CVE-2026-63888
Received
Received - Intake
Heap-based Double-Free in Linux Kernel iSCSI Target
Vulnerability report for CVE-2026-63888, including description, CVSS score, EPSS score, affected products, exploitability, helpful resources, and attack-flow context.
Publication date: 2026-07-19
Last updated on: 2026-07-19
Assigner: kernel.org
Description
Description
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.
CVSS Scores
EPSS Scores
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Meta Information
Affected Vendors & Products
| Vendor | Product | Version / Range |
|---|---|---|
| linux_kernel | linux_kernel | * |
Helpful Resources
Exploitability
CWE
KEV
| CWE ID | Description |
|---|---|
| CWE-UNKNOWN |
