Catalin Marinas <[email protected]> writes: > On Wed, Sep 23, 2026 at 11:23:27AM +0530, Aneesh Kumar K.V wrote: >> Catalin Marinas <[email protected]> writes: >> > On Mon, Sep 21, 2026 at 08:18:36PM +0530, Aneesh Kumar K.V (Arm) wrote: >> >> +int alloc_cc_shared_pages_node(int nid, gfp_t gfp, >> >> + size_t requested, struct cc_shared_pages *mem) >> >> +{ >> >> + struct cc_shared_layout layout; >> >> + struct page *page; >> >> + unsigned int order; >> >> + bool zero = gfp & __GFP_ZERO; >> >> + int ret; >> >> + >> >> + if (!mem) >> >> + return -EINVAL; >> >> + >> >> + ret = cc_shared_calc_layout(requested, &layout); >> >> + if (ret) >> >> + return ret; >> >> + >> >> + order = get_order(layout.shared_size); >> >> + if (order > MAX_PAGE_ORDER) >> >> + return -EINVAL; >> >> + >> >> + /* >> >> + * State transitions require a linear-map address and may modify memory. >> >> + * Allocate from low memory and defer requested zeroing until >> >> afterwards. >> >> + */ >> >> + gfp &= ~(__GFP_HIGHMEM | __GFP_ZERO); >> >> + if (nid == NUMA_NO_NODE) >> >> + page = alloc_pages(gfp, order); >> >> + else >> >> + page = alloc_pages_node(nid, gfp, order); >> >> + if (!page) >> >> + return -ENOMEM; >> >> + >> >> + ret = cc_make_shared(page_address(page), layout.shared_size); >> >> + if (ret) { >> >> + if (!cc_make_private(page_address(page), layout.shared_size)) >> >> + __free_pages(page, order); >> >> + else >> >> + pr_warn_ratelimited("leaking %zu bytes with uncertain >> >> shared state\n", >> >> + layout.shared_size); >> >> + return ret; >> >> + } >> >> + >> >> + if (zero) >> >> + memset(page_address(page), 0, layout.shared_size); >> > >> > Does the memset() post sharing logic work for pKVM as well? If nothing >> > clears it, we have a small window where guest data is leaked to the >> > host. >> > >> > Is there a case where we *do not* need the memory cleared? If not, maybe >> > we can move the logic in the arch set_memory_decrypted(). >> > >> >> I don't think every architecture or platform can unconditionally zero >> memory in set_memory_decrypted(). Some callers may need to share valid >> contents with the host. > > Is there any? That would be a bad assumptions in the caller. Most > set_memory_* backends don't preserve the content as they change the > encryption key. So properly written code shouldn't rely on this unless > it knows specifically it's only running on pKVM for example. The only > use-case I see to avoid explicit zeroing is when the caller doesn't care > about the page initialisation and wants to save some cycles. The > encryption key change would take care of the security aspect. >
I checked this, and you are right. We cannot expect the contents to remain valid across sharing; set_memory_decrypted() is destructive in that sense. Since pKVM does not rely on memory encryption, it needs to zero the memory unconditionally in set_memory_decrypted() to avoid exposing existing guest data. Other CoCo implementations may omit the memset(0). We still need to zero the memory when the caller requests __GFP_ZERO, where the memory location is expected to be zero. We could either zero the memory unconditionally or pass a flag to allow this micro-optimization. We would also need to audit all call paths to avoid redundant zeroing after set_memory_decrypted(). Let me know if you have a preference for either approach. -aneesh
