A local kernel crash on invalid USB device requiring the visor driver was reported. The treoattach() function of the [visor] driver, which is called during the driver initialization process, was dereferencing the bulk-in and interrupt-in urbs without first making sure they had been allocated by core. Due to an incomplete sanity check, the visor driver tries to dereference null-pointers, which results in crash.

Vulnerable code:

CentOS-Kernel linux-3.10.0-229.14.1.el7 (drivers/usb/serial/visor.c) ... 554 #define COPYPORT(dest, src) \ 555 do { \ 556 int i; \ 557 \ 558 for (i = 0; i < ARRAYSIZE(src->readurbs); ++i) { \ 559 dest->readurbs[i] = src->readurbs[i]; \ / Possible Nullpointer-Dereference / 560 dest->readurbs[i]->context = dest; \ 561 dest->bulkinbuffers[i] = src->bulkinbuffers[i]; \ 562 } \ 563 dest->readurb = src->readurb; \ 564 dest->bulkinendpointAddress = src->bulkinendpointAddress;\ 565 dest->bulkinbuffer = src->bulkinbuffer; \ 566 dest->bulkinsize = src->bulkinsize; \ 567 dest->interruptinurb = src->interruptinurb; \ 568 dest->interruptinurb->context = dest; \ 569 dest->interruptinendpointAddress = \ 570 src->interruptinendpointAddress;\ 571 dest->interruptinbuffer = src->interruptinbuffer; \ 572 } while (0); 573 574 swapport = kmalloc(sizeof(swapport), GFPKERNEL); 575 if (!swapport) 576 return -ENOMEM; 577 COPYPORT(swapport, serial->port[0]); / no sanity-check! / 578 COPYPORT(serial->port[0], serial->port[1]); / no sanity-check! / 579 COPYPORT(serial->port[1], swapport); / no sanity-check! / ...

Reproducer can be found in original bug report: https://bugzilla.redhat.com/showbug.cgi?id=1283374

An upstream patch: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=cb3232138e37129e88240a98a1d2aba2187ff57c

Public via: http://seclists.org/bugtraq/2016/Mar/86

CVE-ID request and assignment: http://seclists.org/oss-sec/2016/q1/456 http://seclists.org/oss-sec/2016/q1/458

ISSUE DESCRIPTION =================

Xen PCI backend driver does not perform proper sanity checks on the device's state.

Which in turn allows the generic MSI code (called by Xen PCI backend) to be called incorrectly leading to hitting BUG conditions or causing NULL pointer exceptions in the MSI code.

To exploit this the guest can craft specific sequence of XENPCIOP operations which will trigger this.

Furthermore the frontend can also craft an continous stream of XENPCIOPenablemsi which will trigger an continous stream of WARN() messages triggered by the MSI code leading to the logging in the initial domain to exhaust disk space.

Lastly there is also missing check to verify whether the device has memory decoding enabled set at the start of the day leading the initial domain "accesses to the respective MMIO or I/O port ranges would - - on PCI Express devices - [which can] lead to Unsupported Request responses. The treatment of such errors is platform specific." (from XSA-120). Note that if XSA-120 'addendum' patch has been applied this particular sub-issue is not exploitable.

IMPACT ======

Malicious guest administrators can cause denial of service. If driver domains are not in use, the impact is a host crash.

Only x86 systems are vulnerable. ARM systems are not vulnerable.

VULNERABLE SYSTEMS ==================

This bug affects systems using Linux as the driver domain, including non-disaggregated systems using Linux as dom0.

Linux versions v3.1 and onwards are vulnerable due to supporting PCI pass-through backend driver.

PV and HVM guests which have been granted access to physical PCI devices (PCI passthrough') can take advantage of this vulnerability.

Furthermore, the vulnerability is only applicable when the passed-through PCI devices are MSI-capable or MSI-X. (Most modern devices are).

MITIGATION ==========

Not using PCI passthrough for PV and HVM guests. Note that for HVM guests QEMU is used for PCI passthrough - however the toolstack sets up also the 'PV' PCI which the guest can utilize if it chooses to do so.

External References:

http://xenbits.xen.org/xsa/advisory-157.html

Acknowledgements:

Red Hat would like to thank the Xen project for reporting this issue.

Last updated 24 July 2024

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