From: Claudio Imbrenda imbrenda@linux.ibm.com
commit c0573ba5c5a2244dc02060b1f374d4593c1d20b7 upstream.
When handling the SCK instruction, the kvm lock is taken, even though the vcpu lock is already being held. The normal locking order is kvm lock first and then vcpu lock. This is can (and in some circumstances does) lead to deadlocks.
The function kvm_s390_set_tod_clock is called both by the SCK handler and by some IOCTLs to set the clock. The IOCTLs will not hold the vcpu lock, so they can safely take the kvm lock. The SCK handler holds the vcpu lock, but will also somehow need to acquire the kvm lock without relinquishing the vcpu lock.
The solution is to factor out the code to set the clock, and provide two wrappers. One is called like the original function and does the locking, the other is called kvm_s390_try_set_tod_clock and uses trylock to try to acquire the kvm lock. This new wrapper is then used in the SCK handler. If locking fails, -EAGAIN is returned, which is eventually propagated to userspace, thus also freeing the vcpu lock and allowing for forward progress.
This is not the most efficient or elegant way to solve this issue, but the SCK instruction is deprecated and its performance is not critical.
The goal of this patch is just to provide a simple but correct way to fix the bug.
Fixes: 6a3f95a6b04c ("KVM: s390: Intercept SCK instruction") Signed-off-by: Claudio Imbrenda imbrenda@linux.ibm.com Reviewed-by: Christian Borntraeger borntraeger@linux.ibm.com Reviewed-by: Janis Schoetterl-Glausch scgl@linux.ibm.com Link: https://lore.kernel.org/r/20220301143340.111129-1-imbrenda@linux.ibm.com Cc: stable@vger.kernel.org Signed-off-by: Christian Borntraeger borntraeger@linux.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- arch/s390/kvm/kvm-s390.c | 19 ++++++++++++++++--- arch/s390/kvm/kvm-s390.h | 4 ++-- arch/s390/kvm/priv.c | 15 ++++++++++++++- 3 files changed, 32 insertions(+), 6 deletions(-)
--- a/arch/s390/kvm/kvm-s390.c +++ b/arch/s390/kvm/kvm-s390.c @@ -3869,14 +3869,12 @@ retry: return 0; }
-void kvm_s390_set_tod_clock(struct kvm *kvm, - const struct kvm_s390_vm_tod_clock *gtod) +static void __kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) { struct kvm_vcpu *vcpu; union tod_clock clk; unsigned long i;
- mutex_lock(&kvm->lock); preempt_disable();
store_tod_clock_ext(&clk); @@ -3897,7 +3895,22 @@ void kvm_s390_set_tod_clock(struct kvm *
kvm_s390_vcpu_unblock_all(kvm); preempt_enable(); +} + +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) +{ + mutex_lock(&kvm->lock); + __kvm_s390_set_tod_clock(kvm, gtod); + mutex_unlock(&kvm->lock); +} + +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod) +{ + if (!mutex_trylock(&kvm->lock)) + return 0; + __kvm_s390_set_tod_clock(kvm, gtod); mutex_unlock(&kvm->lock); + return 1; }
/** --- a/arch/s390/kvm/kvm-s390.h +++ b/arch/s390/kvm/kvm-s390.h @@ -349,8 +349,8 @@ int kvm_s390_handle_sigp(struct kvm_vcpu int kvm_s390_handle_sigp_pei(struct kvm_vcpu *vcpu);
/* implemented in kvm-s390.c */ -void kvm_s390_set_tod_clock(struct kvm *kvm, - const struct kvm_s390_vm_tod_clock *gtod); +void kvm_s390_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod); +int kvm_s390_try_set_tod_clock(struct kvm *kvm, const struct kvm_s390_vm_tod_clock *gtod); long kvm_arch_fault_in_page(struct kvm_vcpu *vcpu, gpa_t gpa, int writable); int kvm_s390_store_status_unloaded(struct kvm_vcpu *vcpu, unsigned long addr); int kvm_s390_vcpu_store_status(struct kvm_vcpu *vcpu, unsigned long addr); --- a/arch/s390/kvm/priv.c +++ b/arch/s390/kvm/priv.c @@ -102,7 +102,20 @@ static int handle_set_clock(struct kvm_v return kvm_s390_inject_prog_cond(vcpu, rc);
VCPU_EVENT(vcpu, 3, "SCK: setting guest TOD to 0x%llx", gtod.tod); - kvm_s390_set_tod_clock(vcpu->kvm, >od); + /* + * To set the TOD clock the kvm lock must be taken, but the vcpu lock + * is already held in handle_set_clock. The usual lock order is the + * opposite. As SCK is deprecated and should not be used in several + * cases, for example when the multiple epoch facility or TOD clock + * steering facility is installed (see Principles of Operation), a + * slow path can be used. If the lock can not be taken via try_lock, + * the instruction will be retried via -EAGAIN at a later point in + * time. + */ + if (!kvm_s390_try_set_tod_clock(vcpu->kvm, >od)) { + kvm_s390_retry_instr(vcpu); + return -EAGAIN; + }
kvm_s390_set_psw_cc(vcpu, 0); return 0;