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A race condition was found in the way the Linux kernel's memory subsystem handled the copy-on-write (COW) breakage of private read-only memory mappings. All the information we have so far is included in this page.
The bug has existed since around 2.6.22 (released in 2007) and was fixed on Oct 18, 2016. List of patched versions here
There are proof of concepts available here.
- An unprivileged local user could use this flaw to gain write access to otherwise read-only memory mappings and thus increase their privileges on the system.
- This flaw allows an attacker with a local system account to modify on-disk binaries, bypassing the standard permission mechanisms that would prevent modification without an appropriate permission set.
faultin_page handle_mm_fault __handle_mm_fault handle_pte_fault do_fault <- pte is not present do_cow_fault <- FAULT_FLAG_WRITE alloc_set_pte maybe_mkwrite(pte_mkdirty(entry), vma) <- mark the page dirty but keep it RO # Returns with 0 and retry follow_page_mask follow_page_pte (flags & FOLL_WRITE) && !pte_write(pte) <- retry fault faultin_page handle_mm_fault __handle_mm_fault handle_pte_fault FAULT_FLAG_WRITE && !pte_write do_wp_page PageAnon() <- this is CoWed page already reuse_swap_page <- page is exclusively ours wp_page_reuse maybe_mkwrite <- dirty but RO again ret = VM_FAULT_WRITE ((ret & VM_FAULT_WRITE) && !(vma->vm_flags & VM_WRITE)) <- we drop FOLL_WRITE # Returns with 0 and retry as a read fault cond_resched -> different thread will now unmap via madvise follow_page_mask !pte_present && pte_none faultin_page handle_mm_fault __handle_mm_fault handle_pte_fault do_fault <- pte is not present do_read_fault <- this is a read fault and we will get pagecache page!
- The In The Wild exploit relied on writing to /proc/self/mem on one side of the race.
- ptrace(PTRACE_POKEDATA) can write to readonly mappings.
- The attack relies on racing the madvise(MADV_DONTNEED) system call while having the page of the executable mmapped in memory.
commit 4ceb5db9757aaeadcf8fbbf97d76bd42aa4df0d6 Author: Linus Torvalds <email@example.com> Date: Mon Aug 1 11:14:49 2005 -0700 Fix get_user_pages() race for write access There's no real guarantee that handle_mm_fault() will always be able to break a COW situation - if an update from another thread ends up modifying the page table some way, handle_mm_fault() may end up requiring us to re-try the operation. That's normally fine, but get_user_pages() ended up re-trying it as a read, and thus a write access could in theory end up losing the dirty bit or be done on a page that had not been properly COW'ed. This makes get_user_pages() always retry write accesses as write accesses by making "follow_page()" require that a writable follow has the dirty bit set. That simplifies the code and solves the race: if the COW break fails for some reason, we'll just loop around and try again. commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 Author: Linus Torvalds <firstname.lastname@example.org> Date: Thu Oct 13 20:07:36 2016 GMT This is an ancient bug that was actually attempted to be fixed once (badly) by me eleven years ago in commit 4ceb5db9757a ("Fix get_user_pages() race for write access") but that was then undone due to problems on s390 by commit f33ea7f404e5 ("fix get_user_pages bug"). In the meantime, the s390 situation has long been fixed, and we can now fix it by checking the pte_dirty() bit properly (and do it better). The s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement software dirty bits") which made it into v3.9. Earlier kernels will have to look at the page state itself. Also, the VM has become more scalable, and what used a purely theoretical race back then has become easier to trigger. To fix it, we introduce a new internal FOLL_COW flag to mark the "yes, we already did a COW" rather than play racy games with FOLL_WRITE that is very fundamental, and then use the pte dirty flag to validate that the FOLL_COW flag is still valid.