On 12/16/19 2:47 PM, Peter Zijlstra wrote: > On Mon, Dec 16, 2019 at 02:30:44PM +0100, Vitaly Kuznetsov wrote: >> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes: >> >>> 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 >>> >> >> Yes, Well, that commit fixes the "page table can be freed under us" part. But this patch is about the "get_page() will succeed on a page that's being freed" part. Upstream fixed that unknowingly in 4.13 by a gup.c refactoring that would be too risky to backport fully. >> but HAVE_RCU_TABLE_FREE was enabled on x86 only in 4.14: >> >> commit 9e52fc2b50de3a1c08b44f94c610fbe998c0031a >> Author: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> >> Date: Mon Aug 28 10:22:51 2017 +0200 >> >> x86/mm: Enable RCU based page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y) >> >> and, if I understood correctly, Ajay is suggesting the patch for older >> stable kernels (4.9 and 4.4 I would guess). > > It wasn't at all clear this was targeted at old kernels (I only got this > one patch). > > And why can't those necro kernels do backports of the upstream solution? >
