linux-kernel - Re: [PATCH v2 24/25] KVM: x86: Introduce KVM_TDX_GET

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Message-ID: <cd9eb487a433533e892f16be2a55b0abe4088456.camel@intel.com>
Date: Tue, 21 Jan 2025 23:19:20 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"francescolavra.fl@...il.com" <francescolavra.fl@...il.com>
CC: "Huang, Kai" <kai.huang@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "Chatre, Reinette" <reinette.chatre@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "tony.lindgren@...ux.intel.com"
	<tony.lindgren@...ux.intel.com>
Subject: Re: [PATCH v2 24/25] KVM: x86: Introduce KVM_TDX_GET_CPUID

On Fri, 2025-01-10 at 12:47 +0800, Xiaoyao Li wrote:
> > +		output_e = &td_cpuid->entries[i];
> > +		i += tdx_vcpu_get_cpuid_leaf(vcpu, leaf,
> > +					     KVM_MAX_CPUID_ENTRIES - i - 1,
> > +					     output_e);
> > +	}
> > +
> > +	for (leaf = 0x80000000; leaf <= 0x80000008; leaf++) {
> > +		output_e = &td_cpuid->entries[i];
> > +		i += tdx_vcpu_get_cpuid_leaf(vcpu, leaf,
> > +					     KVM_MAX_CPUID_ENTRIES - i - 1,
> > +					     output_e);
> 
> Though what gets passed in for max_cnt is
> 
>    KVM_MAX_CPUID_ENTRIES - i - 1
> 
> tdx_vcpu_get_cpuid_leaf() can return "max_cnt+1", i.e., 
> KVM_MAX_CPUID_ENTRIES - i.
> 
> Then, it makes next round i to be KVM_MAX_CPUID_ENTRIES, and
> 
>    output_e = &td_cpuid->entries[i];
> 
> will overflow the buffer and access illegal memory.
> 
> Similar issue inside tdx_vcpu_get_cpuid_leaf() as I replied in [*]
> 
> [*] 
> https://lore.kernel.org/all/7574968a-f0e2-49d5-b740-2454a0f70bb6@intel.com/

Per Francesco's comment in the other thread, I'm not sure there is an off-by-one
bug here. But in any case the code is too sensitive to issues like that.

In line with Francesco's other comment to move the subleaf checking into
tdx_read_cpuid(), I just changed it to pass around the real index and check
KVM_MAX_CPUID_ENTRIES in tdx_read_cpuid() too. It seems less elegant but easier
to read.