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FEXLoader: Allocate the second 4GB of virtual memory when executing 32-bit

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Spurred on by #3421. To ensure that applications don't take advantage of
small address wrap around, allocate the second 4GB of virtual memory.

Some context. Linux always reserves the first 16KB of virtual address
space (unless you tinker with some settings which nobody should do).

Example of 32-bit code:
lea eax, [0xffff_0000]
mov ebx, [eax + 0x1_0000]

The address calculated by the mov will wrap around to 0x0 which will
result in SIGSEGV. If FEX messes up zero extensions then it would try to
access 0x1_0000_0000 instead.

This could result in a 32-bit application potentially accessing some FEX
memory instead of crashing.
Add this safety net which will still SIGSEGV and we will be able to see
the crash.
This commit is contained in:
Ryan Houdek 2024-02-21 15:38:43 -08:00
parent 3d671cba10
commit be96cb7bd0
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1 changed files with 8 additions and 0 deletions
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8
Source/Tools/FEXLoader/FEXLoader.cpp
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@ -417,11 +417,17 @@ int main(int argc, char **argv, char **const envp) {
fextl::unique_ptr<FEX::HLE::MemAllocator> Allocator;
fextl::vector<FEXCore::Allocator::MemoryRegion> Base48Bit;
fextl::vector<FEXCore::Allocator::MemoryRegion> Low4GB;
if (Loader.Is64BitMode()) {
// Destroy the 48th bit if it exists
Base48Bit = FEXCore::Allocator::Steal48BitVA();
} else {
// Reserve [0x1_0000_0000, 0x2_0000_0000).
// Safety net if 32-bit address calculation overflows in to 64-bit range.
constexpr uint64_t First64BitAddr = 0x1'0000'0000ULL;
Low4GB = FEXCore::Allocator::StealMemoryRegion(First64BitAddr, First64BitAddr + First64BitAddr);
// Setup our userspace allocator
FEXCore::Allocator::SetupHooks();
Allocator = FEX::HLE::CreatePassthroughAllocator();
@ -578,6 +584,8 @@ int main(int argc, char **argv, char **const envp) {
FEXCore::Allocator::ClearHooks();
FEXCore::Allocator::ReclaimMemoryRegion(Base48Bit);
FEXCore::Allocator::ReclaimMemoryRegion(Low4GB);
// Allocator is now original system allocator
FEXCore::Telemetry::Shutdown(Program.ProgramName);
FEXCore::Profiler::Shutdown();