Re: [kernel-hardening] Re: [PATCH 4/4] refcount: Report failures through CHECK_DATA_CORRUPTION
From: Peter Zijlstra
Date: Wed Feb 08 2017 - 04:13:18 EST
On Tue, Feb 07, 2017 at 05:55:42PM +0000, Mark Rutland wrote:
> On Tue, Feb 07, 2017 at 06:30:36PM +0100, Peter Zijlstra wrote:
> > On Tue, Feb 07, 2017 at 04:03:01PM +0000, Mark Rutland wrote:
> > > For x86 it's a little painful due to '%' in the register names, but it looks
> > > possible. The below appears to do the mangling correctly (then screams due to
> > > the mangled result being nonexistent).
> > > asm(
> > > " .macro reg_to_offset r\n"
> > > " .irp rs,eax,ebx,ecx,edx\n"
> > > " .ifc \\r, %\\rs\n"
> > > " __offset_of_\\rs\n"
> > > " .endif\n"
> > > " .endr\n"
> > > " .endm\n"
> > > );
> > >
> > > #define asm_sym(var) asm volatile("reg_to_offset %0\n" : : "r" (var))
> > Oh gawd that's a most gnarly hack.
> > Do we want to go do that for all archs or somehow cook a generic
> > fallback that ends up doing a full function call or something?
> Given the arch-specific reg->blah mapping is so "fun", I guess a generic
> fallback would be a good start.
> I haven't figured out all the plumbing details. It'd be nice to reuse
> the bug infrastructure so that arches don't have to implement another
> trap and callback pair, but I guess the reg details need to live in
> another data structure.
On x86 have have __ex_table and __bug_table. The former is used for all
sorts of things, including fixing up faults.
Now, our struct exception_table_entry has a third field used to specify
a handler, see commit:
548acf19234d ("x86/mm: Expand the exception table logic to allow new handling options")
Also, given we trigger things with a known instruction at these sites,
the ->to field is reusable and can be used to encode the register
Still, if we want to allow a generic implementation that does a function
call, the handler prototype should probably look like:
void exception_value(unsigned long value);
Which means the arch bits need a trampoline and we also need to encode
that. The best I've come up with is having nr_regs trampolines and
stuffing the trampoline function in the ->handler field and then using
the ->to field to encode the actual handler.
#define EX_REG_HANDLER(_reg) \
bool ex_handler_value_##_reg(const struct exception_table_entry *fixup, \
struct pt_regs *regs, int trapnr) \
void (*handler)(unsigned long) = \
(void *)((unsigned long)&fixup->to + fixup->to); \
if (trapnr != X86_TRAP_UD) \
return false; \
regs->ip += 2; /* size of UD2 instruction */ \
return true; \
" .macro reg_to_handler r\n"
" .irp rs,bx,cx,...,ss\n"
" .ifc \\r, %\\rs\n"
#define EXCEPTION_VALUE(val, handler) \
asm volatile ("1: ud2" \
_ASM_EXTABLE_HANDLE(1b, handler, \
reg_to_handler %0) \
: : "r" (val))
Where the generic version can simply be:
#define EXCEPTION_VALUE(val, handler) handler((unsigned long)val)