CVE-2021-47012

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

RDMA/siw: Fix a use after free in siw_alloc_mr

Our code analyzer reported a UAF.

In siw_alloc_mr(), it calls siw_mr_add_mem(mr,..). In the implementation of siw_mr_add_mem(), mem is assigned to mr->mem and then mem is freed via kfree(mem) if xa_alloc_cyclic() failed. Here, mr->mem still point to a freed object. After, the execution continue up to the err_out branch of siw_alloc_mr, and the freed mr->mem is used in siw_mr_drop_mem(mr).

My patch moves mr->mem = mem behind the if (xa_alloc_cyclic(..)<0) {} section, to avoid the uaf.

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/3093ee182f01689b89e9f8797b321603e5de4f63
• https://git.kernel.org/stable/c/30b9e92d0b5e5d5dc1101ab856c17009537cbca4
• https://git.kernel.org/stable/c/3e22b88e02c194f6c80867abfef5cc09383461f4
• https://git.kernel.org/stable/c/608a4b90ece039940e9425ee2b39c8beff27e00c
• https://git.kernel.org/stable/c/ad9ce7188432650469a6c7625bf479f5ed0b6155
• https://git.kernel.org/stable/c/3093ee182f01689b89e9f8797b321603e5de4f63
• https://git.kernel.org/stable/c/30b9e92d0b5e5d5dc1101ab856c17009537cbca4
• https://git.kernel.org/stable/c/3e22b88e02c194f6c80867abfef5cc09383461f4
• https://git.kernel.org/stable/c/608a4b90ece039940e9425ee2b39c8beff27e00c
• https://git.kernel.org/stable/c/ad9ce7188432650469a6c7625bf479f5ed0b6155