In the Linux kernel, the following vulnerability has been resolved: ext4: fix timer use-after-free on failed mount Syzbot has found an ODEBUG bug in ext4_fill_super The del_timer_sync function cancels the s_err_report timer, which reminds about filesystem errors daily. We should guarantee the timer is no longer active before kfree(sbi). When filesystem mounting fails, the flow goes to failed_mount3, where an error occurs when ext4_stop_mmpd is called, causing a read I/O failure. This triggers the ext4_handle_error function that ultimately re-arms the timer, leaving the s_err_report timer active before kfree(sbi) is called. Fix the issue by canceling the s_err_report timer after calling ext4_stop_mmpd.
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.