[PATCH 0/2] Compile-time stack frame pointer validation

[Josh Poimboeuf]

[ This is a repost of a previous RFC, rebased onto linux-next. ]

In discussions around the live kernel patching consistency model RFC [1], Peter
and Ingo correctly pointed out that stack traces aren't reliable.  And as Ingo
said, there's no "strong force" which ensures we can rely on them.

So I've been thinking about how to fix that.  My goal is to eventually make
stack traces reliable.  Or at the very least, to be able to detect at runtime
when a given stack trace *might* be unreliable.  But improved stack traces
would broadly benefit the entire kernel, regardless of the outcome of the live
kernel patching consistency model discussions.

This patch set is just the first in a series of proposed stack trace
reliability improvements.  Future proposals will include runtime stack
reliability checking, as well as compile-time and runtime DWARF validations.

As far as I can tell, there are two main obstacles which prevent frame pointer
based stack traces from being reliable:

1) Missing frame pointer logic: currently, most assembly functions don't set up
   the frame pointer.

2) Interrupts: if a function is interrupted before it can save and set up
   the frame pointer, its caller won't show up in the stack trace.

This patch set aims to remove the first obstacle by enforcing that all asm
functions honor CONFIG_FRAME_POINTER.  This is done with a new stackvalidate
host tool which is automatically run for every compiled .S file and which
validates that every asm function does the proper frame pointer setup.

Also, to make sure somebody didn't forget to annotate their callable asm code
as a function, flag an error for any return instructions which are hiding
outside of a function.  In almost all cases, return instructions are part of
callable functions and should be annotated as such so that we can validate
their frame pointer usage.  A whitelist mechanism exists for those few return
instructions which are not actually in callable code.

It currently only supports x86_64.  It *almost* supports x86_32, but the
stackvalidate code doesn't yet know how to deal with 32-bit REL
relocations for the return whitelists.  I tried to make the code generic
so that support for other architectures can be plugged in pretty easily.

As a first step, all reported non-compliances result in warnings.  Right
now I'm seeing 200+ warnings.  Once we get them all cleaned up, we can
change the warnings to build errors so the asm code can stay clean.

The patches are based on linux-next.  Patch 1 adds the stackvalidate host tool.
Patch 2 adds some helper macros for asm functions so that they can comply with
stackvalidate.


[1] https://lkml.org/lkml/2015/2/9/475

Josh Poimboeuf (2):
  x86, stackvalidate: Compile-time stack frame pointer validation
  x86, stackvalidate: Add asm frame pointer setup macros

 MAINTAINERS                           |   6 +
 arch/Kconfig                          |   4 +
 arch/x86/Kconfig                      |   1 +
 arch/x86/Makefile                     |   6 +-
 arch/x86/include/asm/func.h           |  82 ++++++++
 lib/Kconfig.debug                     |  11 ++
 scripts/Makefile                      |   1 +
 scripts/Makefile.build                |  22 ++-
 scripts/stackvalidate/Makefile        |  17 ++
 scripts/stackvalidate/arch-x86.c      | 134 +++++++++++++
 scripts/stackvalidate/arch.h          |  10 +
 scripts/stackvalidate/elf.c           | 341 ++++++++++++++++++++++++++++++++++
 scripts/stackvalidate/elf.h           |  56 ++++++
 scripts/stackvalidate/list.h          | 217 ++++++++++++++++++++++
 scripts/stackvalidate/stackvalidate.c | 226 ++++++++++++++++++++++
 15 files changed, 1131 insertions(+), 3 deletions(-)
 create mode 100644 arch/x86/include/asm/func.h
 create mode 100644 scripts/stackvalidate/Makefile
 create mode 100644 scripts/stackvalidate/arch-x86.c
 create mode 100644 scripts/stackvalidate/arch.h
 create mode 100644 scripts/stackvalidate/elf.c
 create mode 100644 scripts/stackvalidate/elf.h
 create mode 100644 scripts/stackvalidate/list.h
 create mode 100644 scripts/stackvalidate/stackvalidate.c

-- 
2.1.0

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