On Tue, Dec 17, 2019 at 02:15:48AM +0530, Ajay Kaher wrote: > From: Vlastimil Babka <vbabka@xxxxxxx> > > The x86 version of get_user_pages_fast() relies on disabled interrupts to > synchronize gup_pte_range() between gup_get_pte(ptep); and get_page() against > a parallel munmap. The munmap side nulls the pte, then flushes TLBs, then > releases the page. As TLB flush is done synchronously via IPI disabling > interrupts blocks the page release, and get_page(), which assumes existing > reference on page, is thus safe. > However when TLB flush is done by a hypercall, e.g. in a Xen PV guest, there is > no blocking thanks to disabled interrupts, and get_page() can succeed on a page > that was already freed or even reused. > > We have recently seen this happen with our 4.4 and 4.12 based kernels, with > userspace (java) that exits a thread, where mm_release() performs a futex_wake() > on tsk->clear_child_tid, and another thread in parallel unmaps the page where > tsk->clear_child_tid points to. The spurious get_page() succeeds, but futex code > immediately releases the page again, while it's already on a freelist. Symptoms > include a bad page state warning, general protection faults acessing a poisoned > list prev/next pointer in the freelist, or free page pcplists of two cpus joined > together in a single list. Oscar has also reproduced this scenario, with a > patch inserting delays before the get_page() to make the race window larger. > > Fix this by removing the dependency on TLB flush interrupts the same way as the This is suppsed to be fixed by: arch/x86/Kconfig: select HAVE_RCU_TABLE_FREE if PARAVIRT
