Patch "KVM: s390: fix setting of fpc register" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    KVM: s390: fix setting of fpc register

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     kvm-s390-fix-setting-of-fpc-register.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 9f62ccb74f6c467c52435b3323b97f2f92308523
Author: Heiko Carstens <hca@xxxxxxxxxxxxx>
Date:   Thu Nov 30 18:56:00 2023 +0100

    KVM: s390: fix setting of fpc register
    
    [ Upstream commit b988b1bb0053c0dcd26187d29ef07566a565cf55 ]
    
    kvm_arch_vcpu_ioctl_set_fpu() allows to set the floating point control
    (fpc) register of a guest cpu. The new value is tested for validity by
    temporarily loading it into the fpc register.
    
    This may lead to corruption of the fpc register of the host process:
    if an interrupt happens while the value is temporarily loaded into the fpc
    register, and within interrupt context floating point or vector registers
    are used, the current fp/vx registers are saved with save_fpu_regs()
    assuming they belong to user space and will be loaded into fp/vx registers
    when returning to user space.
    
    test_fp_ctl() restores the original user space / host process fpc register
    value, however it will be discarded, when returning to user space.
    
    In result the host process will incorrectly continue to run with the value
    that was supposed to be used for a guest cpu.
    
    Fix this by simply removing the test. There is another test right before
    the SIE context is entered which will handles invalid values.
    
    This results in a change of behaviour: invalid values will now be accepted
    instead of that the ioctl fails with -EINVAL. This seems to be acceptable,
    given that this interface is most likely not used anymore, and this is in
    addition the same behaviour implemented with the memory mapped interface
    (replace invalid values with zero) - see sync_regs() in kvm-s390.c.
    
    Reviewed-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
    Reviewed-by: Claudio Imbrenda <imbrenda@xxxxxxxxxxxxx>
    Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
    Signed-off-by: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index eb97db59b236..5526f782249c 100644
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -3700,10 +3700,6 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
 
 	vcpu_load(vcpu);
 
-	if (test_fp_ctl(fpu->fpc)) {
-		ret = -EINVAL;
-		goto out;
-	}
 	vcpu->run->s.regs.fpc = fpu->fpc;
 	if (MACHINE_HAS_VX)
 		convert_fp_to_vx((__vector128 *) vcpu->run->s.regs.vrs,
@@ -3711,7 +3707,6 @@ int kvm_arch_vcpu_ioctl_set_fpu(struct kvm_vcpu *vcpu, struct kvm_fpu *fpu)
 	else
 		memcpy(vcpu->run->s.regs.fprs, &fpu->fprs, sizeof(fpu->fprs));
 
-out:
 	vcpu_put(vcpu);
 	return ret;
 }