On Thu, Mar 30, 2023 at 02:56:50PM +0100, Steven Price wrote: > > On Wed, Mar 29, 2023 at 02:55:49AM +0000, Qun-wei Lin (林群崴) wrote: > >> Having compared the differences between Kernel-5.15 and Kernel-6.1, > >> We found the order of swap_free() and set_pte_at() is changed in > >> do_swap_page(). > >> > >> When fault in, do_swap_page() will call swap_free() first: > >> do_swap_page() -> swap_free() -> __swap_entry_free() -> > >> free_swap_slot() -> swapcache_free_entries() -> swap_entry_free() -> > >> swap_range_free() -> arch_swap_invalidate_page() -> > >> mte_invalidate_tags_area() -> mte_invalidate_tags() -> xa_erase() > >> > >> and then call set_pte_at(): > >> do_swap_page() -> set_pte_at() -> __set_pte_at() -> mte_sync_tags() -> > >> mte_sync_page_tags() -> mte_restore_tags() -> xa_load() > >> > >> This means that the swap slot is invalidated before pte mapping, and > >> this will cause the mte tag in XArray to be released before tag > >> restore. > > This analysis looks correct to me. The MTE swap code works on the > assumption that the set_pte_at() will restore the tags to the page > before the swap entry is removed. The reordering which has happened > since has broken this assumption and as you observed can cause the tags > to be unavailable by the time set_pte_at() is called. > > >> After I moved swap_free() to the next line of set_pte_at(), the problem > >> is disappeared. > >> > >> We suspect that the following patches, which have changed the order, do > >> not consider the mte tag restoring in page fault flow: > >> https://lore.kernel.org/all/20220131162940.210846-5-david@xxxxxxxxxx/ > > I'm not sure I entirely follow the reasoning in this patch, so I'm not > sure whether it's safe to just move swap_free() down to below > set_pte_at() or if that reintroduces the information leak. > > I also wonder if sparc has a similar issue as the arch_do_swap() > callback is located next to set_pte_at(). SPARC has a potential race here since the page is made visible to the user but the tags are not restored yet (I raised this before). But even ignoring this small window, arch_do_swap() needs to have the metadata available. > >> Any suggestion is appreciated. > > The other possibility is to add a(nother) callback for MTE in > arch_do_swap() that calls mte_restore_tags() on the page before the > swap_free() call rather than depending on the hook in set_pte_at(). I think we should move arch_do_swap_page() earlier before swap_free() and in arm64 we copy the tags to pte_page(pte). I don't think SPARC would have any issues with this change (and it also fixes their race). -- Catalin
