CVE-2026-72135 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
tpm: Make the TPM character devices non-seekable
The TPM character devices expose a sequential command/response interface, but their open handlers leave FMODE_PREAD and FMODE_PWRITE enabled.
After a command leaves a response pending, pread(fd, buf, 16, 0x1400) passes 0x1400 as *off to tpm_common_read(). The transfer length is bounded by response_length, but the offset is used unchecked when forming data_buffer + *off. A sufficiently large offset therefore causes an out-of-bounds heap read through copy_to_user() and, if the copy succeeds, an out-of-bounds zero-write through the following memset().
Positional I/O does not provide coherent semantics for this interface. An arbitrary pread offset cannot represent how much of a response has been consumed sequentially. The write callback always stores a command at the start of data_buffer, while pwrite() does not update file->f_pos and can leave the sequential read cursor stale.
Call nonseekable_open() from both open handlers. This removes FMODE_PREAD and FMODE_PWRITE, causing positional reads and writes to fail with -ESPIPE before reaching the TPM callbacks, and explicitly marks the files non-seekable. Normal read() and write() continue to use the existing sequential f_pos cursor, leaving the response state machine unchanged.
Tested on Linux 6.12 with KASAN and a swtpm TPM2 device:
- sequential partial reads returned the complete response - pread() and preadv() with offset 0x1400 returned -ESPIPE - pwrite() and pwritev() with offset zero returned -ESPIPE - the pending response remained intact after the rejected operations - a subsequent normal command/response cycle completed normally - no KASAN report was produced.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability identified in the Linux kernel relates to improper handling of TPM character device operations, specifically concerning seekable file descriptor behavior. The TPM (Trusted Platform Module) character devices implement a sequential command/response interface that should only support ordered data access patterns. However, the implementation failed to properly restrict file descriptor operations by leaving FMODE_PREAD and FMODE_PWRITE flags enabled during the open handlers, creating potential security risks through improper input validation.
The technical flaw manifests when a user-space application attempts positional reads using pread() with arbitrary offsets. The kernel's tpm_common_read() function processes these requests without proper bounds checking on the offset parameter, allowing an attacker to specify large offset values such as 0x1400. This unchecked offset value gets used directly in calculations involving data_buffer + *off, leading to out-of-bounds heap reads through copy_to_user() operations. If successful, subsequent memset() operations may cause out-of-bounds zero-writes, potentially corrupting adjacent memory regions and affecting system stability or enabling privilege escalation attacks.
This vulnerability directly maps to CWE-129 Input Validation and CWE-787 Out-of-bounds Write categories within the Common Weakness Enumeration framework. The improper handling of file positioning creates a path for attackers to bypass normal sequential command processing, violating fundamental security assumptions about how TPM interfaces should operate. The design flaw stems from conflating sequential access semantics with random-access capabilities, which is incompatible with TPM's state machine architecture.
The operational impact of this vulnerability extends beyond simple memory corruption, potentially enabling attackers to extract sensitive information from kernel memory or manipulate TPM response states in ways that could compromise system security. Since the TPM interface handles cryptographic operations and security-critical data, such vulnerabilities could lead to broader system compromise if exploited effectively. The sequential nature of TPM commands makes proper state management critical, and this flaw undermines the integrity of command/response cycles.
The mitigation strategy implemented involves calling nonseekable_open() from both open handlers for TPM character devices, which explicitly removes FMODE_PREAD and FMODE_PWRITE flags from the file descriptor. This change ensures that any attempt to perform positional reads or writes using pread(), preadv(), pwrite(), or pwritev() operations fails immediately with -ESPIPE error codes before reaching the TPM callback functions. The solution maintains the existing sequential read/write behavior through normal read() and write() operations while preventing malicious offset-based access patterns.
The fix addresses fundamental security principles outlined in the ATT&CK framework under T1059 Command and Scripting Interpreter and T1566 Credential Access, as it prevents unauthorized memory access patterns that could be used to extract sensitive TPM data or manipulate cryptographic operations. The solution's effectiveness was validated through testing on Linux 6.12 with KASAN enabled and a swtpm TPM2 device, demonstrating that sequential partial reads still function correctly while preventing the problematic pread() behavior with large offsets. No KASAN reports were produced after implementing the fix, confirming that the out-of-bounds access patterns have been eliminated without disrupting normal TPM functionality. The approach preserves the integrity of the response state machine while explicitly marking these character devices as non-seekable, aligning with industry best practices for secure device interface design and preventing similar classes of vulnerabilities in related subsystems.