On 1/3/19 12:14 PM, Alexander Potapenko wrote: > On Thu, Jan 3, 2019 at 9:42 AM Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: >> >> On Thu, Jan 3, 2019 at 9:36 AM Vlastimil Babka <vbabka@xxxxxxx> wrote: >>> >>> >>> On 12/31/18 8:51 AM, syzbot wrote: >>>> Hello, >>>> >>>> syzbot found the following crash on: >>>> >>>> HEAD commit: 79fc24ff6184 kmsan: highmem: use kmsan_clear_page() in cop.. >>>> git tree: kmsan >>>> console output: https://syzkaller.appspot.com/x/log.txt?x=13c48b67400000 >>>> kernel config: https://syzkaller.appspot.com/x/.config?x=901dd030b2cc57e7 >>>> dashboard link: https://syzkaller.appspot.com/bug?extid=b19c2dc2c990ea657a71 >>>> compiler: clang version 8.0.0 (trunk 349734) >>>> >>>> Unfortunately, I don't have any reproducer for this crash yet. >>>> >>>> IMPORTANT: if you fix the bug, please add the following tag to the commit: >>>> Reported-by: syzbot+b19c2dc2c990ea657a71@xxxxxxxxxxxxxxxxxxxxxxxxx >>>> >>>> ================================================================== >>>> BUG: KMSAN: uninit-value in mpol_rebind_policy mm/mempolicy.c:353 [inline] >>>> BUG: KMSAN: uninit-value in mpol_rebind_mm+0x249/0x370 mm/mempolicy.c:384 >>> >>> The report doesn't seem to indicate where the uninit value resides in >>> the mempolicy object. >> >> Yes, it doesn't and it's not trivial to do. The tool reports uses of >> unint _values_. Values don't necessary reside in memory. It can be a >> register, that come from another register that was calculated as a sum >> of two other values, which may come from a function argument, etc. >> >>> I'll have to guess. mm/mempolicy.c:353 contains: >>> >>> if (!mpol_store_user_nodemask(pol) && >>> nodes_equal(pol->w.cpuset_mems_allowed, *newmask)) >>> >>> "mpol_store_user_nodemask(pol)" is testing pol->flags, which I couldn't >>> see being uninitialized after leaving mpol_new(). So I'll guess it's >>> actually about accessing pol->w.cpuset_mems_allowed on line 354. >>> >>> For w.cpuset_mems_allowed to be not initialized and the nodes_equal() >>> reachable for a mempolicy where mpol_set_nodemask() is called in >>> do_mbind(), it seems the only possibility is a MPOL_PREFERRED policy >>> with empty set of nodes, i.e. MPOL_LOCAL equivalent. Let's see if the >>> patch below helps. This code is a maze to me. Note the uninit access >>> should be benign, rebinding this kind of policy is always a no-op. > If I'm reading mempolicy.c right, `pol->flags & MPOL_F_LOCAL` doesn't > imply `pol->mode == MPOL_PREFERRED`, shouldn't we check for both here? I think it does? Only preferred mempolicies set it, including default_policy, and MPOL_LOCAL is converted to MPOL_PREFERRED internally. Anyway we would need the opposite implication here to be safe, and that's also true. >>> ----8<---- >>> From ff0ca29da6bc2572d7b267daa77ced6083e3f02d Mon Sep 17 00:00:00 2001 >>> From: Vlastimil Babka <vbabka@xxxxxxx> >>> Date: Thu, 3 Jan 2019 09:31:59 +0100 >>> Subject: [PATCH] mm, mempolicy: fix uninit memory access >>> >>> --- >>> mm/mempolicy.c | 2 +- >>> 1 file changed, 1 insertion(+), 1 deletion(-) >>> >>> diff --git a/mm/mempolicy.c b/mm/mempolicy.c >>> index d4496d9d34f5..a0b7487b9112 100644 >>> --- a/mm/mempolicy.c >>> +++ b/mm/mempolicy.c >>> @@ -350,7 +350,7 @@ static void mpol_rebind_policy(struct mempolicy *pol, const nodemask_t *newmask) >>> { >>> if (!pol) >>> return; >>> - if (!mpol_store_user_nodemask(pol) && >>> + if (!mpol_store_user_nodemask(pol) && !(pol->flags & MPOL_F_LOCAL) && >>> nodes_equal(pol->w.cpuset_mems_allowed, *newmask)) >>> return; >>> >>> -- >>> 2.19.2 >>> >>> -- >>> You received this message because you are subscribed to the Google Groups "syzkaller-bugs" group. >>> To unsubscribe from this group and stop receiving emails from it, send an email to syzkaller-bugs+unsubscribe@xxxxxxxxxxxxxxxx. >>> To view this discussion on the web visit https://groups.google.com/d/msgid/syzkaller-bugs/a71997c3-e8ae-a787-d5ce-3db05768b27c%40suse.cz. >>> For more options, visit https://groups.google.com/d/optout. > > >
