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
