Re: [PATCH v2] mm: hwpoison: coredump: support recovery from dump_user_range()

[Kefeng Wang]

On 2023/4/19 15:25, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Tue, Apr 18, 2023 at 05:45:06PM +0800, Kefeng Wang wrote:
> > On 2023/4/18 11:13, HORIGUCHI NAOYA(堀口 直也) wrote:
> > > On Mon, Apr 17, 2023 at 12:53:23PM +0800, Kefeng Wang wrote:
> > > > The dump_user_range() is used to copy the user page to a coredump file,
> > > > but if a hardware memory error occurred during copy, which called from
> > > > __kernel_write_iter() in dump_user_range(), it crashes,
> > > >
> > > >     CPU: 112 PID: 7014 Comm: mca-recover Not tainted 6.3.0-rc2 #425
> > > >     pc : __memcpy+0x110/0x260
> > > >     lr : _copy_from_iter+0x3bc/0x4c8
> > > >     ...
> > > >     Call trace:
> > > >      __memcpy+0x110/0x260
> > > >      copy_page_from_iter+0xcc/0x130
> > > >      pipe_write+0x164/0x6d8
> > > >      __kernel_write_iter+0x9c/0x210
> > > >      dump_user_range+0xc8/0x1d8
> > > >      elf_core_dump+0x308/0x368
> > > >      do_coredump+0x2e8/0xa40
> > > >      get_signal+0x59c/0x788
> > > >      do_signal+0x118/0x1f8
> > > >      do_notify_resume+0xf0/0x280
> > > >      el0_da+0x130/0x138
> > > >      el0t_64_sync_handler+0x68/0xc0
> > > >      el0t_64_sync+0x188/0x190
> > > >
> > > > Generally, the '->write_iter' of file ops will use copy_page_from_iter()
> > > > and copy_page_from_iter_atomic(), change memcpy() to copy_mc_to_kernel()
> > > > in both of them to handle #MC during source read, which stop coredump
> > > > processing and kill the task instead of kernel panic, but the source
> > > > address may not always a user address, so introduce a new copy_mc flag in
> > > > struct iov_iter{} to indicate that the iter could do a safe memory copy,
> > > > also introduce the helpers to set/cleck the flag, for now, it's only
> > > > used in coredump's dump_user_range(), but it could expand to any other
> > > > scenarios to fix the similar issue.
> > > >
> > > > Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > > Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> > > > Cc: Naoya Horiguchi <naoya.horiguchi@xxxxxxx>
> > > > Cc: Tong Tiangen <tongtiangen@xxxxxxxxxx>
> > > > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > > > Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
> > > > ---
> > > > v2:
> > > > - move the helper functions under pre-existing CONFIG_ARCH_HAS_COPY_MC
> > > > - reposition the copy_mc in struct iov_iter for easy merge, suggested
> > > >     by Andrew Morton
> > > > - drop unnecessary clear flag helper
> > > > - fix checkpatch warning
> > > >    fs/coredump.c       |  1 +
> > > >    include/linux/uio.h | 16 ++++++++++++++++
> > > >    lib/iov_iter.c      | 17 +++++++++++++++--
> > > >    3 files changed, 32 insertions(+), 2 deletions(-)
> > > ...
> > > > @@ -371,6 +372,14 @@ size_t _copy_mc_to_iter(const void *addr, size_t bytes, struct iov_iter *i)
> > > >    EXPORT_SYMBOL_GPL(_copy_mc_to_iter);
> > > >    #endif /* CONFIG_ARCH_HAS_COPY_MC */
> > > > +static void *memcpy_from_iter(struct iov_iter *i, void *to, const void *from,
> > > > +				 size_t size)
> > > > +{
> > > > +	if (iov_iter_is_copy_mc(i))
> > > > +		return (void *)copy_mc_to_kernel(to, from, size);
> > >
> > > Is it helpful to call memory_failure_queue() if copy_mc_to_kernel() fails
> > > due to a memory error?
> >
> > For dump_user_range(), the task is dying, if copy incomplete size, the
> > coredump will fail and task will exit, also memory_failure will
> > be called by kill_me_maybe(),
> >
> >   CPU: 0 PID: 1418 Comm: test Tainted: G   M               6.3.0-rc5 #29
> >   Call Trace:
> >    <TASK>
> >    dump_stack_lvl+0x37/0x50
> >    memory_failure+0x51/0x970
> >    kill_me_maybe+0x5b/0xc0
> >    task_work_run+0x5a/0x90
> >    exit_to_user_mode_prepare+0x194/0x1a0
> >    irqentry_exit_to_user_mode+0x9/0x30
> >    noist_exc_machine_check+0x40/0x80
> >    asm_exc_machine_check+0x33/0x40
>
> Is this call trace printed out when copy_mc_to_kernel() failed by finding
> a memory error (or in some testcase using error injection)?

I add dump_stack() into memory_failure() to check whether the poisoned
memory is called or not, and the call trace shows it do call
memory_failure()， but I get confused when do the test.

> In my understanding, an MCE should not be triggered when MC-safe copy tries
> to access to a memory error.  So I feel that we might be talking about
> different scenarios.
>
> When I questioned previously, I thought about the following scenario:
>
>    - a process terminates abnormally for any reason like segmentation fault,
>    - then, kernel tries to create a coredump,
>    - during this, the copying routine accesses to corrupted page to read.
Yes, we tested like your described,

1) inject memory error into a process
2) send a SIGABT/SIGBUS to process to trigger the coredump

Without patch, the system panic, and with patch only process exits.

> In this case the corrupted page should not be handled by memory_failure()
> yet (because otherwise properly handled hwpoisoned page should be ignored
> by coredump process).  The coredump process would exit with failure with
> your patch, but then, the corrupted page is still left unhandled and can
> be reused, so any other thread can easily access to it again.

As shown above, the corrupted page will be handled by memory_failure(), 
but what I'm wondering,
1) memory_failure() is not always called
2) look at the above call trace, it looks like from asynchronous
   interrupt, not from synchronous exception, right?

> You can find a few other places (like __wp_page_copy_user and ksm_might_need_to_copy)
> to call memory_failure_queue() to cope with such unhandled error pages.
> So does memcpy_from_iter() do the same?

I add some debug print in do_machine_check() on x86:

1) COW,
  m.kflags: MCE_IN_KERNEL_RECOV
  fixup_type: EX_TYPE_DEFAULT_MCE_SAFE

  CPU: 11 PID: 2038 Comm: einj_mem_uc
  Call Trace:
   <#MC>
   dump_stack_lvl+0x37/0x50
   do_machine_check+0x7ad/0x840
   exc_machine_check+0x5a/0x90
   asm_exc_machine_check+0x1e/0x40
  RIP: 0010:copy_mc_fragile+0x35/0x62

  if (m.kflags & MCE_IN_KERNEL_RECOV) {
          if (!fixup_exception(regs, X86_TRAP_MC, 0, 0))
                  mce_panic("Failed kernel mode recovery", &m, msg);
  }

  if (m.kflags & MCE_IN_KERNEL_COPYIN)
          queue_task_work(&m, msg, kill_me_never);

There is no memory_failure() called when
EX_TYPE_DEFAULT_MCE_SAFE, also EX_TYPE_FAULT_MCE_SAFE too,
so we manually add a memory_failure_queue() to handle with
the poisoned page.

2） Coredump,  nothing print about m.kflags and fixup_type,
with above check, add a memory_failure_queue() or memory_failure() seems
to be needed for memcpy_from_iter(), but it is totally different from
the COW scenario


Another question, other copy_mc_to_kernel() callers, eg,
nvdimm/dm-writecache/dax, there are not call memory_failure_queue(),
should they need a memory_failure_queue(), if so, why not add it into
do_machine_check() ?

Thanks.



> Thanks,
> Naoya Horiguchi