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Paolo Bonzini
4578a747f3
In function kvm_guest_time_update(), __scale_tsc() is used to calculate
a TSC *frequency* rather than a TSC value. With low-enough ratios,
a TSC value that is less than 1 would underflow to 0 and to an infinite
while loop in kvm_get_time_scale():
kvm_guest_time_update(struct kvm_vcpu *v)
if (kvm_caps.has_tsc_control)
tgt_tsc_khz = kvm_scale_tsc(tgt_tsc_khz,
v->arch.l1_tsc_scaling_ratio);
__scale_tsc(u64 ratio, u64 tsc)
ratio=122380531, tsc=2299998, N=48
ratio*tsc >> N = 0.999... -> 0
Later in the function:
Call Trace:
<TASK>
kvm_get_time_scale arch/x86/kvm/x86.c:2458 [inline]
kvm_guest_time_update+0x926/0xb00 arch/x86/kvm/x86.c:3268
vcpu_enter_guest.constprop.0+0x1e70/0x3cf0 arch/x86/kvm/x86.c:10678
vcpu_run+0x129/0x8d0 arch/x86/kvm/x86.c:11126
kvm_arch_vcpu_ioctl_run+0x37a/0x13d0 arch/x86/kvm/x86.c:11352
kvm_vcpu_ioctl+0x56b/0xe60 virt/kvm/kvm_main.c:4188
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0x12d/0x190 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x59/0x110 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x78/0xe2
This can really happen only when fuzzing, since the TSC frequency
would have to be nonsensically low.
Fixes: 35181e86df ("KVM: x86: Add a common TSC scaling function")
Reported-by: Yuntao Liu <liuyuntao12@huawei.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.