CVE-2021-47646

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

Revert Revert block, bfq: honor already-setup queue merges

A crash [1] happened to be triggered in conjunction with commit 2d52c58b9c9b (block, bfq: honor already-setup queue merges). The latter was then reverted by commit ebc69e897e17 (Revert block, bfq: honor already-setup queue merges). Yet, the reverted commit was not the one introducing the bug. In fact, it actually triggered a UAF introduced by a different commit, and now fixed by commit d29bd41428cf (block, bfq: reset last_bfqq_created on group change).

So, there is no point in keeping commit 2d52c58b9c9b (block, bfq: honor already-setup queue merges) out. This commit restores it.

[1] https://bugzilla.kernel.org/show_bug.cgi?id=214503

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/15729ff8143f8135b03988a100a19e66d7cb7ecd
• https://git.kernel.org/stable/c/4083925bd6dc89216d156474a8076feec904e607
• https://git.kernel.org/stable/c/65d8a737452e88f251fe5d925371de6d606df613
• https://git.kernel.org/stable/c/931aff627469a75c77b9fd3823146d0575afffd6
• https://git.kernel.org/stable/c/abc2129e646af7b43025d90a071f83043f1ae76c
• https://git.kernel.org/stable/c/cc051f497eac9d8a0d816cd4bffa3415f2724871
• https://git.kernel.org/stable/c/f990f0985eda59d4f29fc83fcf300c92b1225d39