MEMGUARD(9) FreeBSD Kernel Developer's Manual MEMGUARD(9)


MemGuardmemory allocator for debugging purposes




MemGuard is a simple and small replacement memory allocator designed to help detect tamper-after-free scenarios. These problems are more and more common and likely with multithreaded kernels where race conditions are more prevalent.

MemGuard can take over malloc(), realloc() and free() for a single malloc type. Alternatively MemGuard can take over uma_zalloc(), uma_zalloc_arg() and uma_free() for a single uma zone. Also MemGuard can guard all allocations larger than PAGE_SIZE, and can guard a random fraction of all allocations. There is also a knob to prevent allocations smaller than a specified size from being guarded, to limit memory waste.


To use MemGuard for a memory type, either add an entry to /boot/loader.conf:


Or set the vm.memguard.desc sysctl(8) variable at run-time:

sysctl vm.memguard.desc=<memory_type>

Where memory_type can be either a short description of the memory type to monitor, either name of uma(9) zone. Only allocations from that memory_type made after vm.memguard.desc is set will potentially be guarded. If vm.memguard.desc is modified at run-time then only allocations of the new memory_type will potentially be guarded once the sysctl(8) is set. Existing guarded allocations will still be properly released by either free(9) or uma_zfree(9), depending on what kind of allocation was taken over.

To determine short description of a malloc(9) type one can either take it from the first column of vmstat(8) -m output, or to find it in the kernel source. It is the second argument to MALLOC_DEFINE(9) macro. To determine name of uma(9) zone one can either take it from the first column of vmstat(8) -z output, or to find it in the kernel source. It is the first argument to the uma_zcreate(9) function.

The vm.memguard.divisor boot-time tunable is used to scale how much of the system's physical memory MemGuard is allowed to consume. The default is 10, so up to cnt.v_page_count/10 pages can be used. MemGuard will reserve vm_kmem_max / vm.memguard.divisor bytes of virtual address space, limited by twice the physical memory size. The physical limit is reported as vm.memguard.phys_limit and the virtual space reserved for MemGuard is reported as vm.memguard.mapsize.

MemGuard will not do page promotions for any allocation smaller than vm.memguard.minsize bytes. The default is 0, meaning all allocations can potentially be guarded. MemGuard can guard sufficiently large allocations randomly, with average frequency of every one in 100000 / vm.memguard.frequency allocations. The default is 0, meaning no allocations are randomly guarded.

MemGuard can optionally add unmapped guard pages around each allocation to detect overflow and underflow, if vm.memguard.options has the 1 bit set. This option is enabled by default. MemGuard will optionally guard all allocations of PAGE_SIZE or larger if vm.memguard.options has the 2 bit set. This option is off by default. By default MemGuard doesn't guard those uma(9) zones that have been initialized with the UMA_ZONE_NOFREE flag set, since it can produce false positives on them. However, this safety measure can be turned off by setting bit 3 of the vm.memguard.options tunable.


MemGuard first appeared in FreeBSD 6.0.


MemGuard was originally written by Bosko Milekic <bmilekic@FreeBSD.org>. This manual page was originally written by Christian Brueffer <brueffer@FreeBSD.org>. Additions have been made by Matthew Fleming <mdf@FreeBSD.org> and Gleb Smirnoff <glebius@FreeBSD.org> to both the implementation and the documentation.


It is not possible to guard allocations that really expect themselves to be allocated from uma(9), utilizing additional interfaces apart from uma_zalloc() and uma_free(), for example uma_find_refcnt(). For the moment of writing only mbuf(9) cluster zones belong to that kind of allocations. Attempt to guard them would lead to kernel panic.
October 21, 2011 FreeBSD