[syzbot] [bpf?] [net?] WARNING in skb_ensure_writable

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 4ac828960a60 Merge branch 'eee-linkmode-bitmaps'
git tree: net-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=14710a54180000
kernel config: https://syzkaller.appspot.com/x/.config?x=57c41f64f37f51c5
dashboard link: https://syzkaller.appspot.com/bug?extid=0c4150bff9fff3bf023c
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17cc49d8180000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12c88616180000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/f1bd74942969/disk-4ac82896.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c99cbac61b8b/vmlinux-4ac82896.xz
kernel image: https://storage.googleapis.com/syzbot-assets/fa3d589c2a1c/bzImage-4ac82896.xz

The issue was bisected to:

commit 219eee9c0d16f1b754a8b85275854ab17df0850a
Author: Florian Westphal <f...@strlen.de>
Date: Fri Feb 16 11:36:57 2024 +0000

net: skbuff: add overflow debug check to pull/push helpers

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=166b3a4c180000
final oops: https://syzkaller.appspot.com/x/report.txt?x=156b3a4c180000
console output: https://syzkaller.appspot.com/x/log.txt?x=116b3a4c180000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0c4150...@syzkaller.appspotmail.com
Fixes: 219eee9c0d16 ("net: skbuff: add overflow debug check to pull/push helpers")

------------[ cut here ]------------
WARNING: CPU: 1 PID: 5074 at include/linux/skbuff.h:2723 pskb_may_pull_reason include/linux/skbuff.h:2723 [inline]
WARNING: CPU: 1 PID: 5074 at include/linux/skbuff.h:2723 pskb_may_pull include/linux/skbuff.h:2739 [inline]
WARNING: CPU: 1 PID: 5074 at include/linux/skbuff.h:2723 skb_ensure_writable+0x2ef/0x440 net/core/skbuff.c:6103
Modules linked in:
CPU: 1 PID: 5074 Comm: syz-executor365 Not tainted 6.8.0-rc5-syzkaller-01654-g4ac828960a60 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
RIP: 0010:pskb_may_pull_reason include/linux/skbuff.h:2723 [inline]
RIP: 0010:pskb_may_pull include/linux/skbuff.h:2739 [inline]
RIP: 0010:skb_ensure_writable+0x2ef/0x440 net/core/skbuff.c:6103
Code: e8 b6 f7 57 f8 4c 89 ef 31 f6 31 d2 b9 20 08 00 00 48 83 c4 28 5b 41 5c 41 5d 41 5e 41 5f 5d e9 17 04 fe ff e8 92 f7 57 f8 90 <0f> 0b 90 e9 3e fd ff ff 44 89 f7 44 89 e6 e8 3e f9 57 f8 45 39 e6
RSP: 0018:ffffc900039ef8f8 EFLAGS: 00010293
RAX: ffffffff893b75de RBX: ffff88802c8a1000 RCX: ffff88802a155940
RDX: 0000000000000000 RSI: 00000000fb6014e4 RDI: 0000000000000000
RBP: 00000000fb6014e4 R08: ffffffff893b7317 R09: 1ffffffff1f0bcb5
R10: dffffc0000000000 R11: ffffffffa0000954 R12: 00000000fb6014e4
R13: ffff88802c8a1000 R14: ffffc90000b06030 R15: dffffc0000000000
FS: 0000555556fa9380(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000001b7b398 CR3: 00000000241e8000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__bpf_try_make_writable net/core/filter.c:1665 [inline]
bpf_try_make_writable net/core/filter.c:1671 [inline]
____bpf_skb_pull_data net/core/filter.c:1862 [inline]
bpf_skb_pull_data+0x7c/0x230 net/core/filter.c:1851
bpf_prog_ca74b6b79e086095+0x1d/0x1f
bpf_dispatcher_nop_func include/linux/bpf.h:1235 [inline]
__bpf_prog_run include/linux/filter.h:651 [inline]
bpf_prog_run include/linux/filter.h:658 [inline]
bpf_test_run+0x408/0x900 net/bpf/test_run.c:423
bpf_prog_test_run_skb+0xaf9/0x13a0 net/bpf/test_run.c:1056
bpf_prog_test_run+0x33a/0x3b0 kernel/bpf/syscall.c:4188
__sys_bpf+0x48d/0x810 kernel/bpf/syscall.c:5591
__do_sys_bpf kernel/bpf/syscall.c:5680 [inline]
__se_sys_bpf kernel/bpf/syscall.c:5678 [inline]
__x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5678
do_syscall_64+0xf9/0x240
entry_SYSCALL_64_after_hwframe+0x6f/0x77
RIP: 0033:0x7f6a0471c4a9
Code: 48 83 c4 28 c3 e8 37 17 00 00 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffcdcead7e8 EFLAGS: 00000246 ORIG_RAX: 0000000000000141
RAX: ffffffffffffffda RBX: 00007ffcdcead9b8 RCX: 00007f6a0471c4a9
RDX: 0000000000000050 RSI: 0000000020000080 RDI: 000000000000000a
RBP: 00007f6a0478f610 R08: 0000000000000000 R09: 00007ffcdcead9b8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00007ffcdcead9a8 R14: 0000000000000001 R15: 0000000000000001
</TASK>


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[Eric Dumazet]

Hmm... Not sure how to deal with this one... this is a 'false positive'

diff --git a/net/core/filter.c b/net/core/filter.c
index 58e8e1a70aa752a2c045117e00d8797478da4738..a7cea6d717ef321215bc4cf9ab3b83535c4eec98
100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1662,6 +1662,11 @@ static DEFINE_PER_CPU(struct bpf_scratchpad, bpf_sp);
static inline int __bpf_try_make_writable(struct sk_buff *skb,
unsigned int write_len)
{
+#if defined(CONFIG_DEBUG_NET)
+ /* Avoid a splat in pskb_may_pull_reason() */
+ if (write_len > INT_MAX)
+ return -EINVAL;
+#endif
return skb_ensure_writable(skb, write_len);
}

[Florian Westphal]

Eric Dumazet <edum...@google.com> wrote:
> Hmm... Not sure how to deal with this one... this is a 'false positive'
>
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 58e8e1a70aa752a2c045117e00d8797478da4738..a7cea6d717ef321215bc4cf9ab3b83535c4eec98
> 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -1662,6 +1662,11 @@ static DEFINE_PER_CPU(struct bpf_scratchpad, bpf_sp);
> static inline int __bpf_try_make_writable(struct sk_buff *skb,
> unsigned int write_len)
> {
> +#if defined(CONFIG_DEBUG_NET)
> + /* Avoid a splat in pskb_may_pull_reason() */
> + if (write_len > INT_MAX)
> + return -EINVAL;
> +#endif
> return skb_ensure_writable(skb, write_len);
> }
>

LGTM, thanks Eric. I think the current 'warn on > INT_MAX' makes sense
for normal (non-bpf) callers.