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Most buffer overflows overwrite memory from lower to higher memory addresses, so in order to overwrite the return pointer (and thus take control of the process) the canary value must also be overwritten. This value is checked to make sure it has not changed before a routine uses the return pointer on the stack. [2]
Canaries or canary words or stack cookies are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will usually be the canary, and a failed verification of the canary data will therefore alert of an overflow, which can then be handled, for example, by invalidating the corrupted data.
Visualization of a software buffer overflow. Data is written into A, but is too large to fit within A, so it overflows into B.. In programming and information security, a buffer overflow or buffer overrun is an anomaly whereby a program writes data to a buffer beyond the buffer's allocated memory, overwriting adjacent memory locations.
Exploitation is performed by corrupting this data in specific ways to cause the application to overwrite internal structures such as linked list pointers. The canonical heap overflow technique overwrites dynamic memory allocation linkage (such as malloc metadata) and uses the resulting pointer exchange to overwrite a program function pointer.
There are many binaries used in Linux which are read-only, and can only be modified or written to by a user of higher permissions, such as the root. When privileges are escalated, whether by genuine or malicious means – such as by using the Dirty COW exploit – the user can modify usually unmodifiable binaries and files.
Copy-on-write (COW), also called implicit sharing [1] or shadowing, [2] is a resource-management technique [3] used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared between programs until one tries to modify it.
File locking is a mechanism that restricts access to a computer file, or to a region of a file, by allowing only one user or process to modify or delete it at a specific time, and preventing reading of the file while it's being modified or deleted.
Thanks to the copy-on-write nature of Btrfs, the original versions of the file data blocks are preserved during all file modifications. Until the conversion becomes permanent, only the blocks that were marked as free in ext2/3/4 are used to hold new Btrfs modifications, meaning that the conversion can be undone at any time (although doing so ...