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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
Notes: sideshowbarker 2024-07-19 13:08:06 +09:00 
Author: https://github.com/rburchell
Commit: https://github.com/SerenityOS/serenity/commit/2d4d465206d
Pull-request: https://github.com/SerenityOS/serenity/pull/346
3 changed files with 13 additions and 10 deletions
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6
Kernel/Process.cpp
View File

@ -2408,7 +2408,7 @@ int Process::sys$create_shared_buffer(int size, void** buffer)
int shared_buffer_id = ++s_next_shared_buffer_id;
auto shared_buffer = make<SharedBuffer>(shared_buffer_id, size);
shared_buffer->share_with(m_pid);
*buffer = shared_buffer->get_address(*this);
*buffer = shared_buffer->ref_for_process_and_get_address(*this);
ASSERT((int)shared_buffer->size() >= size);
#ifdef SHARED_BUFFER_DEBUG
kprintf("%s(%u): Created shared buffer %d @ %p (%u bytes, vmo is %u)\n", name().characters(), pid(), shared_buffer_id, *buffer, size, shared_buffer->size());
@ -2447,7 +2447,7 @@ int Process::sys$release_shared_buffer(int shared_buffer_id)
#ifdef SHARED_BUFFER_DEBUG
kprintf("%s(%u): Releasing shared buffer %d, buffer count: %u\n", name().characters(), pid(), shared_buffer_id, shared_buffers().resource().size());
#endif
shared_buffer.release(*this);
shared_buffer.deref_for_process(*this);
return 0;
}
@ -2463,7 +2463,7 @@ void* Process::sys$get_shared_buffer(int shared_buffer_id)
#ifdef SHARED_BUFFER_DEBUG
kprintf("%s(%u): Retaining shared buffer %d, buffer count: %u\n", name().characters(), pid(), shared_buffer_id, shared_buffers().resource().size());
#endif
return shared_buffer.get_address(*this);
return shared_buffer.ref_for_process_and_get_address(*this);
}
int Process::sys$seal_shared_buffer(int shared_buffer_id)

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

7
Kernel/SharedBuffer.h
View File

@ -12,7 +12,7 @@ private:
}
pid_t pid;
unsigned count { 1 };
unsigned count { 0 };
Region* region { nullptr };
};
public:
@ -33,9 +33,9 @@ public:
}
bool is_shared_with(pid_t peer_pid);
void* get_address(Process& process);
void* ref_for_process_and_get_address(Process& process);
void share_with(pid_t peer_pid);
void release(Process& process);
void deref_for_process(Process& process);
void disown(pid_t pid);
size_t size() const { return m_vmo->size(); }
void destroy_if_unused();
@ -45,6 +45,7 @@ public:
bool m_writable { true };
NonnullRefPtr<VMObject> m_vmo;
Vector<Reference, 2> m_refs;
unsigned m_total_refs { 0 };
};
Lockable<HashMap<int, OwnPtr<SharedBuffer>>>& shared_buffers();