CVE-2020-8835

Published: 30 March 2020

In the Linux kernel 5.5.0 and newer, the bpf verifier (kernel/bpf/verifier.c) did not properly restrict the register bounds for 32-bit operations, leading to out-of-bounds reads and writes in kernel memory. The vulnerability also affects the Linux 5.4 stable series, starting with v5.4.7, as the introducing commit was backported to that branch. This vulnerability was fixed in 5.6.1, 5.5.14, and 5.4.29. (issue is aka ZDI-CAN-10780)

From the Ubuntu Security Team

Manfred Paul discovered that the bpf verifier in the Linux kernel did not properly calculate register bounds for certain operations. A local attacker could use this to expose sensitive information (kernel memory) or gain administrative privileges.

Notes

introduced by upstream commit 581738a681b6, which
was mistakenly backported to upstream stable 5.4 kernel
(b4de258dede528f88f401259aab3147fb6da1ddf). Ubuntu's 5.3 kernels
are affected because 5.4 stable backport commits were pulled into
Ubuntu's 5.3 kernels.

Mitigation

Mitigation for this vulnerability is available by setting the
kernel.unprivileged_bpf_disabled sysctl to 1:
   $ sudo sysctl kernel.unprivileged_bpf_disabled=1
   $ echo kernel.unprivileged_bpf_disabled=1 | \
     sudo tee /etc/sysctl.d/90-CVE-2020-8835.conf
This issue is also mitigated on systems that use secure
boot, thanks to the kernel lockdown feature which blocks
BPF program loading.

Status

Severity score breakdown

References

• https://www.thezdi.com/blog/2020/3/19/pwn2own-2020-day-one-results
• https://lore.kernel.org/bpf/20200330160324.15259-1-daniel@iogearbox.net/T/
• https://www.openwall.com/lists/oss-security/2020/03/30/3
• https://ubuntu.com/security/notices/USN-4313-1
• https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f2d67fec0b43edce8c416101cdc52e71145b5fef
• https://www.zerodayinitiative.com/advisories/ZDI-20-350/
• https://www.cve.org/CVERecord?id=CVE-2020-8835
• NVD
• Launchpad
• Debian