Re: [PATCH v5 08/10] ARM: uaccess: add __{get,put}_kernel_nofault

[Chen Zhongjin]

Hi,

It seems that the problem has not been solved so far.

I found that "echo t > /proc/sysrq-trigger" causes the same fault 
because "print_worker_info()" also calls "copy_from_kernel_nofault()", 
but "worker->current_pwq" can be zero when copying.

Stack trace:

[   15.303013] 8<--- cut here ---
[   15.303315] Unhandled fault: page domain fault (0x01b) at 0x00000004
[   15.303538] [00000004] *pgd=6338f831, *pte=00000000, *ppte=00000000
[   15.304367] Internal error: : 1b [#1] SMP ARM
[   15.304721] Modules linked in:
[   15.305107] CPU: 0 PID: 89 Comm: sh Not tainted 5.19.0-rc5-dirty #332
[   15.305373] Hardware name: ARM-Versatile Express
[   15.305529] PC is at copy_from_kernel_nofault+0xf0/0x174
[   15.305712] LR is at copy_from_kernel_nofault+0x30/0x174
[   15.305873] pc : [<c0448ea4>]    lr : [<c0448de4>]    psr: 20000013
[   15.306078] sp : eac4dde8  ip : 0000bff4  fp : eac4de74
[   15.306233] r10: 00000007  r9 : 00000000  r8 : c1a09700
[   15.306397] r7 : c1a04cc8  r6 : 00000004  r5 : eac4de18  r4 : 00000004
[   15.306586] r3 : 00000000  r2 : c2440000  r1 : 00000004  r0 : 00000001
[   15.306831] Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  
Segment none
[   15.307120] Control: 10c5387d  Table: 633f006a  DAC: 00000051
...
[   15.318121]  copy_from_kernel_nofault from print_worker_info+0xd0/0x15c
[   15.318343]  print_worker_info from sched_show_task+0x134/0x180
[   15.318534]  sched_show_task from show_state_filter+0x74/0xa8
[   15.318714]  show_state_filter from sysrq_handle_showstate+0xc/0x14
[   15.318902]  sysrq_handle_showstate from __handle_sysrq+0x88/0x138
[   15.319173]  __handle_sysrq from write_sysrq_trigger+0x4c/0x5c
[   15.319356]  write_sysrq_trigger from proc_reg_write+0xa8/0xd0
[   15.319541]  proc_reg_write from vfs_write+0xb4/0x388
[   15.319708]  vfs_write from ksys_write+0x58/0xd0
[   15.319851]  ksys_write from ret_fast_syscall+0x0/0x54

> On Thu, Jan 13, 2022 at 12:14:50PM +0100, Arnd Bergmann wrote:
> > On Thu, Jan 13, 2022 at 10:47 AM Daniel Thompson
> > <daniel.thompson@xxxxxxxxxx> wrote:
> > > On Wed, Jan 12, 2022 at 06:08:17PM +0000, Russell King (Oracle) 
> wrote:
> > >
> > > > The kernel attempted to access an address that is in the userspace
> > > > domain (NULL pointer) and took an exception.
> > > >
> > > > I suppose we should handle a domain fault more gracefully - what 
> are
> > > > the required semantics if the kernel attempts a userspace access
> > > > using one of the _nofault() accessors?
> > >
> > > I think the best answer might well be that, if the arch provides
> > > implementations of hooks such as copy_from_kernel_nofault_allowed()
> > > then the kernel should never attempt a userspace access using the
> > > _nofault() accessors. That means they can do whatever they like!
> > >
> > > In other words something like the patch below looks like a promising
> > > approach.
> >
> > Right, it seems this is the same as on x86.
>
> Hmnn...
>
> Looking a bit deeper into copy_from_kernel_nofault() there is an odd
> asymmetry between copy_to_kernel_nofault(). Basically there is
> copy_from_kernel_nofault_allowed() but no corresponding
> copy_to_kernel_nofault_allowed() which means we cannot defend memory
> pokes using a helper function.
>
> I checked the behaviour of copy_to_kernel_nofault() on arm, arm64, mips,
> powerpc, riscv, x86 kernels (which is pretty much everything where I
> know how to fire up qemu). All except arm gracefully handle an
> attempt to write to userspace (well, NULL actually) with
> copy_to_kernel_nofault() so I think there still a few more changes
> to fully fix this.
>
> Looks like we would need a slightly more assertive change, either adding
> a copy_to_kernel_nofault_allowed() or modifying the arm dabt handlers to
> avoid faults on userspace access.
>
> Any views on which is better?
I've tested the copy_from_kernel_nofault_allowed() and agree that it's a 
enough simple and effective solution. There is only one little gap 
compared to other arch that it returns -ERANGE while actually it should 
be a -EFAULT (refer to other arches).

Anyway if we want to modify the FSR handlers I guess it's also easy 
because not we do nothing special for Domain Fault now.

> Daniel.
>
> >
> > > From f66a63b504ff582f261a506c54ceab8c0e77a98c Mon Sep 17 00:00:00 
> 2001
> > > From: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> > > Date: Thu, 13 Jan 2022 09:34:45 +0000
> > > Subject: [PATCH] arm: mm: Implement 
> copy_from_kernel_nofault_allowed()
> > >
> > > Currently copy_from_kernel_nofault() can actually fault (due to 
> software
> > > PAN) if we attempt userspace access. In any case, the documented
> > > behaviour for this function is to return -ERANGE if we attempt an 
> access
> > > outside of kernel space.
> > >
> > > Implementing copy_from_kernel_nofault_allowed() solves both these
> > > problems.
> > >
> > > Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
> >
> > Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>

Tested-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>

Best,

Chen