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Technical
CVE-2026-64149 is a vulnerability in the Linux kernel’s DMA mapping subsystem, specifically within the `dma_map_resource()` function. This issue arises from an incorrect memory validation approach that uses `pfn_valid()` to determine whether a physical frame number (PFN) represents actual RAM or a memory-mapped I/O (MMIO) region.
The core of the problem lies in the assumption that `pfn_valid()` is sufficient for this distinction. While `pfn_valid()` checks for the availability of a memory map for a PFN, it does not guarantee that the PFN is actually backed by RAM. This becomes a critical flaw on architectures like ARM64 that use the SPARSEMEM memory model with a 128MB section granularity. In such configurations, MMIO addresses can share a memory section with actual RAM.
A real-world example of this is on the Raspberry Pi 4. During the `spi_bcm2835` driver probe, the SPI FIFO register at address `0xfe204004` was found to fall within the same sparsemem section (section 31, ranging from `0xf8000000` to 0xffffffff) as the end of RAM (0xf8000000 to 0xfbffffff). This architectural overlap caused `pfn_valid()` to return a false positive for the MMIO address.
Consequently, the kernel would incorrectly flag the legitimate MMIO region as problematic, triggering a `WARN_ON_ONCE` and causing `dma_map_resource()` to return DMA_MAPPING_ERROR. This leads to a failure in the device’s initialization process. The vulnerability affects not just the Raspberry Pi 4 but any ARM64 system with SPARSEMEM where MMIO regions share memory sections with RAM.
The fix, which has been applied, moves the sanity check from `dma_map_resource()` into the debug function debug_dma_map_phys(). It replaces the unreliable `pfn_valid()` check with pfn_valid() && !PageReserved(), which correctly identifies actual usable RAM by also verifying that the page is not reserved. This prevents false positives for MMIO regions that happen to have struct pages.
DailyCVE Form
Platform: Linux Kernel
Version: 6.18 to 6.18.34
Vulnerability: DMA Mapping Logic
Severity: Medium
date: 2026-07-19
Prediction: 2026-08-15
What Undercode Say:
Analytics
Check if the system is vulnerable by inspecting the kernel version uname -r Check for the presence of the vulnerable commit git log --oneline | grep f7326196a781622b33bfbdabb00f5e72b5fb5679 Check for the fix commit git log --oneline | grep 181e67bc11c5ec5b87c6c512c2078752b23ca8d4
Exploit: (Educational Purposes!)
A local user can trigger this vulnerability by causing the kernel to map an MMIO address that shares a sparsemem section with RAM. On a vulnerable system, this can be simulated by attempting to use a device driver that performs DMA mapping on such an address, like the `spi_bcm2835` driver on a Raspberry Pi 4. The driver’s probe will fail, and the kernel log will show a `WARNING` and a DMA_MAPPING_ERROR.
// This is a simplified representation of the vulnerable code path.
// In a real scenario, this is triggered by a device driver.
dma_addr_t dma_handle;
dma_handle = dma_map_resource(dev, phys_addr, size, dir, attrs);
if (dma_mapping_error(dev, dma_handle)) {
// Handle the error, which would happen on a vulnerable system.
pr_err("DMA mapping failed!\n");
}
Protection
To protect against this vulnerability, it is essential to update the Linux kernel to a version that includes the fix. The vulnerability is fixed in the following kernel versions:
6.18.34 (with commit `181e67bc11c5ec5b87c6c512c2078752b23ca8d4`)
7.0.11 (with commit `004a777879ff629f6e0ca3d09ad09fa3452bcc4d`)
7.1 (with commit `af0c3f05866237f7592219bfe05387bc3bfc99b5`)
Users should apply the latest security updates from their distribution’s repository.
Impact
The primary impact of this vulnerability is a denial of service. It can cause device initialization to fail, leading to hardware being unavailable or non-functional. This is particularly disruptive for systems relying on affected drivers for critical operations. The issue is triggered locally, requiring a user to have access to the system to initiate the vulnerable code path. The vulnerability does not allow for privilege escalation or arbitrary code execution.
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Sources:
Reported By: nvd.nist.gov
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