SB2020042446 - Operation on a resource after expiration or release in Linux kernel

Description

This security bulletin contains information about 1 security vulnerability.

1) Operation on a resource after expiration or release (CVE-ID: CVE-2019-15794)

The vulnerability allows a local privileged user to execute arbitrary code.

Overlayfs in the Linux kernel and shiftfs, a non-upstream patch to the Linux kernel included in the Ubuntu 5.0 and 5.3 kernel series, both replace vma->vm_file in their mmap handlers. On error the original value is not restored, and the reference is put for the file to which vm_file points. On upstream kernels this is not an issue, as no callers dereference vm_file following after call_mmap() returns an error. However, the aufs patchs change mmap_region() to replace the fput() using a local variable with vma_fput(), which will fput() vm_file, leading to a refcount underflow.

Remediation

Install update from vendor's website.

References

• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/eoan/commit/?id=270d16ae48a4dbf1c7e25e94cc3e38b4bea37635
• https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/eoan/commit/?id=ef81780548d20a786cc77ed4203fca146fd81ce3
• https://usn.ubuntu.com/usn/usn-4208-1
• https://usn.ubuntu.com/usn/usn-4209-1