CVE-2021-47081

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

habanalabs/gaudi: Fix a potential use after free in gaudi_memset_device_memory

Our code analyzer reported a uaf.

In gaudi_memset_device_memory, cb is get via hl_cb_kernel_create() with 2 refcount. If hl_cs_allocate_job() failed, the execution runs into release_cb branch. One ref of cb is dropped by hl_cb_put(cb) and could be freed if other thread also drops one ref. Then cb is used by cb->id later, which is a potential uaf.

My patch add a variable id to accept the value of cb->id before the hl_cb_put(cb) is called, to avoid the potential 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/115726c5d312b462c9d9931ea42becdfa838a076
• https://git.kernel.org/stable/c/b49f5af30b0e4064fbd91e83823a4bfcb2c7a3e7
• https://git.kernel.org/stable/c/115726c5d312b462c9d9931ea42becdfa838a076
• https://git.kernel.org/stable/c/b49f5af30b0e4064fbd91e83823a4bfcb2c7a3e7