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Commit 73267498 authored by Ard Biesheuvel's avatar Ard Biesheuvel Committed by Mark Rutland
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arm64: unwind: reference pt_regs via embedded stack frame 



As it turns out, the unwind code is slightly broken, and probably has
been for a while. The problem is in the dumping of the exception stack,
which is intended to dump the contents of the pt_regs struct at each
level in the call stack where an exception was taken and routed to a
routine marked as __exception (which means its stack frame is right
below the pt_regs struct on the stack).

'Right below the pt_regs struct' is ill defined, though: the unwind
code assigns 'frame pointer + 0x10' to the .sp member of the stackframe
struct at each level, and dump_backtrace() happily dereferences that as
the pt_regs pointer when encountering an __exception routine. However,
the actual size of the stack frame created by this routine (which could
be one of many __exception routines we have in the kernel) is not known,
and so frame.sp is pretty useless to figure out where struct pt_regs
really is.

So it seems the only way to ensure that we can find our struct pt_regs
when walking the stack frames is to put it at a known fixed offset of
the stack frame pointer that is passed to such __exception routines.
The simplest way to do that is to put it inside pt_regs itself, which is
the main change implemented by this patch. As a bonus, doing this allows
us to get rid of a fair amount of cruft related to walking from one stack
to the other, which is especially nice since we intend to introduce yet
another stack for overflow handling once we add support for vmapped
stacks. It also fixes an inconsistency where we only add a stack frame
pointing to ELR_EL1 if we are executing from the IRQ stack but not when
we are executing from the task stack.

To consistly identify exceptions regs even in the presence of exceptions
taken from entry code, we must check whether the next frame was created
by entry text, rather than whether the current frame was crated by
exception text.

To avoid backtracing using PCs that fall in the idmap, or are controlled
by userspace, we must explcitly zero the FP and LR in startup paths, and
must ensure that the frame embedded in pt_regs is zeroed upon entry from
EL0. To avoid these NULL entries showin in the backtrace, unwind_frame()
is updated to avoid them.

Signed-off-by: default avatarArd Biesheuvel <ard.biesheuvel@linaro.org>
[Mark: compare current frame against .entry.text, avoid bogus PCs]
Signed-off-by: default avatarMark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
parent c7365330

arch/arm64/include/asm/irq.h

+0 −25
Original line number Diff line number Diff line
@@ -16,31 +16,6 @@ struct pt_regs; 


DECLARE_PER_CPU(unsigned long [IRQ_STACK_SIZE/sizeof(long)], irq_stack);



/*

 * The highest address on the stack, and the first to be used. Used to

 * find the dummy-stack frame put down by el?_irq() in entry.S, which

 * is structured as follows:

 *

 *       ------------

 *       |          |  <- irq_stack_ptr

 *   top ------------

 *       |   x19    | <- irq_stack_ptr - 0x08

 *       ------------

 *       |   x29    | <- irq_stack_ptr - 0x10

 *       ------------

 *

 * where x19 holds a copy of the task stack pointer where the struct pt_regs

 * from kernel_entry can be found.

 *

 */

#define IRQ_STACK_PTR() ((unsigned long)raw_cpu_ptr(irq_stack) + IRQ_STACK_START_SP)



/*

 * The offset from irq_stack_ptr where entry.S will store the original

 * stack pointer. Used by unwind_frame() and dump_backtrace().

 */

#define IRQ_STACK_TO_TASK_STACK(ptr) (*((unsigned long *)((ptr) - 0x08)))



extern void set_handle_irq(void (*handle_irq)(struct pt_regs *));



static inline int nr_legacy_irqs(void)

arch/arm64/include/asm/ptrace.h

+1 −0
Original line number Diff line number Diff line
@@ -119,6 +119,7 @@ struct pt_regs { 
	u64 syscallno;

	u64 orig_addr_limit;

	u64 unused;	// maintain 16 byte alignment

	u64 stackframe[2];

};



#define MAX_REG_OFFSET offsetof(struct pt_regs, pstate)

arch/arm64/include/asm/traps.h

+5 −0
Original line number Diff line number Diff line
@@ -60,4 +60,9 @@ static inline int in_exception_text(unsigned long ptr) 
	return in ? : __in_irqentry_text(ptr);

}



static inline int in_entry_text(unsigned long ptr)

{

	return ptr >= (unsigned long)&__entry_text_start &&

	       ptr < (unsigned long)&__entry_text_end;

}

#endif

arch/arm64/kernel/asm-offsets.c

+1 −0
Original line number Diff line number Diff line
@@ -75,6 +75,7 @@ int main(void) 
  DEFINE(S_ORIG_X0,		offsetof(struct pt_regs, orig_x0));

  DEFINE(S_SYSCALLNO,		offsetof(struct pt_regs, syscallno));

  DEFINE(S_ORIG_ADDR_LIMIT,	offsetof(struct pt_regs, orig_addr_limit));

  DEFINE(S_STACKFRAME,		offsetof(struct pt_regs, stackframe));

  DEFINE(S_FRAME_SIZE,		sizeof(struct pt_regs));

  BLANK();

  DEFINE(MM_CONTEXT_ID,		offsetof(struct mm_struct, context.id.counter));

arch/arm64/kernel/entry.S

+12 −8
Original line number Diff line number Diff line
@@ -111,6 +111,18 @@ 
	mrs	x23, spsr_el1

	stp	lr, x21, [sp, #S_LR]



	/*

	 * In order to be able to dump the contents of struct pt_regs at the

	 * time the exception was taken (in case we attempt to walk the call

	 * stack later), chain it together with the stack frames.

	 */

	.if \el == 0

	stp	xzr, xzr, [sp, #S_STACKFRAME]

	.else

	stp	x29, x22, [sp, #S_STACKFRAME]

	.endif

	add	x29, sp, #S_STACKFRAME



#ifdef CONFIG_ARM64_SW_TTBR0_PAN

	/*

	 * Set the TTBR0 PAN bit in SPSR. When the exception is taken from
@@ -265,14 +277,6 @@ alternative_else_nop_endif 


	/* switch to the irq stack */

	mov	sp, x26



	/*

	 * Add a dummy stack frame, this non-standard format is fixed up

	 * by unwind_frame()

	 */

	stp     x29, x19, [sp, #-16]!

	mov	x29, sp



9998:

	.endm