CVE-2021-47254

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

gfs2: Fix use-after-free in gfs2_glock_shrink_scan

The GLF_LRU flag is checked under lru_lock in gfs2_glock_remove_from_lru() to remove the glock from the lru list in __gfs2_glock_put().

On the shrink scan path, the same flag is cleared under lru_lock but because of cond_resched_lock(&lru_lock) in gfs2_dispose_glock_lru(), progress on the put side can be made without deleting the glock from the lru list.

Keep GLF_LRU across the race window opened by cond_resched_lock(&lru_lock) to ensure correct behavior on both sides - clear GLF_LRU after list_del under lru_lock.

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/0364742decb0f02bc183404868b82896f7992595
• https://git.kernel.org/stable/c/094bf5670e762afa243d2c41a5c4ab71c7447bf4
• https://git.kernel.org/stable/c/1ab19c5de4c537ec0d9b21020395a5b5a6c059b2
• https://git.kernel.org/stable/c/38ce329534500bf4ae71f81df6a37a406cf187b4
• https://git.kernel.org/stable/c/86fd5b27db743a0ce0cc245e3a34813b2aa6ec1d
• https://git.kernel.org/stable/c/92869945cc5b78ee8a1ef90336fe070893e3458a
• https://git.kernel.org/stable/c/a61156314b66456ab6a291ed5deba1ebd002ab3c
• https://git.kernel.org/stable/c/e87ef30fe73e7e10d2c85bdcc778dcec24dca553
• https://git.kernel.org/stable/c/0364742decb0f02bc183404868b82896f7992595
• https://git.kernel.org/stable/c/094bf5670e762afa243d2c41a5c4ab71c7447bf4
• https://git.kernel.org/stable/c/1ab19c5de4c537ec0d9b21020395a5b5a6c059b2
• https://git.kernel.org/stable/c/38ce329534500bf4ae71f81df6a37a406cf187b4
• https://git.kernel.org/stable/c/86fd5b27db743a0ce0cc245e3a34813b2aa6ec1d
• https://git.kernel.org/stable/c/92869945cc5b78ee8a1ef90336fe070893e3458a
• https://git.kernel.org/stable/c/a61156314b66456ab6a291ed5deba1ebd002ab3c
• https://git.kernel.org/stable/c/e87ef30fe73e7e10d2c85bdcc778dcec24dca553