CVE-2026-63888 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
scsi: target: iscsi: Fix CRC overread and double-free in iscsit_handle_text_cmd()
Two latent bugs in the Text-phase handler, both present since the original LIO integration in commit e48354ce078c ("iscsi-target: Add iSCSI fabric support for target v4.1"):
1) DataDigest CRC buffer overread (4 bytes past text_in).
text_in is kzalloc()'d at ALIGN(payload_length, 4). rx_size is then incremented by ISCSI_CRC_LEN to make room for the received DataDigest in the iovec, but the same (now-bumped) rx_size is passed as the buffer length to iscsit_crc_buf():
if (conn->conn_ops->DataDigest) {
... rx_size += ISCSI_CRC_LEN; } ... if (conn->conn_ops->DataDigest) {
data_crc = iscsit_crc_buf(text_in, rx_size, 0, NULL);
iscsit_crc_buf() walks rx_size bytes of text_in with crc32c(), so when DataDigest is negotiated it reads 4 bytes past the end of the text_in allocation. KASAN reproduces this directly on the unpatched mainline tree as slab-out-of-bounds in crc32c() called from the Text PDU path. The OOB bytes feed crc32c() and are then compared against the initiator-supplied checksum, so the value does not flow back to the attacker, but the kernel does read past the buffer on every Text PDU with DataDigest=CRC32C.
Fix by passing the actual padded payload length (ALIGN(payload_length, 4)) that was used for the kzalloc().
2) Stale cmd->text_in_ptr re-free (double-free) on ERL>0 bad DataDigest drop.
On DataDigest mismatch with ErrorRecoveryLevel > 0 the handler silently drops the PDU and lets the initiator plug the CmdSN gap:
kfree(text_in); return 0;
cmd->text_in_ptr still points at the freed buffer. The next Text Request on the same ITT re-enters iscsit_setup_text_cmd(), which unconditionally does
kfree(cmd->text_in_ptr); cmd->text_in_ptr = NULL;
freeing the same pointer a second time. Session teardown via iscsit_release_cmd() has the same shape and hits the same double-free if the connection is dropped before a second Text Request arrives.
On an unmodified mainline tree the bug-1 CRC overread fires first on the initial valid Text Request and perturbs the subsequent state, so #4 was isolated by building a kernel with only the bug-1 hunk of this patch applied plus temporary printk() observability around the three relevant kfree() sites. The observability prints are not part of this patch. On that build, a three-PDU Text Request sequence after login produces two back-to-back splats:
BUG: KASAN: double-free in iscsit_setup_text_cmd+0x?? BUG: KASAN: double-free in iscsit_release_cmd+0x??
showing the same pointer freed in the ERL>0 drop path and again in iscsit_setup_text_cmd() (next Text Request on the same ITT) and once more in iscsit_release_cmd() (session teardown). On distro kernels with CONFIG_SLAB_FREELIST_HARDENED=y (default) the double-free becomes a remote kernel BUG(); on non-hardened kernels it corrupts the slab freelist.
Fix by clearing cmd->text_in_ptr after the kfree() in the ERL>0 drop path. With both hunks applied #4 is directly observable on the stock tree without observability printks; fixing bug-1 alone would mask #4 less, not more, so the hunks are submitted together.
Both fixes are one-liners. The Text PDU state machine is unchanged and the wire protocol is unaffected.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability resides within the Linux kernel's iSCSI target implementation, specifically in the SCSI target subsystem's text-phase handler for iSCSI fabric support. This flaw affects the iscsit_handle_text_cmd() function and stems from two distinct but related memory management errors that have persisted since the original LIO integration in commit e48354ce078c. The first issue manifests as a buffer overread during CRC calculation, while the second results in a double-free condition that can lead to kernel memory corruption or remote exploitation.
The primary technical flaw involves an incorrect buffer size parameter passed to the crc32c() function during DataDigest validation. When processing Text PDUs with DataDigest enabled, the system allocates memory using kzalloc() with padding to align the payload length to 4-byte boundaries. However, the code incorrectly passes the increased rx_size value—which includes space for the DataDigest—into iscsit_crc_buf(), causing a 4-byte overread beyond the allocated text_in buffer. This memory corruption occurs on every Text PDU that negotiates CRC32C DataDigest, creating a persistent vulnerability that KASAN can detect as a slab-out-of-bounds error in the crc32c() function call chain.
The second vulnerability presents a double-free condition that occurs when processing bad DataDigest values with ErrorRecoveryLevel greater than zero. When such a mismatch is detected, the system silently drops the PDU and frees the text_in buffer without clearing the corresponding cmd->text_in_ptr pointer. Subsequent Text Requests using the same ITT (Initiator Task Tag) re-enter the command setup routine where the stale pointer gets freed again, followed by session teardown also attempting to free the same memory location. This creates a cascading double-free scenario that can be triggered through normal iSCSI session operations and is particularly dangerous in hardened kernel configurations where such corruption results in immediate kernel BUG() conditions rather than silent memory corruption.
Both vulnerabilities are classified under CWE-125 (Out-of-bounds Read) and CWE-415 (Double Free) respectively, and align with ATT&CK techniques involving privilege escalation through kernel memory corruption. The fix addresses the overread by using the correct buffer size parameter that matches the actual allocation size, while the double-free is resolved by clearing the command pointer after freeing the buffer. These one-line patches preserve all existing functionality while eliminating the memory safety issues, ensuring that iSCSI target operations remain stable and secure without affecting wire protocol behavior or the underlying text-phase state machine logic.
The operational impact of these vulnerabilities extends beyond simple memory corruption, as they create potential attack vectors for remote kernel exploitation. The overread condition can cause system instability through memory access violations, while the double-free opens possibilities for heap-based attacks that could be leveraged to gain unauthorized privileges. System administrators should prioritize applying these patches to maintain kernel stability and security, particularly in environments where iSCSI target services are actively used or exposed to untrusted network traffic. The vulnerabilities affect all Linux kernel versions containing the LIO iSCSI target implementation and demonstrate the importance of careful memory management in kernel subsystems handling network protocol processing.