[CVE-2021-29155] Linux kernel protection for sequences of pointer arithmetic operations against speculatively out-of-bounds loads can be bypassed to leak content of kernel memory

                							

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[CVE-2021-29155] Linux kernel protection for sequences of pointer arithmetic operations against speculatively out-of-bounds loads can be bypassed to leak content of kernel memory

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From: Piotr Krysiuk &lt;piotras () gmail com&gt;

Date: Sun, 18 Apr 2021 13:16:39 +0100

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An issue has been discovered in the Linux kernel mechanism to mitigate
speculatively out-of-bounds loads (Spectre mitigation).

Unprivileged BPF programs running on affected systems can bypass
the protection and execute speculatively out-of-bounds loads from
the kernel memory. This can be abused to extract contents of kernel
memory via side-channel.

The identified gap is that when protecting sequences of pointer
arithmetic operations against speculatively out-of-bounds loads,
the pointer modification performed by the first operation is not
correctly accounted for when restricting subsequent operations.

I developed a PoC that allows unprivileged local users to extract
contents of 31 KByte window within the kernel memory.

The PoC has been shared privately with &lt;security () kernel org&gt; to
assist with fix development.

The patches are available from Linux kernel mainline public git
repository.

The upstream fix depends on refactoring of the BPF verifier logic.
The full patch series is as follows:

* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=9601148392520e2e134936e76788fc2a6371e7be
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=6f55b2f2a1178856c19bbce2f71449926e731914
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=24c109bb1537c12c02aeed2d51a347b4d6a9b76e
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=b658bbb844e28f1862867f37e8ca11a8e2aa94a3
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=a6aaece00a57fa6f22575364b3903dfbccf5345d
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=073815b756c51ba9d8384d924c5d1c03ca3d1ae4
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=f528819334881fd622fdadeddb3f7edaed8b7c9b
* 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/patch/kernel/bpf/verifier.c?id=7fedb63a8307dda0ec3b8969a3b233a1dd7ea8e0

# Discoverers

Piotr Krysiuk &lt;piotras () gmail com&gt;
Benedict Schlueter (independent report)

# References

CVE-2021-29155 (reserved via https://cveform.mitre.org/)

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[CVE-2021-29155] Linux kernel protection for sequences of pointer arithmetic operations against speculatively out-of-bounds loads can be bypassed to leak content of kernel memory Piotr Krysiuk (Apr 18)

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