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Message-ID: <CAFzhf4qipkzzR1r8mowFaMNmUxXMhR9agw=gqJs1CSLNF0=rWA@mail.gmail.com>
Date: Tue, 4 May 2021 11:06:52 +0100
From: Piotr Krysiuk <piotras@...il.com>
To: oss-security@...ts.openwall.com
Subject: [CVE-2021-31829] Linux kernel protection of stack pointer against
speculative pointer arithmetic can be bypassed to leak content of kernel memory
An issue has been discovered in the Linux kernel mechanism to mitigate
speculative loads (Spectre mitigation).
Unprivileged BPF programs running on affected systems can bypass
the protection and execute speculative loads from the kernel stack.
This can be abused to extract contents of the stack via side-channel.
The extracted contents may include addresses of kernel structures
that could be used to defeat Kernel Address Space Layout Randomization
(KASLR) to facilitate exploitation of other vulnerabilities.
The identified gap is that when protecting BPF stack pointer against
speculative pointer arithmetic, the BPF stack area itself is not
protected against speculative loads. This could be abused to perform
speculative loads from any location within the BPF stack. And so
any restricted data from the BPF stack could be disclosed, such as
addresses of data structures referred by the BPF program. Further,
the original content of kernel memory is not wiped when allocating
the BPF stack, and could be disclosed as well.
I developed a PoC that allows unprivileged local users to extract
contents of 511 bytes from the BPF stack.
The PoC has been shared privately with <security@...nel.org> to assist
with fix development.
The patches are available from the BPF subsystem public git repository.
The fix has dependency of another recent commit fixing a separate
issue. The full patch series is as follows:
* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/patch/?id=b9b34ddbe2076ade359cd5ce7537d5ed019e9807
* https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf.git/patch/?id=801c6058d14a82179a7ee17a4b532cac6fad067f
# Discoverers
Piotr Krysiuk <piotras@...il.com>
# References
CVE-2021-31829 (reserved via https://cveform.mitre.org/)
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