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Kernel: Limit free space between randomized memory allocations

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Tim Schumacher 2022-04-12 02:28:39 +02:00 • committed by Andreas Kling
parent 908d5a2853
commit 71f2c342c4
1 changed files with 20 additions and 0 deletions
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20
Kernel/Memory/RegionTree.cpp
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@ -121,6 +121,26 @@ ErrorOr<VirtualRange> RegionTree::allocate_range_randomized(size_t size, size_t
if (!m_total_range.contains(random_address, size)) if (!m_total_range.contains(random_address, size))
continue; continue;
#if ARCH(I386)
// Attempt to limit the amount of wasted address space on platforms with small address sizes (read: i686).
// This works by only allowing arbitrary random allocations until a certain threshold, to create more possibilities for placing mappings.
// After the threshold has been reached, new allocations can only be placed randomly within a certain range from the adjacent allocations.
VirtualAddress random_address_end { random_address.get() + size };
constexpr size_t max_allocations_until_limited = 200;
constexpr size_t max_space_between_allocations = 1 * MiB;
if (m_regions.size() >= max_allocations_until_limited) {
auto* lower_allocation = m_regions.find_largest_not_above(random_address.get());
auto* upper_allocation = m_regions.find_smallest_not_below(random_address_end.get());
bool lower_in_range = (!lower_allocation || random_address - lower_allocation->range().end() <= VirtualAddress(max_space_between_allocations));
bool upper_in_range = (!upper_allocation || upper_allocation->range().base() - random_address_end <= VirtualAddress(max_space_between_allocations));
if (!upper_in_range && !lower_in_range)
continue;
}
#endif
auto range_or_error = allocate_range_specific(random_address, size); auto range_or_error = allocate_range_specific(random_address, size);
if (!range_or_error.is_error()) if (!range_or_error.is_error())
return range_or_error.release_value(); return range_or_error.release_value();