Re: [PATCH v2 4/4] KVM: x86: Fix stack-out-of-bounds memory access from ioapic_write_indirect()

[Eduardo Habkost]

On Mon, Aug 23, 2021 at 04:30:28PM +0200, Vitaly Kuznetsov wrote:
> KASAN reports the following issue:
> 
>  BUG: KASAN: stack-out-of-bounds in kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
>  Read of size 8 at addr ffffc9001364f638 by task qemu-kvm/4798
> 
>  CPU: 0 PID: 4798 Comm: qemu-kvm Tainted: G               X --------- ---
>  Hardware name: AMD Corporation DAYTONA_X/DAYTONA_X, BIOS RYM0081C 07/13/2020
>  Call Trace:
>   dump_stack+0xa5/0xe6
>   print_address_description.constprop.0+0x18/0x130
>   ? kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
>   __kasan_report.cold+0x7f/0x114
>   ? kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
>   kasan_report+0x38/0x50
>   kasan_check_range+0xf5/0x1d0
>   kvm_make_vcpus_request_mask+0x174/0x440 [kvm]
>   kvm_make_scan_ioapic_request_mask+0x84/0xc0 [kvm]
>   ? kvm_arch_exit+0x110/0x110 [kvm]
>   ? sched_clock+0x5/0x10
>   ioapic_write_indirect+0x59f/0x9e0 [kvm]
>   ? static_obj+0xc0/0xc0
>   ? __lock_acquired+0x1d2/0x8c0
>   ? kvm_ioapic_eoi_inject_work+0x120/0x120 [kvm]
> 
> The problem appears to be that 'vcpu_bitmap' is allocated as a single long
> on stack and it should really be KVM_MAX_VCPUS long. We also seem to clear
> the lower 16 bits of it with bitmap_zero() for no particular reason (my
> guess would be that 'bitmap' and 'vcpu_bitmap' variables in
> kvm_bitmap_or_dest_vcpus() caused the confusion: while the later is indeed
> 16-bit long, the later should accommodate all possible vCPUs).
> 
> Fixes: 7ee30bc132c6 ("KVM: x86: deliver KVM IOAPIC scan request to target vCPUs")
> Fixes: 9a2ae9f6b6bb ("KVM: x86: Zero the IOAPIC scan request dest vCPUs bitmap")
> Reported-by: Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> ---
>  arch/x86/kvm/ioapic.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index ff005fe738a4..92cd4b02e9ba 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -319,7 +319,7 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
>  	unsigned index;
>  	bool mask_before, mask_after;
>  	union kvm_ioapic_redirect_entry *e;
> -	unsigned long vcpu_bitmap;
> +	unsigned long vcpu_bitmap[BITS_TO_LONGS(KVM_MAX_VCPUS)];

Is there a way to avoid this KVM_MAX_VCPUS-sized variable on the
stack?  This might hit us back when we increase KVM_MAX_VCPUS to
a few thousand VCPUs (I was planning to submit a patch for that
soon).


>  	int old_remote_irr, old_delivery_status, old_dest_id, old_dest_mode;
>  
>  	switch (ioapic->ioregsel) {
> @@ -384,9 +384,9 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
>  			irq.shorthand = APIC_DEST_NOSHORT;
>  			irq.dest_id = e->fields.dest_id;
>  			irq.msi_redir_hint = false;
> -			bitmap_zero(&vcpu_bitmap, 16);
> +			bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS);
>  			kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq,
> -						 &vcpu_bitmap);
> +						 vcpu_bitmap);
>  			if (old_dest_mode != e->fields.dest_mode ||
>  			    old_dest_id != e->fields.dest_id) {
>  				/*
> @@ -399,10 +399,10 @@ static void ioapic_write_indirect(struct kvm_ioapic *ioapic, u32 val)
>  				    kvm_lapic_irq_dest_mode(
>  					!!e->fields.dest_mode);
>  				kvm_bitmap_or_dest_vcpus(ioapic->kvm, &irq,
> -							 &vcpu_bitmap);
> +							 vcpu_bitmap);
>  			}
>  			kvm_make_scan_ioapic_request_mask(ioapic->kvm,
> -							  &vcpu_bitmap);
> +							  vcpu_bitmap);
>  		} else {
>  			kvm_make_scan_ioapic_request(ioapic->kvm);
>  		}
> -- 
> 2.31.1
> 

-- 
Eduardo