Re: [syzbot] [bpf?] [trace?] possible deadlock in force_sig_info_to_task

[Hillf Danton <hdanton@sina.com>]

On Sun, 28 Apr 2024 13:01:19 -0700 Linus Torvalds wrote:
> On Sat, 27 Apr 2024 at 16:13, Hillf Danton <hdanton@sina.com> wrote:
> >
> > > -> #0 (&sighand->siglock){....}-{2:2}:
> > >        check_prev_add kernel/locking/lockdep.c:3134 [inline]
> > >        check_prevs_add kernel/locking/lockdep.c:3253 [inline]
> > >        validate_chain+0x18cb/0x58e0 kernel/locking/lockdep.c:3869
> > >        __lock_acquire+0x1346/0x1fd0 kernel/locking/lockdep.c:5137
> > >        lock_acquire+0x1ed/0x550 kernel/locking/lockdep.c:5754
> > >        __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> > >        _raw_spin_lock_irqsave+0xd5/0x120 kernel/locking/spinlock.c:162
> > >        force_sig_info_to_task+0x68/0x580 kernel/signal.c:1334
> > >        force_sig_fault_to_task kernel/signal.c:1733 [inline]
> > >        force_sig_fault+0x12c/0x1d0 kernel/signal.c:1738
> > >        __bad_area_nosemaphore+0x127/0x780 arch/x86/mm/fault.c:814
> > >        handle_page_fault arch/x86/mm/fault.c:1505 [inline]
> >
> > Given page fault with runqueue locked, bpf makes trouble instead of
> > helping anything in this case.
> 
> That's not the odd thing here.
> 
> Look, the callchain is:
> 
> > >        exc_page_fault+0x612/0x8e0 arch/x86/mm/fault.c:1563
> > >        asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623
> > >        rep_movs_alternative+0x22/0x70 arch/x86/lib/copy_user_64.S:48
> > >        copy_user_generic arch/x86/include/asm/uaccess_64.h:110 [inline]
> > >        raw_copy_from_user arch/x86/include/asm/uaccess_64.h:125 [inline]
> > >        __copy_from_user_inatomic include/linux/uaccess.h:87 [inline]
> > >        copy_from_user_nofault+0xbc/0x150 mm/maccess.c:125
> 
> IOW, this is all doing a copy from user with page faults disabled, and
> it shouldn't have caused a signal to be sent, so the whole
> __bad_area_nosemaphore -> force_sig_fault path is bad.
> 
So is game like copying from/putting to user with runqueue locked
at the first place.

Plus as per another syzbot report [1], bpf could make trouble with
workqueue pool locked.

[1] https://lore.kernel.org/lkml/00000000000051348606171f61a1@google.com/