NAME

kasan — kernel address sanitizer

SYNOPSIS

To compile KASAN into the kernel, place the following line in your kernel configuration file:

void
kasan_mark(const void *addr, size_t size, size_t redzsize, uint8_t code);

DESCRIPTION

kasan is a subsystem which leverages compiler instrumentation to detect invalid memory accesses in the kernel. Currently it is implemented only on the amd64 platform.

When kasan is compiled into the kernel, the compiler is configured to emit function calls upon every memory access. The functions are implemented by kasan and permit run-time detection of several types of bugs including use-after-frees, double frees and frees of invalid pointers, and out-of-bounds accesses. These protections apply to memory allocated by uma(9), malloc(9) and related functions, and kmem_malloc() and related functions, as well as global variables and kernel stacks. kasan is conservative and will not detect all instances of these types of bugs. Memory accesses through the kernel map are sanitized, but accesses via the direct map are not. When kasan is configured, the kernel aims to minimize its use of the direct map.

IMPLEMENTATION NOTES

kasan is implemented using compiler instrumentation and a kernel runtime. When a kernel is built with the KASAN option enabled, the compiler inserts function calls before most memory accesses in the generated code. The runtime implements the corresponding functions, which decide whether a given access is valid. If not, the runtime prints a warning or panics the kernel, depending on the value of the debug.kasan.panic_on_violation sysctl/tunable.

The kasan runtime in a KASAN-configured kernel can be disabled by setting the loader tunable debug.kasan.disable=1.

The kasan runtime works by maintaining a shadow map for the kernel map. There exists a linear mapping between addresses in the kernel map and addresses in the shadow map. The shadow map is used to store information about the current state of allocations from the kernel map. For example, when a buffer is returned by malloc(9), the corresponding region of the shadow map is marked to indicate that the buffer is valid. When it is freed, the shadow map is updated to mark the buffer as invalid. Accesses to the buffer are intercepted by the kasan runtime and validated using the contents of the shadow map.

Upon booting, all kernel memory is marked as valid. Kernel allocators must mark cached but free buffers as invalid, and must mark them valid before freeing the kernel virtual address range. This slightly reduces the effectiveness of kasan but simplifies its maintenance and integration into the kernel.

Updates to the shadow map are performed by calling kasan_mark(). Parameter addr is the address of the buffer whose shadow is to be updated, size is the usable size of the buffer, and redzsize is the full size of the buffer allocated from lower layers of the system. redzsize must be greater than or equal to size. In some cases kernel allocators will return a buffer larger than that requested by the consumer; the unused space at the end is referred to as a red zone and is always marked as invalid. code allows the caller to specify an identifier used when marking a buffer as invalid. The identifier is included in any reports generated by kasan and helps identify the source of the invalid access. For instance, when an item is freed to a uma(9) zone, the item is marked with KASAN_UMA_FREED. See <sys/asan.h> for the available identifiers. If the entire buffer is to be marked valid, i.e., size and redzsize are equal, code should be 0.

SEE ALSO

malloc(9), memguard(9), redzone(9), uma(9)

HISTORY

kasan first appeared in FreeBSD 14.0.

BUGS

Accesses to kernel memory outside of the kernel map are ignored by the kasan runtime. When kasan is configured, the kernel memory allocators are configured to use the kernel map, but some uses of the direct map remain. For example, on amd64, accesses to page table pages are not tracked.

Some kernel memory allocators explicitly permit accesses after an object has been freed. These cannot be sanitized by kasan. For example, memory from all uma(9) zones initialized with the UMA_ZONE_NOFREE flag are not sanitized.