CVE-2021-47259

In the Linux kernel, the following vulnerability has been resolved:

NFS: Fix use-after-free in nfs4_init_client()

KASAN reports a use-after-free when attempting to mount two different exports through two different NICs that belong to the same server.

Olga was able to hit this with kernels starting somewhere between 5.7 and 5.10, but I traced the patch that introduced the clear_bit() call to 4.13. So something must have changed in the refcounting of the clp pointer to make this call to nfs_put_client() the very last one.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/3e3c7ebbfac152d08be75c92802a64a1f6471a15
• https://git.kernel.org/stable/c/42c10b0db064e45f5c5ae7019bbf2168ffab766c
• https://git.kernel.org/stable/c/476bdb04c501fc64bf3b8464ffddefc8dbe01577
• https://git.kernel.org/stable/c/72651c6579a25317a90536181d311c663d0329ab
• https://git.kernel.org/stable/c/c3b6cf64dfe4ef96e7341508d50d6998da7062c7
• https://git.kernel.org/stable/c/c7eab9e2d7b4e983ce280276fb920af649955897