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SharedBuffer: Fix a denial of service

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It's a very bad idea to increment the refcount on behalf of another
process. That process may (for either benign or evil reasons) not
reference the SharedBuffer, and then we'll be stuck with loads of
SharedBuffers until we OOM.

Instead, increment the refcount when the buffer is mapped. That way, a
buffer is only kept if *someone* has explicitly requested it via
get_shared_buffer.

Fixes #341
This commit is contained in:
Robin Burchell 2019-07-19 17:46:21 +02:00 • committed by Andreas Kling
parent f8beb0f665
commit 2d4d465206
3 changed files with 13 additions and 10 deletions
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10
Kernel/SharedBuffer.cpp
View file

@ -21,12 +21,14 @@ bool SharedBuffer::is_shared_with(pid_t peer_pid)
return false;
}
void* SharedBuffer::get_address(Process& process)
void* SharedBuffer::ref_for_process_and_get_address(Process& process)
{
LOCKER(shared_buffers().lock());
ASSERT(is_shared_with(process.pid()));
for (auto& ref : m_refs) {
if (ref.pid == process.pid()) {
ref.count++;
m_total_refs++;
if (ref.region == nullptr) {
ref.region = process.allocate_region_with_vmo(VirtualAddress(), size(), m_vmo, 0, "SharedBuffer", PROT_READ | (m_writable ? PROT_WRITE : 0));
ref.region->set_shared(true);
@ -42,7 +44,7 @@ void SharedBuffer::share_with(pid_t peer_pid)
LOCKER(shared_buffers().lock());
for (auto& ref : m_refs) {
if (ref.pid == peer_pid) {
ref.count++;
// don't increment the reference count yet; let them get_shared_buffer it first.
return;
}
}
@ -50,7 +52,7 @@ void SharedBuffer::share_with(pid_t peer_pid)
m_refs.append(Reference(peer_pid));
}
void SharedBuffer::release(Process& process)
void SharedBuffer::deref_for_process(Process& process)
{
LOCKER(shared_buffers().lock());
for (int i = 0; i < m_refs.size(); ++i) {
@ -94,7 +96,7 @@ void SharedBuffer::disown(pid_t pid)
void SharedBuffer::destroy_if_unused()
{
LOCKER(shared_buffers().lock());
if (m_refs.size() == 0) {
if (m_total_refs == 0) {
#ifdef SHARED_BUFFER_DEBUG
kprintf("Destroying unused SharedBuffer{%p} id: %d\n", this, m_shared_buffer_id);
#endif