linux-kernel - Re: [PATCH] KVM: x86: Fix potential memory access error

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Message-ID: <87mtuis77m.fsf@vitty.brq.redhat.com>
Date:   Thu, 01 Apr 2021 11:08:13 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yang Li <yang.lee@...ux.alibaba.com>
Cc:     pbonzini@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Fix potential memory access error

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Mar 31, 2021, Yang Li wrote:
>> Using __set_bit() to set a bit in an integer is not a good idea, since
>> the function expects an unsigned long as argument, which can be 64bit wide.
>> Coverity reports this problem as
>> 
>> High:Out-of-bounds access(INCOMPATIBLE_CAST)
>> CWE119: Out-of-bounds access to a scalar
>> Pointer "&vcpu->arch.regs_avail" points to an object whose effective
>> type is "unsigned int" (32 bits, unsigned) but is dereferenced as a
>> wider "unsigned long" (64 bits, unsigned). This may lead to memory
>> corruption.
>> 
>> /home/heyuan.shy/git-repo/linux/arch/x86/kvm/kvm_cache_regs.h:
>> kvm_register_is_available
>> 
>> Just use BIT instead.
>
> Meh, we're hosed either way.  Using BIT() will either result in undefined
> behavior due to SHL shifting beyond the size of a u64, or setting random bits
> if the truncated shift ends up being less than 63.
>

A stupid question: why can't we just make 'regs_avail'/'regs_dirty'
'unsigned long' and drop a bunch of '(unsigned long *)' casts? 

-- 
Vitaly