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Fix COW race condition in mem_ptr under concurrent write faults #2706

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Fix COW race condition in mem_ptr under concurrent write faults #2706

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15 changes: 9 additions & 6 deletions kernel/memory.c
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Original file line number Diff line number Diff line change
Expand Up @@ -276,15 +276,18 @@ void *mem_ptr(struct mem *mem, addr_t addr, int type) {
asbestos_invalidate_page(mem->mmu.asbestos, page);
// if page is cow, ~~milk~~ copy it
if (entry->flags & P_COW) {
void *data = (char *) entry->data->data + entry->offset;
void *copy = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);

// copy/paste from above
read_wrunlock(&mem->lock);
write_wrlock(&mem->lock);
memcpy(copy, data, PAGE_SIZE);
pt_map(mem, page, 1, copy, 0, entry->flags &~ P_COW);
// re-fetch entry after lock upgrade, another thread may have COW'd it
entry = mem_pt(mem, page);
if (entry != NULL && (entry->flags & P_COW)) {
void *data = (char *) entry->data->data + entry->offset;
void *copy = mmap(NULL, PAGE_SIZE, PROT_READ | PROT_WRITE,
MAP_PRIVATE | MAP_ANONYMOUS, 0, 0);
memcpy(copy, data, PAGE_SIZE);
pt_map(mem, page, 1, copy, 0, entry->flags &~ P_COW);
}
write_wrunlock(&mem->lock);
read_wrlock(&mem->lock);
}
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