP */ \
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"movl %[next_sp],%%esp\n\t" /* restore ESP */ \
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"movl $1f,%[prev_ip]\n\t" /* save EIP */ \
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"pushl %[next_ip]\n\t" /* restore EIP */ \
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__switch_canary \
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"jmp __switch_to\n" /* regparm call */ \
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"1:\t" \
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"popl %%ebp\n\t" /* restore EBP */ \
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"popfl\n" /* restore flags */ \
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\
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/* output parameters */ \
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: [prev_sp] "=m" (prev->thread.sp), \
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[prev_ip] "=m" (prev->thread.ip), \
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"=a" (last), \
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\
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/* clobbered output registers: */ \
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"=b" (ebx), "=c" (ecx), "=d" (edx), \
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"=S" (esi), "=D" (edi) \
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\
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__switch_canary_oparam \
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\
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/* input parameters: */ \
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: [next_sp] "m" (next->thread.sp), \
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[next_ip] "m" (next->thread.ip), \
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\
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/* regparm parameters for __switch_to(): */ \
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[prev] "a" (prev), \
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[next] "d" (next) \
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\
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__switch_canary_iparam \
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\
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: /* reloaded segment registers */ \
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"memory"); \
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} while (0)
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#else /* CONFIG_X86_32 */
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/* frame pointer must be last for get_wchan */
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#define SAVE_CONTEXT "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
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#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
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#define __EXTRA_CLOBBER \
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, "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
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"r12", "r13", "r14", "r15", "flags"
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#ifdef CONFIG_CC_STACKPROTECTOR
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#define __switch_canary \
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"movq %P[task_canary](%%rsi),%%r8\n\t" \
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"movq %%r8,"__percpu_arg([gs_canary])"\n\t"
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#define __switch_canary_oparam \
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, [gs_canary] "=m" (irq_stack_union.stack_canary)
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#define __switch_canary_iparam \
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, [task_canary] "i" (offsetof(struct task_struct, stack_canary))
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#else /* CC_STACKPROTECTOR */
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#define __switch_canary
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#define __switch_canary_oparam
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#define __switch_canary_iparam
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#endif /* CC_STACKPROTECTOR */
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/*
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* There is no need to save or restore flags, because flags are always
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* clean in kernel mode, with the possible exception of IOPL. Kernel IOPL
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* has no effect.
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*/
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#define switch_to(prev, next, last) \
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asm volatile(SAVE_CONTEXT \
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"movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */ \
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"movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */ \
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"call __switch_to\n\t" \
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"movq "__percpu_arg([current_task])",%%rsi\n\t" \
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__switch_canary \
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"movq %P[thread_info](%%rsi),%%r8\n\t" \
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"movq %%rax,%%rdi\n\t" \
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"testl %[_tif_fork],%P[ti_flags](%%r8)\n\t" \
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"jnz ret_from_fork\n\t" \
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RESTORE_CONTEXT \
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: "=a" (last) \
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__switch_canary_oparam \
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: [next] "S" (next), [prev] "D" (prev), \
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[threadrsp] "i" (offsetof(struct task_struct, thread.sp)), \
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[ti_flags] "i" (offsetof(struct thread_info, flags)), \
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[_tif_fork] "i" (_TIF_FORK), \
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[thread_info] "i" (offsetof(struct task_struct, stack)), \
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[current_task] "m" (current_task) \
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__switch_canary_iparam \
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: "memory", "cc" __EXTRA_CLOBBER)
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#endif /* CONFIG_X86_32 */
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#endif /* _ASM_X86_SWITCH_TO_H */
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