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Commit 946c1911 authored by Josh Poimboeuf's avatar Josh Poimboeuf Committed by Ingo Molnar
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x86/entry/unwind: Create stack frames for saved interrupt registers 



With frame pointers, when a task is interrupted, its stack is no longer
completely reliable because the function could have been interrupted
before it had a chance to save the previous frame pointer on the stack.
So the caller of the interrupted function could get skipped by a stack
trace.

This is problematic for live patching, which needs to know whether a
stack trace of a sleeping task can be relied upon.  There's currently no
way to detect if a sleeping task was interrupted by a page fault
exception or preemption before it went to sleep.

Another issue is that when dumping the stack of an interrupted task, the
unwinder has no way of knowing where the saved pt_regs registers are, so
it can't print them.

This solves those issues by encoding the pt_regs pointer in the frame
pointer on entry from an interrupt or an exception.

This patch also updates the unwinder to be able to decode it, because
otherwise the unwinder would be broken by this change.

Note that this causes a change in the behavior of the unwinder: each
instance of a pt_regs on the stack is now considered a "frame".  So
callers of unwind_get_return_address() will now get an occasional
'regs->ip' address that would have previously been skipped over.

Suggested-by: default avatarAndy Lutomirski <luto@amacapital.net>
Signed-off-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
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/8b9f84a21e39d249049e0547b559ff8da0df0988.1476973742.git.jpoimboe@redhat.com


Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 29a6d796

arch/x86/entry/calling.h

+20 −0
Original line number Diff line number Diff line
@@ -192,6 +192,26 @@ For 32-bit we have the following conventions - kernel is built with 
	.byte 0xf1

	.endm



/*

 * This is a sneaky trick to help the unwinder find pt_regs on the stack.  The

 * frame pointer is replaced with an encoded pointer to pt_regs.  The encoding

 * is just setting the LSB, which makes it an invalid stack address and is also

 * a signal to the unwinder that it's a pt_regs pointer in disguise.

 *

 * NOTE: This macro must be used *after* SAVE_EXTRA_REGS because it corrupts

 * the original rbp.

 */

.macro ENCODE_FRAME_POINTER ptregs_offset=0

#ifdef CONFIG_FRAME_POINTER

	.if \ptregs_offset

		leaq \ptregs_offset(%rsp), %rbp

	.else

		mov %rsp, %rbp

	.endif

	orq	$0x1, %rbp

#endif

.endm



#endif /* CONFIG_X86_64 */



/*

arch/x86/entry/entry_32.S

+29 −4
Original line number Diff line number Diff line
@@ -176,6 +176,22 @@ 
	SET_KERNEL_GS %edx

.endm



/*

 * This is a sneaky trick to help the unwinder find pt_regs on the stack.  The

 * frame pointer is replaced with an encoded pointer to pt_regs.  The encoding

 * is just setting the LSB, which makes it an invalid stack address and is also

 * a signal to the unwinder that it's a pt_regs pointer in disguise.

 *

 * NOTE: This macro must be used *after* SAVE_ALL because it corrupts the

 * original rbp.

 */

.macro ENCODE_FRAME_POINTER

#ifdef CONFIG_FRAME_POINTER

	mov %esp, %ebp

	orl $0x1, %ebp

#endif

.endm



.macro RESTORE_INT_REGS

	popl	%ebx

	popl	%ecx
@@ -641,6 +657,7 @@ common_interrupt: 
	ASM_CLAC

	addl	$-0x80, (%esp)			/* Adjust vector into the [-256, -1] range */

	SAVE_ALL

	ENCODE_FRAME_POINTER

	TRACE_IRQS_OFF

	movl	%esp, %eax

	call	do_IRQ
@@ -652,6 +669,7 @@ ENTRY(name) \ 
	ASM_CLAC;			\

	pushl	$~(nr);			\

	SAVE_ALL;			\

	ENCODE_FRAME_POINTER;		\

	TRACE_IRQS_OFF			\

	movl	%esp, %eax;		\

	call	fn;			\
@@ -786,6 +804,7 @@ END(spurious_interrupt_bug) 
ENTRY(xen_hypervisor_callback)

	pushl	$-1				/* orig_ax = -1 => not a system call */

	SAVE_ALL

	ENCODE_FRAME_POINTER

	TRACE_IRQS_OFF



	/*
@@ -840,6 +859,7 @@ ENTRY(xen_failsafe_callback) 
	jmp	iret_exc

5:	pushl	$-1				/* orig_ax = -1 => not a system call */

	SAVE_ALL

	ENCODE_FRAME_POINTER

	jmp	ret_from_exception



.section .fixup, "ax"
@@ -1067,6 +1087,7 @@ common_exception: 
	pushl	%edx

	pushl	%ecx

	pushl	%ebx

	ENCODE_FRAME_POINTER

	cld

	movl	$(__KERNEL_PERCPU), %ecx

	movl	%ecx, %fs
@@ -1099,6 +1120,7 @@ ENTRY(debug) 
	ASM_CLAC

	pushl	$-1				# mark this as an int

	SAVE_ALL

	ENCODE_FRAME_POINTER

	xorl	%edx, %edx			# error code 0

	movl	%esp, %eax			# pt_regs pointer
@@ -1114,11 +1136,11 @@ ENTRY(debug) 


.Ldebug_from_sysenter_stack:

	/* We're on the SYSENTER stack.  Switch off. */

	movl	%esp, %ebp

	movl	%esp, %ebx

	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esp

	TRACE_IRQS_OFF

	call	do_debug

	movl	%ebp, %esp

	movl	%ebx, %esp

	jmp	ret_from_exception

END(debug)
@@ -1141,6 +1163,7 @@ ENTRY(nmi) 


	pushl	%eax				# pt_regs->orig_ax

	SAVE_ALL

	ENCODE_FRAME_POINTER

	xorl	%edx, %edx			# zero error code

	movl	%esp, %eax			# pt_regs pointer
@@ -1159,10 +1182,10 @@ ENTRY(nmi) 
	 * We're on the SYSENTER stack.  Switch off.  No one (not even debug)

	 * is using the thread stack right now, so it's safe for us to use it.

	 */

	movl	%esp, %ebp

	movl	%esp, %ebx

	movl	PER_CPU_VAR(cpu_current_top_of_stack), %esp

	call	do_nmi

	movl	%ebp, %esp

	movl	%ebx, %esp

	jmp	.Lrestore_all_notrace



#ifdef CONFIG_X86_ESPFIX32
@@ -1179,6 +1202,7 @@ ENTRY(nmi) 
	.endr

	pushl	%eax

	SAVE_ALL

	ENCODE_FRAME_POINTER

	FIXUP_ESPFIX_STACK			# %eax == %esp

	xorl	%edx, %edx			# zero error code

	call	do_nmi
@@ -1192,6 +1216,7 @@ ENTRY(int3) 
	ASM_CLAC

	pushl	$-1				# mark this as an int

	SAVE_ALL

	ENCODE_FRAME_POINTER

	TRACE_IRQS_OFF

	xorl	%edx, %edx			# zero error code

	movl	%esp, %eax			# pt_regs pointer

arch/x86/entry/entry_64.S

+7 −3
Original line number Diff line number Diff line
@@ -469,6 +469,7 @@ END(irq_entries_start) 
	ALLOC_PT_GPREGS_ON_STACK

	SAVE_C_REGS

	SAVE_EXTRA_REGS

	ENCODE_FRAME_POINTER



	testb	$3, CS(%rsp)

	jz	1f
@@ -985,6 +986,7 @@ ENTRY(xen_failsafe_callback) 
	ALLOC_PT_GPREGS_ON_STACK

	SAVE_C_REGS

	SAVE_EXTRA_REGS

	ENCODE_FRAME_POINTER

	jmp	error_exit

END(xen_failsafe_callback)
@@ -1028,6 +1030,7 @@ ENTRY(paranoid_entry) 
	cld

	SAVE_C_REGS 8

	SAVE_EXTRA_REGS 8

	ENCODE_FRAME_POINTER 8

	movl	$1, %ebx

	movl	$MSR_GS_BASE, %ecx

	rdmsr
@@ -1075,6 +1078,7 @@ ENTRY(error_entry) 
	cld

	SAVE_C_REGS 8

	SAVE_EXTRA_REGS 8

	ENCODE_FRAME_POINTER 8

	xorl	%ebx, %ebx

	testb	$3, CS+8(%rsp)

	jz	.Lerror_kernelspace
@@ -1257,6 +1261,7 @@ ENTRY(nmi) 
	pushq	%r13		/* pt_regs->r13 */

	pushq	%r14		/* pt_regs->r14 */

	pushq	%r15		/* pt_regs->r15 */

	ENCODE_FRAME_POINTER



	/*

	 * At this point we no longer need to worry about stack damage
@@ -1270,11 +1275,10 @@ ENTRY(nmi) 


	/*

	 * Return back to user mode.  We must *not* do the normal exit

	 * work, because we don't want to enable interrupts.  Fortunately,

	 * do_nmi doesn't modify pt_regs.

	 * work, because we don't want to enable interrupts.

	 */

	SWAPGS

	jmp	restore_c_regs_and_iret

	jmp	restore_regs_and_iret



.Lnmi_from_kernel:

	/*

arch/x86/include/asm/unwind.h

+15 −1
Original line number Diff line number Diff line
@@ -13,6 +13,7 @@ struct unwind_state { 
	int graph_idx;

#ifdef CONFIG_FRAME_POINTER

	unsigned long *bp;

	struct pt_regs *regs;

#else

	unsigned long *sp;

#endif
@@ -47,7 +48,15 @@ unsigned long *unwind_get_return_address_ptr(struct unwind_state *state) 
	if (unwind_done(state))

		return NULL;



	return state->bp + 1;

	return state->regs ? &state->regs->ip : state->bp + 1;

}



static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)

{

	if (unwind_done(state))

		return NULL;



	return state->regs;

}



#else /* !CONFIG_FRAME_POINTER */
@@ -58,6 +67,11 @@ unsigned long *unwind_get_return_address_ptr(struct unwind_state *state) 
	return NULL;

}



static inline struct pt_regs *unwind_get_entry_regs(struct unwind_state *state)

{

	return NULL;

}



#endif /* CONFIG_FRAME_POINTER */



#endif /* _ASM_X86_UNWIND_H */

arch/x86/kernel/unwind_frame.c

+68 −8
Original line number Diff line number Diff line
@@ -14,6 +14,9 @@ unsigned long unwind_get_return_address(struct unwind_state *state) 
	if (unwind_done(state))

		return 0;



	if (state->regs && user_mode(state->regs))

		return 0;



	addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, *addr_p,

				     addr_p);
@@ -21,6 +24,20 @@ unsigned long unwind_get_return_address(struct unwind_state *state) 
}

EXPORT_SYMBOL_GPL(unwind_get_return_address);



/*

 * This determines if the frame pointer actually contains an encoded pointer to

 * pt_regs on the stack.  See ENCODE_FRAME_POINTER.

 */

static struct pt_regs *decode_frame_pointer(unsigned long *bp)

{

	unsigned long regs = (unsigned long)bp;



	if (!(regs & 0x1))

		return NULL;



	return (struct pt_regs *)(regs & ~0x1);

}



static bool update_stack_state(struct unwind_state *state, void *addr,

			       size_t len)

{
@@ -43,26 +60,59 @@ static bool update_stack_state(struct unwind_state *state, void *addr, 


bool unwind_next_frame(struct unwind_state *state)

{

	unsigned long *next_bp;

	struct pt_regs *regs;

	unsigned long *next_bp, *next_frame;

	size_t next_len;



	if (unwind_done(state))

		return false;



	/* have we reached the end? */

	if (state->regs && user_mode(state->regs))

		goto the_end;



	/* get the next frame pointer */

	if (state->regs)

		next_bp = (unsigned long *)state->regs->bp;

	else

		next_bp = (unsigned long *)*state->bp;



	/* is the next frame pointer an encoded pointer to pt_regs? */

	regs = decode_frame_pointer(next_bp);

	if (regs) {

		next_frame = (unsigned long *)regs;

		next_len = sizeof(*regs);

	} else {

		next_frame = next_bp;

		next_len = FRAME_HEADER_SIZE;

	}



	/* make sure the next frame's data is accessible */

	if (!update_stack_state(state, next_bp, FRAME_HEADER_SIZE))

	if (!update_stack_state(state, next_frame, next_len))

		return false;



	/* move to the next frame */

	if (regs) {

		state->regs = regs;

		state->bp = NULL;

	} else {

		state->bp = next_bp;

		state->regs = NULL;

	}



	return true;



the_end:

	state->stack_info.type = STACK_TYPE_UNKNOWN;

	return false;

}

EXPORT_SYMBOL_GPL(unwind_next_frame);



void __unwind_start(struct unwind_state *state, struct task_struct *task,

		    struct pt_regs *regs, unsigned long *first_frame)

{

	unsigned long *bp, *frame;

	size_t len;



	memset(state, 0, sizeof(*state));

	state->task = task;
@@ -73,12 +123,22 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task, 
	}



	/* set up the starting stack frame */

	state->bp = get_frame_pointer(task, regs);

	bp = get_frame_pointer(task, regs);

	regs = decode_frame_pointer(bp);

	if (regs) {

		state->regs = regs;

		frame = (unsigned long *)regs;

		len = sizeof(*regs);

	} else {

		state->bp = bp;

		frame = bp;

		len = FRAME_HEADER_SIZE;

	}



	/* initialize stack info and make sure the frame data is accessible */

	get_stack_info(state->bp, state->task, &state->stack_info,

	get_stack_info(frame, state->task, &state->stack_info,

		       &state->stack_mask);

	update_stack_state(state, state->bp, FRAME_HEADER_SIZE);

	update_stack_state(state, frame, len);



	/*

	 * The caller can provide the address of the first frame directly