CVE-2021-46959

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

spi: Fix use-after-free with devm_spi_alloc_*

We cant rely on the contents of the devres list during spi_unregister_controller(), as the list is already torn down at the time we perform devres_find() for devm_spi_release_controller. This causes devices registered with devm_spi_alloc_{master,slave}() to be mistakenly identified as legacy, non-devm managed devices and have their reference counters decremented below 0.

————[ cut here ]———— WARNING: CPU: 1 PID: 660 at lib/refcount.c:28 refcount_warn_saturate+0x108/0x174 [] (refcount_warn_saturate) from [] (kobject_put+0x90/0x98) [] (kobject_put) from [] (put_device+0x20/0x24) r4:b6700140 [] (put_device) from [] (devm_spi_release_controller+0x3c/0x40) [] (devm_spi_release_controller) from [] (release_nodes+0x84/0xc4) r5:b6700180 r4:b6700100 [] (release_nodes) from [] (devres_release_all+0x5c/0x60) r8:b1638c54 r7:b117ad94 r6:b1638c10 r5:b117ad94 r4:b163dc10 [] (devres_release_all) from [] (__device_release_driver+0x144/0x1ec) r5:b117ad94 r4:b163dc10 [] (__device_release_driver) from [] (device_driver_detach+0x84/0xa0) r9:00000000 r8:00000000 r7:b117ad94 r6:b163dc54 r5:b1638c10 r4:b163dc10 [] (device_driver_detach) from [] (unbind_store+0xe4/0xf8)

Instead, determine the devm allocation state as a flag on the controller which is guaranteed to be stable during cleanup.

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/001c8e83646ad3b847b18f6ac55a54367d917d74
• https://git.kernel.org/stable/c/28a5529068c51cdf0295ab1e11a99a3a909a03e4
• https://git.kernel.org/stable/c/62bb2c7f2411a0045c24831f11ecacfc35610815
• https://git.kernel.org/stable/c/794aaf01444d4e765e2b067cba01cc69c1c68ed9
• https://git.kernel.org/stable/c/8735248ebb918d25427965f0db07939ed0473ec6
• https://git.kernel.org/stable/c/8bf96425c90f5c1dcf3b7b9df568019a1d4b8a0e
• https://git.kernel.org/stable/c/8e029707f50a82c53172359c686b2536ab54e58c
• https://git.kernel.org/stable/c/c7fabe372a9031acd00498bc718ce27c253abfd1
• https://git.kernel.org/stable/c/cee78aa24578edac8cf00513dca618c0acc17cd7
• https://git.kernel.org/stable/c/001c8e83646ad3b847b18f6ac55a54367d917d74
• https://git.kernel.org/stable/c/28a5529068c51cdf0295ab1e11a99a3a909a03e4
• https://git.kernel.org/stable/c/62bb2c7f2411a0045c24831f11ecacfc35610815
• https://git.kernel.org/stable/c/794aaf01444d4e765e2b067cba01cc69c1c68ed9
• https://git.kernel.org/stable/c/8735248ebb918d25427965f0db07939ed0473ec6
• https://git.kernel.org/stable/c/8bf96425c90f5c1dcf3b7b9df568019a1d4b8a0e
• https://git.kernel.org/stable/c/8e029707f50a82c53172359c686b2536ab54e58c
• https://git.kernel.org/stable/c/c7fabe372a9031acd00498bc718ce27c253abfd1
• https://git.kernel.org/stable/c/cee78aa24578edac8cf00513dca618c0acc17cd7