Re: [syzbot] [net?] KMSAN: uninit-value in ipgre_xmit

[Eric Dumazet]

On Wed, Nov 29, 2023 at 11:00 AM syzbot
<syzbot+2cb7b1bd08dc77ae7f89@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    18d46e76d7c2 Merge tag 'for-6.7-rc3-tag' of git://git.kern..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1412e7e8e80000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=f711bc2a7eb1db25
> dashboard link: https://syzkaller.appspot.com/bug?extid=2cb7b1bd08dc77ae7f89
> compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> userspace arch: i386
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/cb96093de792/disk-18d46e76.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/630ca4e2d778/vmlinux-18d46e76.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/65573a727973/bzImage-18d46e76.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2cb7b1bd08dc77ae7f89@xxxxxxxxxxxxxxxxxxxxxxxxx
>
> =====================================================
> BUG: KMSAN: uninit-value in __gre_xmit net/ipv4/ip_gre.c:469 [inline]
> BUG: KMSAN: uninit-value in ipgre_xmit+0xdc2/0xe20 net/ipv4/ip_gre.c:662
>  __gre_xmit net/ipv4/ip_gre.c:469 [inline]
>  ipgre_xmit+0xdc2/0xe20 net/ipv4/ip_gre.c:662
>  __netdev_start_xmit include/linux/netdevice.h:4940 [inline]
>  netdev_start_xmit include/linux/netdevice.h:4954 [inline]
>  xmit_one net/core/dev.c:3545 [inline]
>  dev_hard_start_xmit+0x247/0xa10 net/core/dev.c:3561
>  __dev_queue_xmit+0x33b8/0x5130 net/core/dev.c:4346
>  dev_queue_xmit include/linux/netdevice.h:3134 [inline]
>  __bpf_tx_skb net/core/filter.c:2133 [inline]
>  __bpf_redirect_no_mac net/core/filter.c:2163 [inline]
>  __bpf_redirect+0xdd7/0x1600 net/core/filter.c:2186
>  ____bpf_clone_redirect net/core/filter.c:2457 [inline]
>  bpf_clone_redirect+0x328/0x470 net/core/filter.c:2429
>  ___bpf_prog_run+0x2180/0xdb80 kernel/bpf/core.c:1958
>  __bpf_prog_run512+0xb5/0xe0 kernel/bpf/core.c:2199
>  bpf_dispatcher_nop_func include/linux/bpf.h:1196 [inline]
>  __bpf_prog_run include/linux/filter.h:651 [inline]
>  bpf_prog_run include/linux/filter.h:658 [inline]
>  bpf_test_run+0x482/0xb00 net/bpf/test_run.c:423
>  bpf_prog_test_run_skb+0x14e5/0x1f20 net/bpf/test_run.c:1045
>  bpf_prog_test_run+0x6af/0xac0 kernel/bpf/syscall.c:4040
>  __sys_bpf+0x649/0xd60 kernel/bpf/syscall.c:5401
>  __do_sys_bpf kernel/bpf/syscall.c:5487 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5485 [inline]
>  __ia32_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5485
>  do_syscall_32_irqs_on arch/x86/entry/common.c:164 [inline]
>  __do_fast_syscall_32+0xa2/0x100 arch/x86/entry/common.c:230
>  do_fast_syscall_32+0x37/0x70 arch/x86/entry/common.c:255
>  do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:293
>  entry_SYSENTER_compat_after_hwframe+0x70/0x7a
>
> Uninit was created at:
>  slab_post_alloc_hook+0x129/0xa70 mm/slab.h:768
>  slab_alloc_node mm/slub.c:3478 [inline]
>  kmem_cache_alloc_node+0x5e9/0xb10 mm/slub.c:3523
>  kmalloc_reserve+0x13d/0x4a0 net/core/skbuff.c:560
>  pskb_expand_head+0x226/0x1a00 net/core/skbuff.c:2098
>  skb_ensure_writable+0x3d3/0x460 net/core/skbuff.c:5958
>  __bpf_try_make_writable net/core/filter.c:1662 [inline]
>  bpf_try_make_writable net/core/filter.c:1668 [inline]
>  bpf_try_make_head_writable net/core/filter.c:1676 [inline]
>  ____bpf_clone_redirect net/core/filter.c:2451 [inline]
>  bpf_clone_redirect+0x17f/0x470 net/core/filter.c:2429
>  ___bpf_prog_run+0x2180/0xdb80 kernel/bpf/core.c:1958
>  __bpf_prog_run512+0xb5/0xe0 kernel/bpf/core.c:2199
>  bpf_dispatcher_nop_func include/linux/bpf.h:1196 [inline]
>  __bpf_prog_run include/linux/filter.h:651 [inline]
>  bpf_prog_run include/linux/filter.h:658 [inline]
>  bpf_test_run+0x482/0xb00 net/bpf/test_run.c:423
>  bpf_prog_test_run_skb+0x14e5/0x1f20 net/bpf/test_run.c:1045
>  bpf_prog_test_run+0x6af/0xac0 kernel/bpf/syscall.c:4040
>  __sys_bpf+0x649/0xd60 kernel/bpf/syscall.c:5401
>  __do_sys_bpf kernel/bpf/syscall.c:5487 [inline]
>  __se_sys_bpf kernel/bpf/syscall.c:5485 [inline]
>  __ia32_sys_bpf+0xa0/0xe0 kernel/bpf/syscall.c:5485
>  do_syscall_32_irqs_on arch/x86/entry/common.c:164 [inline]
>  __do_fast_syscall_32+0xa2/0x100 arch/x86/entry/common.c:230
>  do_fast_syscall_32+0x37/0x70 arch/x86/entry/common.c:255
>  do_SYSENTER_32+0x1f/0x30 arch/x86/entry/common.c:293
>  entry_SYSENTER_compat_after_hwframe+0x70/0x7a
>
> CPU: 1 PID: 8859 Comm: syz-executor.2 Not tainted 6.7.0-rc3-syzkaller-00024-g18d46e76d7c2 #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/10/2023
> =====================================================
>
>
> ---
> This report is generated by a bot. It may contain errors.
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> syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxx.
>
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>
> If the report is already addressed, let syzbot know by replying with:
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>
> If you want to overwrite report's subsystems, reply with:
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>
> If the report is a duplicate of another one, reply with:
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>
> If you want to undo deduplication, reply with:
> #syz undup

Fix under discussion would be:

diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 22a26d1d29a09d234f18ce3b0f329e5047c0c046..5169c3c72cffe49cef613e69889d139db867ff74
100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -635,15 +635,18 @@ static netdev_tx_t ipgre_xmit(struct sk_buff *skb,
        }

        if (dev->header_ops) {
+               int pull_len = tunnel->hlen + sizeof(struct iphdr);
+
                if (skb_cow_head(skb, 0))
                        goto free_skb;

                tnl_params = (const struct iphdr *)skb->data;

-               /* Pull skb since ip_tunnel_xmit() needs skb->data pointing
-                * to gre header.
-                */
-               skb_pull(skb, tunnel->hlen + sizeof(struct iphdr));
+               if (!pskb_network_may_pull(skb, pull_len))
+                       goto free_skb;
+
+               /* ip_tunnel_xmit() needs skb->data pointing to gre header. */
+               skb_pull(skb, pull_len);
                skb_reset_mac_header(skb);

                if (skb->ip_summed == CHECKSUM_PARTIAL &&