CVE-2021-47342

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

ext4: fix possible UAF when remounting r/o a mmp-protected file system

After commit 618f003199c6 (ext4: fix memory leak in ext4_fill_super), after the file system is remounted read-only, there is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to point at freed memory, which the call to ext4_stop_mmpd() can trip over.

Fix this by only allowing kmmpd() to exit when it is stopped via ext4_stop_mmpd().

Bug-Report-Link: 20210629143603.2166962-1-yebin10@huawei.com

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/61bb4a1c417e5b95d9edb4f887f131de32e419cb
• https://git.kernel.org/stable/c/7ed572cdf11081f8f9e07abd4bea56a3f2c4edbd
• https://git.kernel.org/stable/c/b663890d854403e566169f7e90aed5cd6ff64f6b
• https://git.kernel.org/stable/c/61bb4a1c417e5b95d9edb4f887f131de32e419cb
• https://git.kernel.org/stable/c/7ed572cdf11081f8f9e07abd4bea56a3f2c4edbd
• https://git.kernel.org/stable/c/b663890d854403e566169f7e90aed5cd6ff64f6b