CVE-2021-47506

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

nfsd: fix use-after-free due to delegation race

A delegation break could arrive as soon as weve called vfs_setlease. A delegation break runs a callback which immediately (in nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru. If we then exit nfs4_set_delegation without hashing the delegation, it will be freed as soon as the callback is done with it, without ever being removed from del_recall_lru.

Symptoms show up later as use-after-free or list corruption warnings, usually in the laundromat thread.

I suspect aba2072f4523 nfsd: grant read delegations to clients holding writes made this bug easier to hit, but I looked as far back as v3.0 and it looks to me it already had the same problem. So Im not sure where the bug was introduced; it may have been there from the beginning.

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/04a8d07f3d58308b92630045560799a3faa3ebce
• https://git.kernel.org/stable/c/148c816f10fd11df27ca6a9b3238cdd42fa72cd3
• https://git.kernel.org/stable/c/2becaa990b93cbd2928292c0b669d3abb6cf06d4
• https://git.kernel.org/stable/c/33645d3e22720cac1e4548f8fef57bf0649536ee
• https://git.kernel.org/stable/c/348714018139c39533c55661a0c7c990671396b4
• https://git.kernel.org/stable/c/548ec0805c399c65ed66c6641be467f717833ab5
• https://git.kernel.org/stable/c/e0759696de6851d7536efddfdd2dfed4c4df1f09
• https://git.kernel.org/stable/c/eeb0711801f5e19ef654371b627682aed3b11373
• https://git.kernel.org/stable/c/04a8d07f3d58308b92630045560799a3faa3ebce
• https://git.kernel.org/stable/c/148c816f10fd11df27ca6a9b3238cdd42fa72cd3
• https://git.kernel.org/stable/c/2becaa990b93cbd2928292c0b669d3abb6cf06d4
• https://git.kernel.org/stable/c/33645d3e22720cac1e4548f8fef57bf0649536ee
• https://git.kernel.org/stable/c/348714018139c39533c55661a0c7c990671396b4
• https://git.kernel.org/stable/c/548ec0805c399c65ed66c6641be467f717833ab5
• https://git.kernel.org/stable/c/e0759696de6851d7536efddfdd2dfed4c4df1f09
• https://git.kernel.org/stable/c/eeb0711801f5e19ef654371b627682aed3b11373