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Date: Mon, 6 Nov 2017 17:14:35 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: David Hildenbrand <david@...hat.com>,
Nick Desaulniers <nick.desaulniers@...il.com>
Cc: syzbot
<bot+eb276410a47365e399ee50d68e73650cac178e0f@...kaller.appspotmail.com>,
borntraeger@...ibm.com, hpa@...or.com, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, lstoakes@...il.com, mhocko@...e.com,
mingo@...hat.com, rkrcmar@...hat.com,
syzkaller-bugs@...glegroups.com, tglx@...utronix.de, x86@...nel.org
Subject: Re: BUG: sleeping function called from invalid context at
./include/linux/uaccess.h:LINE
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. 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@...hat.com
>> <mailto:david@...hat.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@...hat.com <mailto:david@...hat.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@...hat.com
>> <mailto:david@...hat.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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