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Date: Wed, 13 Jan 2021 16:40:06 +0100
From: Marcus Meissner <>
Subject: Re: CVE-2020-28374: Linux SCSI target (LIO)
 unrestricted copy offload


For tcmu-runner Mitre suggested that we use a different CVE as its not the same codebase.

Please use CVE-2021-3139 for tcmu-runner.

Ciao, Marcus

On Tue, Jan 12, 2021 at 07:01:34PM +0100, David Disseldorp wrote:
> ===============================================================================
> == Subject:     Linux SCSI target (LIO) unrestricted copy offload
> ==
> ==
> == CVE ID#:     CVE-2020-28374
> ==
> == Versions:    Linux: v3.12 and later
> ==              tcmu-runner: v1.3.0 and later
> ==
> == Summary:     An attacker with access to a LUN and knowledge of Unit Serial
> ==              Number assignments can read and write to any LIO backstore,
> ==              regardless of SCSI transport settings.
> ===============================================================================
> Description
> -----------
> SCSI "EXTENDED COPY" (XCOPY) requests sent to a Linux SCSI target (LIO) allow an
> attacker to read or write anywhere on any LIO backstore configured on the
> host, provided the attacker has access to one LUN and knowledge of the victim
> backstore's vpd_unit_serial (AKA "wwn"). This is possible regardless of the
> transport/HBA settings for the victim backstore.
> - with vhost-scsi this can allow VM guests to read or write to images assigned
>   to other qemu processes
> - with iSCSI this allows CHAP, ACL and network portal isolation bypass
> - backstores with no corresponding transport LUN mapping remain vulnerable
> - all other LIO transports and backstores which allow for XCOPY processing by
>   LIO's target_core_xcopy handler should be considered vulnerable
> - tcmu-runner based user backstores are also vulnerable via a similar logic bug
> This is due to the way that LIO behaves when processing XCOPY
> copy-source/copy-destination (CSCD) descriptors; when attempting to match
> CSCD descriptors with corresponding se_devices, target_xcopy_locate_se_dev_e4()
> iterates over LIO's global devices list, which includes all configured
> backstores, instead of only considering backstores which are exposed to the
> initiator via transport layer ACL settings.
> Similarly, when LIO is configured to forward SCSI requests to the user-space
> tcmu-runner daemon (via target_core_user), tcmu-runner's xcopy_locate_udev()
> iterates over all tcmu-runner devices, without considering any transport layer
> restrictions.
> Exploitation
> ------------
> The attacker sends an XCOPY request with two CSCD descriptors.
> One CSCD descriptor must correspond to the NAA IEEE identifier for the LUN to
> which the attacker has access. The other (victim) CSCD descriptor must be an
> NAA IEEE identifier which matches another configured backstore within LIO's
> global device inventory.
> For successful exploitation of this bug an attacker must be able to provide a
> matching NAA identifier for the victim backstore.
> Affected Versions
> -----------------
> Linux Kernel (LIO target_core_xcopy)
> - Exploitable as of
>   f99715ac8d6f ("target: Enable global EXTENDED_COPY setup/release")
>   + mainline v3.12-rc1 and later
> tcmu-runner (user-space SCSI target, coupled with LIO's target_core_user)
> - Exploitable as of 9c86bd0db97a ("tcmur: Add emulate XCOPY command support")
>   + tcmu-runner v1.3.0 and later
> Relevance
> ---------
> Linux kernel LIO deployments are affected under the following conditions:
> - Linux kernel with f99715ac8d6f, i.e. mainline v3.12-rc1 or later
> - LIO SCSI target (target_core_mod) loaded
> - at least two configured backstores
> - one "attacker backstore" must be exposed via a SCSI transport (e.g. iSCSI)
>   which permits access to a potential attacker
>   + the attacker backstore must allow and use in-kernel LIO XCOPY command
>     emulation
>     - all backstores except special "pscsi" passthrough and "user" types
>     - emulate_3pc=1 must be set (default)
> - one or more "victim backstores" must be configured
>   + transport settings for victim backstores are irrelevant
>   + all backstore types are vulnerable, including "iblock", "fileio", "rd_mcp",
>     "pscsi" and "user"
>   + emulate_3pc=1 must be set the victim backstore (default)
> tcmu-runner deployments are affected under the following conditions:
> - tcmu-runner with 9c86bd0db97a, i.e. v1.3.0 or later
> - one "attacker backstore" must be exposed via a SCSI transport (e.g. iSCSI)
>   which permits access to a potential attacker
> - one or more "victim backstores" must be configured
>   + transport settings for victim backstores are irrelevant
>   + the victim backstore must also be managed by the same tcmu-runner instance
>     - e.g. an XCOPY request to a "user"+tcmu-runner backstore can't be used to
>       read or write to an "iblock" backstore, only to other backstores handled
>       by the same tcmu-runner instance.
> Mitigation
> ----------
> Caveat: instructions below do *not* affect XCOPY requests sent to tcmu-runner
>         based backstores. They are only suitable for disabling kernel
> 	(target_core_xcopy) support for XCOPY requests.
> Requires acb3f2600eb8 ("target: Reject EXTENDED_COPY when emulate_3pc is disabled")
> - v3.12-rc7 or later
> XCOPY support is enabled by default, but can be disabled via:
>   echo 0 > /sys/kernel/config/target/core/<backstore>/<name>/attrib/emulate_3pc
> or
>   targetcli /backstores/<backstore>/<name> set attribute emulate_3pc=0
> ...where <backstore> and <name> should be filled appropriately.
> Fixes
> -----
> Linux kernel and tcmu-runner fixes will be provided following the coordinated
> release date: 2021-01-12 10:00 Pacific Standard Time.
> Credits
> -------
> Research and patches by David Disseldorp of SUSE.
> Patch review by Mike Christie of Oracle, and Lee Duncan of SUSE.

Marcus Meissner,SUSE LINUX GmbH; Maxfeldstrasse 5; D-90409 Nuernberg; Zi. 3.1-33,+49-911-740 53-432,,serv=loki,mail=wotan,type=real <>

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