Re: [syzbot] upstream test error: KASAN: invalid-access Read in __entry_tramp_text_end

[Mark Rutland]

On Mon, Sep 27, 2021 at 04:27:30PM +0200, Dmitry Vyukov wrote:
> On Tue, 21 Sept 2021 at 18:51, Mark Rutland <mark.rutland@xxxxxxx> wrote:
> >
> > Hi Dmitry,
> >
> > The good news is that the bad unwind is a known issue, the bad news is
> > that we don't currently have a way to fix it (and I'm planning to talk
> > about this at the LPC "objtool on arm64" talk this Friday).
> >
> > More info below: the gist is we can produce spurious entries at an
> > exception boundary, but shouldn't miss a legitimate value, and there's a
> > plan to make it easier to spot when entries are not legitimate.
> >
> > On Fri, Sep 17, 2021 at 05:03:48PM +0200, Dmitry Vyukov wrote:
> > > > Call trace:
> > > >  dump_backtrace+0x0/0x1ac arch/arm64/kernel/stacktrace.c:76
> > > >  show_stack+0x18/0x24 arch/arm64/kernel/stacktrace.c:215
> > > >  __dump_stack lib/dump_stack.c:88 [inline]
> > > >  dump_stack_lvl+0x68/0x84 lib/dump_stack.c:105
> > > >  print_address_description+0x7c/0x2b4 mm/kasan/report.c:256
> > > >  __kasan_report mm/kasan/report.c:442 [inline]
> > > >  kasan_report+0x134/0x380 mm/kasan/report.c:459
> > > >  __do_kernel_fault+0x128/0x1bc arch/arm64/mm/fault.c:317
> > > >  do_bad_area arch/arm64/mm/fault.c:466 [inline]
> > > >  do_tag_check_fault+0x74/0x90 arch/arm64/mm/fault.c:737
> > > >  do_mem_abort+0x44/0xb4 arch/arm64/mm/fault.c:813
> > > >  el1_abort+0x40/0x60 arch/arm64/kernel/entry-common.c:357
> > > >  el1h_64_sync_handler+0xb0/0xd0 arch/arm64/kernel/entry-common.c:408
> > > >  el1h_64_sync+0x78/0x7c arch/arm64/kernel/entry.S:567
> > > >  __entry_tramp_text_end+0xdfc/0x3000
> > >
> > > /\/\/\/\/\/\/\
> > >
> > > This is broken unwind on arm64. d_lookup statically calls __d_lookup,
> > > not __entry_tramp_text_end (which is not even a function).
> > > See the following thread for some debugging details:
> > > https://lore.kernel.org/lkml/CACT4Y+ZByJ71QfYHTByWaeCqZFxYfp8W8oyrK0baNaSJMDzoUw@xxxxxxxxxxxxxx/

Looking at this again (and as you point out below), my initial analysis
was wrong, and this isn't to do with the LR -- this value should be the
PC at the time the exception boundary.

Per `aarch64-linux-objdump -t vmlinux | sort -n`, in my kernel
Image I have:

ffff800011799000 g       .text  0000000000000000 __entry_tramp_text_end
ffff8000117a0000 g     O .rodata        0000000000000008 kimage_vaddr
ffff8000117a0000 g       .rodata        0000000000000000 _etext
ffff8000117a0000 g       .rodata        0000000000000000 __kvm_nvhe___start_rodata
ffff8000117a0000 g       .rodata        0000000000000000 __start_rodata
ffff8000117a0000 l    d  .rodata        0000000000000000 .rodata

... and per vmlinux.lds.S, the space between __entry_tramp_text_end and _etext
(all 0x7000 of it in my case) is formed of:

	*(.fixup)
	*(.gnu.warning)
	*(.got)
	.got.plt : { *(.got.plt) }

... so whatever is faulting in your configuration is likely in one of
those, and as it's anonymous, the kernel ends up symbolizing it as
__entry_tramp_text_end, as that's the immediately prior symbol.

In my kernel, the +0xdfc offset would be a fixup, and very few of those
perform faulting instructions. THe ones which do are 

... where I'd expext the 


> >
> > The problem here is that our calling convention (AAPCS64) only allows us
> > to reliably unwind at function call boundaries, where the state of both
> > the Link Register (LR/x30) and Frame Pointer (FP/x29) are well-defined.
> > Within a function, we don't know whether to start unwinding from the LR
> > or FP, and we currently start from the LR, which can produce spurious
> > entries (but ensures we don't miss anything legitimte).
> >
> > In the short term, I have a plan to make the unwinder indicate when an
> > entry might not be legitimate, with the usual stackdump code printing an
> > indicator like '?' on x86.
> >
> > In the longer term, we might be doing things with objtool or asking for
> > some toolchain help such that we can do better in these cases.
> 
> Hi Mark,
> 
> Any updates after the LPC session?

Nothing concrete. We have a few things to go looking at, e.g. processing
DWARF into something usable, or using CTF unwind info (which is
currently being spec'd).

> If the dumper adds " ? ", then syzkaller will strip these frames
> (required for x86).
> However, I am worried that we can remove the true top frame then and
> attribute crashes to wrong frames again?

Yes -- since we don't know either way whether the frame is legitimate or
bogus, always removing the frame would also be misleading.

So it probably wouldn't make sense for Syzkaller to strip such frames.

> Some naive questions:
> 1. Shouldn't the top frame for synchronous faults be in the PC/IP
> register (I would assume LR/FP contains the caller of the current
> frame)?

Yes; you're right, and this appears to be a distinct issue. Looking at
vmlinux.lds.S, it seems likely this is a fixup.

it seems like we must be taking a fault somewhere unexpected.
trampoline itself...

> 2. How __entry_tramp_text_end, which is not a function, even ended up
> in LR? shouldn't it always contain some code pointer (even if stale)?

IIUC the LR is only required to be a code pointer at function call
boundaries, and at other times can be used as a scratch register.

That said, I'll go digging to find a concrete explanation.

> 3. Isn't there already something in the debug info to solve this
> problem? Userspace programs don't use objtool, but I assume that can
> print crash stacks somehow (?).

IIUC this can be solved with DWARF, but in-kernel DWARF support has been
NAK'd due ot the complexity and performance concerns, so we don't have
an in-kernel solution at present.