Re: BUG: sleeping function called from invalid context at ./include/linux/uaccess.h:LINE

Dmitry Vyukov
2017-11-06T19:05:21Z

On Mon, Nov 6, 2017 at 5:14 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 06/11/2017 17:01, David Hildenbrand wrote:
>> On 06.11.2017 16:10, Nick Desaulniers wrote:
>>> Does it have to be stack allocated?
>>
>> We can't use kmalloc and friends in emulate.c. We would have to
>> introduce new emulator callbacks.
>>
>> a) for malloc and free. hmmm.
>> b) for carrying out the fxrstr/fixup.
>>
>> Paolo, what do you suggest?
>
> You can use kmalloc. Any userspace user of emulate.c would have to
> write a wrapper.

Can you please tell me more about this? Is it used for testing? Is
there an example code that builds and tests this in user-space?

> But I'm not sure it's useful... maybe the
> asm_safe+memcpy could be moved to a separate noinline function, so that
> segmented_read_std is invoked with a leaner stack.
>
> Paolo
>
>>>
>>> On Nov 6, 2017 3:52 AM, "David Hildenbrand" <david@redhat.com
>>> <mailto:david@redhat.com>> wrote:
>>>
>>> On 31.10.2017 12:34, syzbot wrote:
>>> > Hello,
>>> >
>>> > syzkaller hit the following crash on
>>> > 91dfed74eabcdae9378131546c446442c29bf769
>>> >
>>> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
>>> <http://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master>
>>> > compiler: gcc (GCC) 7.1.1 20170620
>>> > .config is attached
>>> > Raw console output is attached.
>>> > C reproducer is attached
>>> > syzkaller reproducer is attached. See https://goo.gl/kgGztJ
>>> > for information about syzkaller reproducers
>>> >
>>> >
>>> > in_atomic(): 1, irqs_disabled(): 0, pid: 2909, name: syzkaller879109
>>> > 2 locks held by syzkaller879109/2909:
>>> > #0: (&vcpu->mutex){+.+.}, at: [<ffffffff8106222c>]
>>> vcpu_load+0x1c/0x70
>>> > arch/x86/kvm/../../../virt/kvm/kvm_main.c:154
>>> > #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>] vcpu_enter_guest
>>> > arch/x86/kvm/x86.c:6983 [inline]
>>> > #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>] vcpu_run
>>> > arch/x86/kvm/x86.c:7061 [inline]
>>> > #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>]
>>> > kvm_arch_vcpu_ioctl_run+0x1bc2/0x58b0 arch/x86/kvm/x86.c:7222
>>> > CPU: 1 PID: 2909 Comm: syzkaller879109 Not tainted
>>> 4.13.0-rc4-next-20170811
>>> > #1
>>> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
>>> 01/01/2011
>>> > Call Trace:
>>> > __dump_stack lib/dump_stack.c:16 [inline]
>>> > dump_stack+0x194/0x257 lib/dump_stack.c:52
>>> > ___might_sleep+0x2b2/0x470 kernel/sched/core.c:6014
>>> > __might_sleep+0x95/0x190 kernel/sched/core.c:5967
>>> > __might_fault+0xab/0x1d0 mm/memory.c:4383
>>> > __copy_from_user include/linux/uaccess.h:71 [inline]
>>> > __kvm_read_guest_page+0x58/0xa0
>>> > arch/x86/kvm/../../../virt/kvm/kvm_main.c:1771
>>> > kvm_vcpu_read_guest_page+0x44/0x60
>>> > arch/x86/kvm/../../../virt/kvm/kvm_main.c:1791
>>> > kvm_read_guest_virt_helper+0x76/0x140 arch/x86/kvm/x86.c:4407
>>> > kvm_read_guest_virt_system+0x3c/0x50 arch/x86/kvm/x86.c:4466
>>> > segmented_read_std+0x10c/0x180 arch/x86/kvm/emulate.c:819
>>> > em_fxrstor+0x27b/0x410 arch/x86/kvm/emulate.c:4022
>>>
>>>
>>> In em_fxrstor, we do a get_fpu(), which in return disables preemption.
>>>
>>> With preempt_disable(), we do a
>>> segmented_read_std()->kvm_vcpu_read_guest_page(), triggering the
>>> warning.
>>>
>>> > x86_emulate_insn+0x55d/0x3c50 arch/x86/kvm/emulate.c:5471
>>> > x86_emulate_instruction+0x411/0x1ca0 arch/x86/kvm/x86.c:5698
>>> > kvm_mmu_page_fault+0x18b/0x2c0 arch/x86/kvm/mmu.c:4854
>>> > handle_ept_violation+0x1fc/0x5e0 arch/x86/kvm/vmx.c:6400
>>> > vmx_handle_exit+0x281/0x1ab0 arch/x86/kvm/vmx.c:8718
>>> > vcpu_enter_guest arch/x86/kvm/x86.c:6999 [inline]
>>> > vcpu_run arch/x86/kvm/x86.c:7061 [inline]
>>> > kvm_arch_vcpu_ioctl_run+0x1cee/0x58b0 arch/x86/kvm/x86.c:7222
>>> > kvm_vcpu_ioctl+0x64c/0x1010
>>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:2591
>>> > vfs_ioctl fs/ioctl.c:45 [inline]
>>> > do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:685
>>> > SYSC_ioctl fs/ioctl.c:700 [inline]
>>> > SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
>>> > entry_SYSCALL_64_fastpath+0x1f/0xbe
>>>
>>> I don't really see a way to avoid two fxstate variables. Unloading the
>>> fpu in between the fxstore/fxrstr could lead to host values getting
>>> overwritten. Loading/saving the fpu in kvm_arch_vcpu_ioctl_run() would
>>> most probably also not work, as the relevant portions of fxregs_state
>>> would not get saved/restored. So the preemption would still be needed.
>>>
>>>
>>> So all I can offer for now is the following (untested, can send as
>>> proper patch if needed):
>>>
>>>
>>> From f32d06c8c621c6d68e073e9bdb81a6280b6c9544 Mon Sep 17 00:00:00 2001
>>> From: David Hildenbrand <david@redhat.com <mailto:david@redhat.com>>
>>> Date: Mon, 6 Nov 2017 12:35:39 +0100
>>> Subject: [PATCH v1] KVM: x86: fix em_fxstor sleeping while in atomic
>>>
>>> Commit 9d643f63128b tried to optimize the stack size, but introduced a
>>> guest memory access which might sleep while in atomic.
>>>
>>> Let's undo that part of the commit but keep the cleanups.
>>>
>>> Reported by syzbot:
>>>
>>> in_atomic(): 1, irqs_disabled(): 0, pid: 2909, name: syzkaller879109
>>> 2 locks held by syzkaller879109/2909:
>>> #0: (&vcpu->mutex){+.+.}, at: [<ffffffff8106222c>]
>>> vcpu_load+0x1c/0x70
>>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:154
>>> #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>] vcpu_enter_guest
>>> arch/x86/kvm/x86.c:6983 [inline]
>>> #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>] vcpu_run
>>> arch/x86/kvm/x86.c:7061 [inline]
>>> #1: (&kvm->srcu){....}, at: [<ffffffff810dd162>]
>>> kvm_arch_vcpu_ioctl_run+0x1bc2/0x58b0 arch/x86/kvm/x86.c:7222
>>> CPU: 1 PID: 2909 Comm: syzkaller879109 Not tainted
>>> 4.13.0-rc4-next-20170811
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs
>>> 01/01/2011
>>> Call Trace:
>>> __dump_stack lib/dump_stack.c:16 [inline]
>>> dump_stack+0x194/0x257 lib/dump_stack.c:52
>>> ___might_sleep+0x2b2/0x470 kernel/sched/core.c:6014
>>> __might_sleep+0x95/0x190 kernel/sched/core.c:5967
>>> __might_fault+0xab/0x1d0 mm/memory.c:4383
>>> __copy_from_user include/linux/uaccess.h:71 [inline]
>>> __kvm_read_guest_page+0x58/0xa0
>>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1771
>>> kvm_vcpu_read_guest_page+0x44/0x60
>>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1791
>>> kvm_read_guest_virt_helper+0x76/0x140 arch/x86/kvm/x86.c:4407
>>> kvm_read_guest_virt_system+0x3c/0x50 arch/x86/kvm/x86.c:4466
>>> segmented_read_std+0x10c/0x180 arch/x86/kvm/emulate.c:819
>>> em_fxrstor+0x27b/0x410 arch/x86/kvm/emulate.c:4022
>>> x86_emulate_insn+0x55d/0x3c50 arch/x86/kvm/emulate.c:5471
>>> x86_emulate_instruction+0x411/0x1ca0 arch/x86/kvm/x86.c:5698
>>> kvm_mmu_page_fault+0x18b/0x2c0 arch/x86/kvm/mmu.c:4854
>>> handle_ept_violation+0x1fc/0x5e0 arch/x86/kvm/vmx.c:6400
>>> vmx_handle_exit+0x281/0x1ab0 arch/x86/kvm/vmx.c:8718
>>> vcpu_enter_guest arch/x86/kvm/x86.c:6999 [inline]
>>> vcpu_run arch/x86/kvm/x86.c:7061 [inline]
>>> kvm_arch_vcpu_ioctl_run+0x1cee/0x58b0 arch/x86/kvm/x86.c:7222
>>> kvm_vcpu_ioctl+0x64c/0x1010
>>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:2591
>>> vfs_ioctl fs/ioctl.c:45 [inline]
>>> do_vfs_ioctl+0x1b1/0x1520 fs/ioctl.c:685
>>> SYSC_ioctl fs/ioctl.c:700 [inline]
>>> SyS_ioctl+0x8f/0xc0 fs/ioctl.c:691
>>> entry_SYSCALL_64_fastpath+0x1f/0xbe
>>> RIP: 0033:0x437fc9
>>> RSP: 002b:00007ffc7b4d5ab8 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
>>> RAX: ffffffffffffffda RBX: 00000000004002b0 RCX: 0000000000437fc9
>>> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005
>>> RBP: 0000000000000086 R08: 0000000000000000 R09: 0000000020ae8000
>>> R10: 0000000000009120 R11: 0000000000000206 R12: 0000000000000000
>>> R13: 0000000000000004 R14: 0000000000000004 R15: 0000000020077000
>>>
>>> Fixes: 9d643f63128b ("KVM: x86: avoid large stack allocations in
>>> em_fxrstor")
>>> Signed-off-by: David Hildenbrand <david@redhat.com
>>> <mailto:david@redhat.com>>
>>> ---
>>> arch/x86/kvm/emulate.c | 16 +++++++++-------
>>> 1 file changed, 9 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>>> index fb0055953fbc..d87f01a2d6f4 100644
>>> --- a/arch/x86/kvm/emulate.c
>>> +++ b/arch/x86/kvm/emulate.c
>>> @@ -4002,7 +4002,7 @@ static int em_fxsave(struct x86_emulate_ctxt
>>> *ctxt)
>>>
>>> static int em_fxrstor(struct x86_emulate_ctxt *ctxt)
>>> {
>>> - struct fxregs_state fx_state;
>>> + struct fxregs_state fx_state, fx_old;
>>> int rc;
>>> size_t size;
>>>
>>> @@ -4010,19 +4010,21 @@ static int em_fxrstor(struct
>>> x86_emulate_ctxt *ctxt)
>>> if (rc != X86EMUL_CONTINUE)
>>> return rc;
>>>
>>> + size = fxstate_size(ctxt);
>>> + rc = segmented_read_std(ctxt, ctxt->memop.addr.mem,
>>> &fx_state, size);
>>> + if (rc != X86EMUL_CONTINUE)
>>> + return rc;
>>> +
>>> ctxt->ops->get_fpu(ctxt);
>>>
>>> - size = fxstate_size(ctxt);
>>> if (size < __fxstate_size(16)) {
>>> - rc = asm_safe("fxsave %[fx]", , [fx] "+m"(fx_state));
>>> + rc = asm_safe("fxsave %[fx]", , [fx] "+m"(fx_old));
>>> if (rc != X86EMUL_CONTINUE)
>>> goto out;
>>> + memcpy(((void *)&fx_state) + size, ((void *)&fx_old)
>>> + size,
>>> + __fxstate_size(16) - size);
>>> }
>>>
>>> - rc = segmented_read_std(ctxt, ctxt->memop.addr.mem,
>>> &fx_state, size);
>>> - if (rc != X86EMUL_CONTINUE)
>>> - goto out;
>>> -
>>> if (fx_state.mxcsr >> 16) {
>>> rc = emulate_gp(ctxt, 0);
>>> goto out;
>>> --
>>> 2.13.6
>>>
>>>
>>>
>>> --
>>>
>>> Thanks,
>>>
>>> David / dhildenb
>>>
>>
>>
>
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