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xmit_check_hhlen() observes the dst for getting the device hard header length to make sure a modified packet can fit. When a helper which changes the dst - such as bpf_skb_set_tunnel_key() - is called as part of the xmit program the accessed dst is no longer valid. This leads to the following splat: BUG: kernel NULL pointer dereference, address: 00000000000000de #PF: supervisor read access in kernel mode #PF: error_code(0x0000) - not-present page PGD 0 P4D 0 Oops: 0000 [#1] PREEMPT SMP PTI CPU: 0 PID: 798 Comm: ping Not tainted 5.18.0-rc2+ #103 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014 RIP: 0010:bpf_xmit+0xfb/0x17f Code: c6 c0 4d cd 8e 48 c7 c7 7d 33 f0 8e e8 42 09 fb ff 48 8b 45 58 48 8b 95 c8 00 00 00 48 2b 95 c0 00 00 00 48 83 e0 fe 48 8b 00 <0f> b7 80 de 00 00 00 39 c2 73 22 29 d0 b9 20 0a 00 00 31 d2 48 89 RSP: 0018:ffffb148c0bc7b98 EFLAGS: 00010282 RAX: 0000000000000000 RBX: 0000000000240008 RCX: 0000000000000000 RDX: 0000000000000010 RSI: 00000000ffffffea RDI: 00000000ffffffff RBP: ffff922a828a4e00 R08: ffffffff8f1350e8 R09: 00000000ffffdfff R10: ffffffff8f055100 R11: ffffffff8f105100 R12: 0000000000000000 R13: ffff922a828a4e00 R14: 0000000000000040 R15: 0000000000000000 FS: 00007f414e8f0080(0000) GS:ffff922afdc00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00000000000000de CR3: 0000000002d80006 CR4: 0000000000370ef0 Call Trace: <TASK> lwtunnel_xmit.cold+0x71/0xc8 ip_finish_output2+0x279/0x520 ? __ip_finish_output.part.0+0x21/0x130 Fix by fetching the device hard header length before running the BPF code. Fixes: 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure") Signed-off-by: Eyal Birger <eyal.birger@gmail.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20220420165219.1755407-1-eyal.birger@gmail.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.