linux-kernel - Re: [PATCH] kvm: Fix warning in__kvm_gpc

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Date: Wed, 26 Jun 2024 21:32:58 -0700
From: Pei Li <peili.dev@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: David Woodhouse <dwmw2@...radead.org>, Paul Durrant <paul@....org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, skhan@...uxfoundation.org,
	linux-kernel-mentees@...ts.linuxfoundation.org,
	syzkaller-bugs@...glegroups.com, llvm@...ts.linux.dev,
	syzbot+fd555292a1da3180fc82@...kaller.appspotmail.com
Subject: Re: [PATCH] kvm: Fix warning in__kvm_gpc_refresh

On Wed, Jun 26, 2024 at 01:08:47PM -0700, Sean Christopherson wrote:
> On Tue, Jun 25, 2024, Pei Li wrote:
> > Check for invalid hva address stored in data before calling
> > kvm_gpc_activate_hva() instead of only compare with 0.
> > 
> > Reported-by: syzbot+fd555292a1da3180fc82@...kaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=fd555292a1da3180fc82
> > Tested-by: syzbot+fd555292a1da3180fc82@...kaller.appspotmail.com
> > Signed-off-by: Pei Li <peili.dev@...il.com>
> > ---
> > Syzbot reports a warning message in __kvm_gpc_refresh(). This warning
> > requires at least one of gpa and uhva to be valid.
> > WARNING: CPU: 0 PID: 5090 at arch/x86/kvm/../../../virt/kvm/pfncache.c:259 __kvm_gpc_refresh+0xf17/0x1090 arch/x86/kvm/../../../virt/kvm/pfncache.c:259
> > 
> > We are calling it from kvm_gpc_activate_hva(). This function always calls
> > __kvm_gpc_activate() with INVALID_GPA. Thus, uhva must be valid to
> > disable this warning.
> > 
> > This patch checks for invalid hva address as well instead of only
> > comparing hva with 0 before calling kvm_gpc_activate_hva()
> > 
> > syzbot has tested the proposed patch and the reproducer did not trigger
> > any issue.
> > 
> > Tested on:
> > 
> > commit:         55027e68 Merge tag 'input-for-v6.10-rc5' of git://git...
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=16ea803a980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=e40800950091403a
> > dashboard link: https://syzkaller.appspot.com/bug?extid=fd555292a1da3180fc82
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > patch:          https://syzkaller.appspot.com/x/patch.diff?x=16eeb53e980000
> > 
> > Note: testing is done by a robot and is best-effort only.
> > ---
> >  arch/x86/kvm/xen.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> > index f65b35a05d91..de5f34492405 100644
> > --- a/arch/x86/kvm/xen.c
> > +++ b/arch/x86/kvm/xen.c
> > @@ -881,7 +881,7 @@ int kvm_xen_vcpu_set_attr(struct kvm_vcpu *vcpu, struct kvm_xen_vcpu_attr *data)
> >  			r = kvm_gpc_activate(&vcpu->arch.xen.vcpu_info_cache,
> >  					     data->u.gpa, sizeof(struct vcpu_info));
> >  		} else {
> > -			if (data->u.hva == 0) {
> > +			if (data->u.hva == 0 || kvm_is_error_hva(data->u.hva)) {
> >  				kvm_gpc_deactivate(&vcpu->arch.xen.vcpu_info_cache);
> >  				r = 0;
> >  				break;
> 
> Hmm, I think what we want is to return -EINVAL in this case, not deactivate the
> region.   I could have sworn KVM does that.  Gah, I caught
> KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA during review, but missed this one.  So to fix
> this immediate bug, and avoid similar issues in the future, this?
> 
> diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
> index 93814d3850eb..622fe24da910 100644
> --- a/arch/x86/kvm/xen.c
> +++ b/arch/x86/kvm/xen.c
> @@ -741,7 +741,7 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
>                 } else {
>                         void __user * hva = u64_to_user_ptr(data->u.shared_info.hva);
>  
> -                       if (!PAGE_ALIGNED(hva) || !access_ok(hva, PAGE_SIZE)) {
> +                       if (!PAGE_ALIGNED(hva)) {
>                                 r = -EINVAL;
>                         } else if (!hva) {
>                                 kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
> diff --git a/virt/kvm/pfncache.c b/virt/kvm/pfncache.c
> index 0ab90f45db37..728d2c1b488a 100644
> --- a/virt/kvm/pfncache.c
> +++ b/virt/kvm/pfncache.c
> @@ -438,6 +438,9 @@ int kvm_gpc_activate(struct gfn_to_pfn_cache *gpc, gpa_t gpa, unsigned long len)
>  
>  int kvm_gpc_activate_hva(struct gfn_to_pfn_cache *gpc, unsigned long uhva, unsigned long len)
>  {
> +       if (!access_ok((void __user *)uhva, len))
> +               return -EINVAL;
> +
>         return __kvm_gpc_activate(gpc, INVALID_GPA, uhva, len);
>  }
> 
Thanks Sean. I’ll test and work on v2 based on your suggestions.

Best regards,

Pei