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

David Hildenbrand
2017-11-06T11:52:03Z

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
> 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>
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>
---
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