linux-kernel - Re: [PATCH] KVM: coalesced_mmio: Fix out-of-bounds write in coalesced_mmio

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Message-ID: <aS3mdTKmTFIpc3ye@google.com>
Date: Mon, 1 Dec 2025 11:03:17 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: redacherkaoui <redacherkaoui67@...il.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	redahack12-glitch <redahack12@...il.com>
Subject: Re: [PATCH] KVM: coalesced_mmio: Fix out-of-bounds write in coalesced_mmio_write()

On Thu, Nov 27, 2025, redacherkaoui wrote:
> From: redahack12-glitch <redahack12@...il.com>
> 
> The coalesced MMIO ring stores each entry's MMIO payload in an 8-byte
> fixed-size buffer (data[8]). However, coalesced_mmio_write() copies
> the payload using memcpy(..., len) without verifying that 'len' does not
> exceed the buffer size.
> 
> A malicious

KVM controls all callers.

> or buggy caller could therefore trigger a write past the end of the data[]
> array and corrupt adjacent kernel memory inside the ring page.

True, but if a caller is buggy, KVM likely has bigger problems because KVM relies
on MMIO (and PIO) accesses being no larger than 8 in a number of locations.  If
we want to harden KVM, kvm_iodevice_{read,write}() would be a better place for a
sanity check.