Messages in this thread | | | Date | Thu, 13 Sep 2018 18:19:29 +0200 | From | Guillaume Nault <> | Subject | Re: KMSAN: uninit-value in pppoe_rcv |
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On Thu, Sep 13, 2018 at 04:12:38PM +0200, Alexander Potapenko wrote: > On Thu, Sep 13, 2018 at 3:57 PM Eric Dumazet <eric.dumazet@gmail.com> wrote: > > > > > > > > On 09/12/2018 03:38 AM, Alexander Potapenko wrote: > > > On Wed, Sep 12, 2018 at 12:24 PM syzbot > > > <syzbot+f5f6080811c849739212@syzkaller.appspotmail.com> wrote: > > >> > > >> Hello, > > >> > > >> syzbot found the following crash on: > > >> > > >> HEAD commit: d2d741e5d189 kmsan: add initialization for shmem pages > > >> git tree: https://github.com/google/kmsan.git/master > > >> console output: https://syzkaller.appspot.com/x/log.txt?x=1465fc37800000 > > >> kernel config: https://syzkaller.appspot.com/x/.config?x=48f9de3384bcd0f > > >> dashboard link: https://syzkaller.appspot.com/bug?extid=f5f6080811c849739212 > > >> compiler: clang version 7.0.0 (trunk 329391) > > >> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14d6e607800000 > > >> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=10a15b5b800000 > > >> > > >> IMPORTANT: if you fix the bug, please add the following tag to the commit: > > >> Reported-by: syzbot+f5f6080811c849739212@syzkaller.appspotmail.com > > >> > > >> IPVS: ftp: loaded support on port[0] = 21 > > >> ================================================================== > > >> BUG: KMSAN: uninit-value in __get_item drivers/net/ppp/pppoe.c:172 [inline] > > >> BUG: KMSAN: uninit-value in get_item drivers/net/ppp/pppoe.c:236 [inline] > > >> BUG: KMSAN: uninit-value in pppoe_rcv+0xcef/0x10e0 > > >> drivers/net/ppp/pppoe.c:450 > > >> CPU: 0 PID: 4543 Comm: syz-executor355 Not tainted 4.16.0+ #87 > > >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > >> Google 01/01/2011 > > >> Call Trace: > > >> __dump_stack lib/dump_stack.c:17 [inline] > > >> dump_stack+0x185/0x1d0 lib/dump_stack.c:53 > > >> kmsan_report+0x142/0x240 mm/kmsan/kmsan.c:1067 > > >> __msan_warning_32+0x6c/0xb0 mm/kmsan/kmsan_instr.c:683 > > >> __get_item drivers/net/ppp/pppoe.c:172 [inline] > > >> get_item drivers/net/ppp/pppoe.c:236 [inline] > > >> pppoe_rcv+0xcef/0x10e0 drivers/net/ppp/pppoe.c:450 > > >> __netif_receive_skb_core+0x47df/0x4a90 net/core/dev.c:4562 > > >> __netif_receive_skb net/core/dev.c:4627 [inline] > > >> netif_receive_skb_internal+0x49d/0x630 net/core/dev.c:4701 > > >> netif_receive_skb+0x230/0x240 net/core/dev.c:4725 > > >> tun_rx_batched drivers/net/tun.c:1555 [inline] > > >> tun_get_user+0x740f/0x7c60 drivers/net/tun.c:1962 > > >> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990 > > >> call_write_iter include/linux/fs.h:1782 [inline] > > >> new_sync_write fs/read_write.c:469 [inline] > > >> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482 > > >> vfs_write+0x463/0x8d0 fs/read_write.c:544 > > >> SYSC_write+0x172/0x360 fs/read_write.c:589 > > >> SyS_write+0x55/0x80 fs/read_write.c:581 > > >> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287 > > >> entry_SYSCALL_64_after_hwframe+0x3d/0xa2 > > >> RIP: 0033:0x4447c9 > > >> RSP: 002b:00007fff64c8fc28 EFLAGS: 00000297 ORIG_RAX: 0000000000000001 > > >> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00000000004447c9 > > >> RDX: 000000000000fd87 RSI: 0000000020000600 RDI: 0000000000000004 > > >> RBP: 00000000006cf018 R08: 00007fff64c8fda8 R09: 00007fff00006bda > > >> R10: 0000000000005fe7 R11: 0000000000000297 R12: 00000000004020d0 > > >> R13: 0000000000402160 R14: 0000000000000000 R15: 0000000000000000 > > >> > > >> Uninit was created at: > > >> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:278 [inline] > > >> kmsan_internal_poison_shadow+0xb8/0x1b0 mm/kmsan/kmsan.c:188 > > >> kmsan_kmalloc+0x94/0x100 mm/kmsan/kmsan.c:314 > > >> kmsan_slab_alloc+0x11/0x20 mm/kmsan/kmsan.c:321 > > >> slab_post_alloc_hook mm/slab.h:445 [inline] > > >> slab_alloc_node mm/slub.c:2737 [inline] > > >> __kmalloc_node_track_caller+0xaed/0x11c0 mm/slub.c:4369 > > >> __kmalloc_reserve net/core/skbuff.c:138 [inline] > > >> __alloc_skb+0x2cf/0x9f0 net/core/skbuff.c:206 > > >> alloc_skb include/linux/skbuff.h:984 [inline] > > >> alloc_skb_with_frags+0x1d4/0xb20 net/core/skbuff.c:5234 > > >> sock_alloc_send_pskb+0xb56/0x1190 net/core/sock.c:2085 > > >> tun_alloc_skb drivers/net/tun.c:1532 [inline] > > >> tun_get_user+0x2242/0x7c60 drivers/net/tun.c:1829 > > >> tun_chr_write_iter+0x1d4/0x330 drivers/net/tun.c:1990 > > >> call_write_iter include/linux/fs.h:1782 [inline] > > >> new_sync_write fs/read_write.c:469 [inline] > > >> __vfs_write+0x7fb/0x9f0 fs/read_write.c:482 > > >> vfs_write+0x463/0x8d0 fs/read_write.c:544 > > >> SYSC_write+0x172/0x360 fs/read_write.c:589 > > >> SyS_write+0x55/0x80 fs/read_write.c:581 > > >> do_syscall_64+0x309/0x430 arch/x86/entry/common.c:287 > > >> entry_SYSCALL_64_after_hwframe+0x3d/0xa2 > > >> ================================================================== > > > I did a little digging before sending the bug upstream. > > > If I add memset(obj, 0xfe, size) to __kmalloc_reserve(), these 0xfe > > > bytes are visible in __get_item() at the place where KMSAN reports an > > > error. > > > > > > The problem is somehow related to tun_get_user() creating a fragmented > > > sk_buff - when I change the call to tun_alloc_skb() so that it > > > allocates a single buffer the bug goes away. > > > > > > > I guess the following patch would fix the issue > > > > (I will submit it more formally) > No, as far as I can see it doesn't. > Saving sid before __skb_pull() is still a good idea, but in this > particular case |ph| doesn't change.
Yes, we probably need to save sid.
But I think the problem found by syzbot is related to eth_hdr(skb)->h_source. PPPoE expects that Ethernet header has already been parsed and is accessible at skb_mac_header(skb). But here skb_mac_header(skb) == skb->data, and we may pull only 6 bytes (sizeof(truct pppoe_hdr)). Therefore eth_hdr(skb)->h_source points past skb's head length.
Not sure if something needs to be changed in tun.c for properly setting skb_mac_header. But PPPoE has no reason to consider packets from non-Ethernet devices anyway.
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