CVE-2026-98479: scsi: libiscsi_tcp: Check the data direction of a Data-In PDU
In the Linux kernel, the following vulnerability has been resolved:
scsi: libiscsitcp: Check the data direction of a Data-In PDU
The Data-In branch of iscsitcphdrdissect() resolves the ITT to a task and copies the PDU's data segment into that command's scatterlist without asking whether the command was reading. iscsitcpr2trsp() in the same file does ask, and rejects an R2T for a command that is not DMATODEVICE.
A target that answers a WRITE command's ITT with a Data-In therefore has the initiator write target-supplied bytes into the pages that write was about to send. Those are the caller's own pinned pages for an ODIRECT write, and page cache pages for a buffered one.
Observed against a test target that emits one 512-byte Data-In naming a 128 KB write's ITT, after the R2T for that write. With ODIRECT the caller's buffer ends up holding 512 bytes of the target's data while pwrite() returns 131072. Buffered is quieter: pwrite() and fsync() both succeed, nothing is logged, and reading those blocks back returns the target's bytes out of the page cache without a command going on the wire.
Check the direction before using the scatterlist, the way the R2T path already does.
Affected Software
Remediation
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In iscsi_tcp_hdr_dissect(), check the resolved task's data direction before copying a Data-In PDU into its scatterlist, and reject Data-In processing unless the command is a read (DMA_FROM_DEVICE), analogous to iscsi_tcp_r2t_rsp() rejecting non-DMA_TO_DEVICE commands.
Event History
Frequently Asked Questions
What conditions are required for exploitation?
The system must use the Linux kernel iSCSI TCP initiator and issue a WRITE command to a target. A malicious or compromised iSCSI target must then send a Data-In PDU using that write command's ITT, including after an R2T response.
What data can be affected?
For O_DIRECT writes, target-supplied data can overwrite bytes in the caller's pinned write buffer while pwrite() still reports the full write length. For buffered writes, it can modify page-cache pages, causing later reads to return the target's bytes without sending a command to the target.
Are failed writes or kernel logs reliable indicators of exploitation?
No. In the observed buffered-write case, pwrite() and fsync() both succeeded and nothing was logged, even though the affected blocks later returned target-supplied bytes from page cache.
What does the fix change?
The fix checks the command data direction before copying a Data-In PDU into the command scatterlist. This prevents Data-In data from being accepted for commands that are not reads.