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Date: Mon,  7 Aug 2017 19:35:51 +0100
From: Mark Rutland <>
Subject: [PATCH 00/14] arm64: VMAP_STACK support


Ard and I have worked together to implement vmap stack support for
arm64. This supersedes our earlier vmap stack RFCs [0,1]. The git author
stats are a little misleading, as I've teased parts out into smaller
patches for review.

The series is based on our stack dump rework [2,3], which can be found
in the arm64/exception-stack branch [4] of my repo. This
series can be found in the arm64/vmap-stack branch [5] of the same repo.

On arm64, there is no double-fault exception, as software saves
exception context to the stack. An erroneous memory access taken during
exception handling results in a data abort, as with any other erroneous
memory access. To avoid taking these recursively, we must detect
overflow by checking the SP before we attempt to store any context to
the stack. Doing this efficiently requires a couple of tricks.

For a naturally aligned stack, bits THREAD_SHIFT-1:0 of a valid SP may
contain any arbitrary value:

	0bXX .. 11111111111111
	0bXX .. 11011001011100
	0bXX .. 00000000000000

By aligning stacks to double their natural alignment, we know that the
THREAD_SHIFT bit of any valid SP must be zero:

	0bXX .. 0 11111111111111
	0bXX .. 0 11011001011100
	0bXX .. 0 00000000000000

... while an overflow will result in this bit flipping, along with
(some) other high-order bits:

	0bXX .. 0 00000000000000
	< SP -= 1 >
	0bXX .. 1 11111111111111

... and thus, we can detect overflows of up to THREAD_SIZE by testing
the THREAD_SHIFT bit of the SP value.

Provided we can get the SP into a general purpose register, we can
perform this test with a single TBNZ instruction. We don't have scratch
space to store a GPR, but we can (partially) swap the SP with a GPR
using arithmetic to perform the test:

	add	sp, sp, x0		// sp' = sp + x0
	sub	x0, sp, x0		// x0' = sp' - x0 = (sp + x0) - x0 = sp
	tbnz	x0, #THREAD_SHIFT, overflow_handler
	sub	x0, sp, x0		// sp' - x0' = (sp + x0) - sp = x0
	sub	sp, sp, x0		// sp' - x0 = (sp + x0) - x0 = sp

This series implements this approach, along with the other requisite
changes required to make this work.

The SP test is performed for all exceptions, after compensating for the
size of the exception registers, allowing the original exception context
to be preserved in entirety. The tests themselves are folded into the
exception vectors, minimizing their impact.

To ensure that IRQ stack overflows are detected and handled, IRQ stacks
are now dynamically allocated, with guard pages.

I've given the series some light testing with LKDTM, Syzkaller, Vince
Weaver's perf fuzzer, and a few combinations of debug options. I haven't
compared performance of the entire series to a baseline kernel, but from
testing so far the cost of the SP test falls in the noise for a kernel
build workload on Cortex-A57.

Many thanks to Ard for putting up with my meddling, and also to Laura
and James for their testing and comments on prior patches.


[4] git:// arm64/exception-stack
[5] git:// arm64/vmap-stack

Ard Biesheuvel (2):
  arm64: kernel: remove {THREAD,IRQ_STACK}_START_SP
  arm64: assembler: allow adr_this_cpu to use the stack pointer

Mark Rutland (12):
  arm64: remove __die()'s stack dump
  fork: allow arch-override of VMAP stack alignment
  arm64: factor out PAGE_* and CONT_* definitions
  arm64: clean up THREAD_* definitions
  arm64: clean up irq stack definitions
  arm64: move SEGMENT_ALIGN to <asm/memory.h>
  efi/arm64: add EFI_KIMG_ALIGN
  arm64: factor out entry stack manipulation
  arm64: use an irq stack pointer
  arm64: add basic VMAP_STACK support
  arm64: add on_accessible_stack()
  arm64: add VMAP_STACK overflow detection

 arch/arm64/Kconfig                        |   1 +
 arch/arm64/include/asm/assembler.h        |   3 +-
 arch/arm64/include/asm/efi.h              |   8 +++
 arch/arm64/include/asm/irq.h              |  25 -------
 arch/arm64/include/asm/memory.h           |  53 ++++++++++++++
 arch/arm64/include/asm/page-def.h         |  34 +++++++++
 arch/arm64/include/asm/page.h             |  12 +---
 arch/arm64/include/asm/processor.h        |   2 +-
 arch/arm64/include/asm/stacktrace.h       |  62 ++++++++++++++++-
 arch/arm64/include/asm/thread_info.h      |  10 +--
 arch/arm64/kernel/entry.S                 | 110 +++++++++++++++++++++++-------
 arch/arm64/kernel/irq.c                   |  40 ++++++++++-
 arch/arm64/kernel/ptrace.c                |   1 +
 arch/arm64/kernel/smp.c                   |   2 +-
 arch/arm64/kernel/stacktrace.c            |   7 +-
 arch/arm64/kernel/traps.c                 |  44 ++++++++++--
 arch/arm64/kernel/           |  18 +----
 drivers/firmware/efi/libstub/arm64-stub.c |   6 +-
 kernel/fork.c                             |   5 +-
 19 files changed, 339 insertions(+), 104 deletions(-)
 create mode 100644 arch/arm64/include/asm/page-def.h


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