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Date: Tue, 12 May 2020 22:53:01 -0400
From: Rich Felker <>
Subject: malloc hardening comparison draft

As a review of hardening measures and whether there are further
improvements that should be made, I've started drafting a comparison
of the properties of oldmalloc, mallocng, and hardened_malloc
(, the latter being the
source from which many of the hardening ideas in mallocng were

Protection of metadata

oldmalloc: in-band; consistency of header/footer only

mallocng: nearly(*) all attack surface is all out-of-band, protected
below by guard pages. some non-attack-surface metadata is in-band,
traps if inconsistent with out-of-band. (*=double-free of a slot
another group has been nested in is only protected in-band.)

hardened_malloc: all out-of-band, randomized location, determined at
startup so that it is known never to have been used for other purposes

Possible further improvements for mallocng: Rather than relying on
ASLR to assign address for out-of-band metadata areas, we could
attempt to choose a candidate address to pass to mmap with heavier
randomization, and hope the kernel will honor or allocate close to it.
For 64-bit archs this is very strong and flexible. For 32-bit archs
doing any heavy randomization sacrifices the ability to utilize
multiple very large allocations (~1GB rough order of magnitude) and
the best we could do would be picking to aim high or low with limited
randomization. (hardened_malloc avoids this by not supporting 32-bit.)
This kind of improvement could also help avoid allocating metadata
areas in address ranges that were previously visible to the

While I don't see any reasonable out-of-band solution to protect
nested groups from double-free using an old pointer held by the
application, it should be possible to make inexpensive transformations
on the in-band metadata to prevent falsifying it without access to
malloc's random secrets.

Invalid free detection

oldmalloc: best-effort via C_INUSE header bit which is overloaded as
mmapped flag and consistency of header/footer.

mallocng: deterministic(*) detection and trap on any attempt to free a
slot that's already free or an address not part of an allocation
obtained by malloc. consistency-check-based detection and trap in case
where slot is in use but last-allocated offset in slot does not match
address passed to free. (*=conditional on attacker not possessing
random secret or write access to oob metadata)

hardened_malloc: deterministic detection of any attempt to free an
address not belonging to the start of a currently valid allocation.

Write-after-free detection

oldmalloc: possible crash or compromise of allocator state if
application overwrites linked list pointers in free chunk

mallocng: possible detection at subsequent call if in-band metadata
has been made inconsistent by the write.

hardened_malloc: "detection of write-after-free for slab allocations
by verifying zero filling is intact at allocation time"

Possible further improvements for mallocng: The same approach used in
hardened_malloc could be used here. Possibly just at low offsets. But
the cycling of current offset within a slot complicates this; the
address assigned by malloc may be deep into the range owned by the
slot, and in order to cover all possibilities free would have to zero
the whole slot (expensive). If the possible offsets for the next slot
are known, however -- I think they're only 0 and previous+16 -- then
it would be easy to zero just these.

Detection of overflows

oldmalloc: via inconsistency of footer with header on free; only when
overflow is by large enough margin to hit footer/boundary with next

mallocng: detection and trap of single-byte overflow with any non-zero
value at realloc/free time. detection and trap of overflow into
subsequent slot header at time of realloc/free of this slot or
malloc/realloc/free of subsequent slot, unless effort is made to match
the in-band metadata clobbered by the overflow.

hardened_malloc: detection and trap of overflows with strong random
canary (value per-slab). not clear if detection is only at free of
the overflowed slot or other times as well.

Possible further improvements for mallocng: For allocations with
significant gap between their size and the slot size, all of the
reserved bytes could be checked, rather than just the first one, and a
nonzero canary could be used. Checking all is probably too expensive,
especially at large sizes or alignments (memalign) where the gap may
be very large. But we could reasonably check up to 8 bytes right after
the zero byte.

Guard pages

oldmalloc: none

mallocng: only below out-of-band metadata areas, but aggressive return
of freed memory to system tends to leave a lot of unmapped/faulting
address ranges.

hardened_malloc: randomly intersperses guard pages between slabs in
slab size class regions, and around metadata.

Possible further improvements for mallocng: Probably none. Further use
of guard pages risks hitting kernel VMA limits. Limited quarantine may
be a possibility though.

Valid-double-free and use-after-free mitigations

oldmalloc: none

mallocng: roughly LRU order to free slot allocation, combined with
cycling of the used offset range within a slot whenever the requested
size is less than the full slot size in its size class, stretches
interval to exact address reuse. attempted free of non-exact address
reuse, even if the same slot was reused, will trap.

hardened_malloc: quarantine of large freed areas so same virtual
address ranges cannot be used again until quarantine fills and pushes
old entries out. random delayed free and random slot selection for
allocations within slabs.

Possible further improvements for mallocng: Quarantine could be
implemented by replacing freed groups with PROT_NONE maps. With a
reasonable limit on quarantine size this should not lead to serious
fregmentation of virtual address space or hit VMA limit. Optionally a
build with increased hardening could always use slots significantly
larger than the requested size n, so that offset cycling period is
guaranteed to be greater than 1.

Freed data leak prevention

oldmalloc: none except as a side effect of clobbering first two
pointer-sized words.

mallocng: none.

hardened_malloc: all freed memory is zeroed.

Possible further improvements for mallocng: It may be practical to
zero some initial segment of the freed memory. This also has benefit
of catching some UAF.


oldmalloc: usually limited to initial brk gap randomization.

mallocng: matching kernel mmap ASLR.

hardened_malloc: strong randomization in 64-bit address space.

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