CVE: CVE-2024-40958
Published: 2024-07-12
Description: In the Linux kernel, the following vulnerability has been resolved:

netns: Make get_net_ns() handle zero refcount net

Syzkaller hit a warning:
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 3 PID: 7890 at lib/refcount.c:25 refcount_warn_saturate+0xdf/0x1d0
Modules linked in:
CPU: 3 PID: 7890 Comm: tun Not tainted 6.10.0-rc3-00100-gcaa4f9578aba-dirty #310
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
RIP: 0010:refcount_warn_saturate+0xdf/0x1d0
Code: 41 49 04 31 ff 89 de e8 9f 1e cd fe 84 db 75 9c e8 76 26 cd fe c6 05 b6 41 49 04 01 90 48 c7 c7 b8 8e 25 86 e8 d2 05 b5 fe 90 <0f> 0b 90 90 e9 79 ff ff ff e8 53 26 cd fe 0f b6 1
RSP: 0018:ffff8881067b7da0 EFLAGS: 00010286
RAX: [PII_REDACTED] RBX: [PII_REDACTED] RCX: ffffffff811c72ac
RDX: ffff8881026a2140 RSI: ffffffff811c72b5 RDI: [PII_REDACTED]
RBP: ffff8881067b7db0 R08: [PII_REDACTED] R09: 205b5d[PII_REDACTED]
R10: [PII_REDACTED] R11: 205d[PII_REDACTED]20 R12: ffff[PII_REDACTED]c4
R13: ffff[PII_REDACTED]c4 R14: ffff[PII_REDACTED]b0 R15: ffff[PII_REDACTED]40
FS:  00007f5b2961a4c0(0000) GS:ffff88823bd00000(0000) knlGS:[PII_REDACTED]
CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
CR2: 000055d7ed36fd18 CR3:[PII_REDACTED]2f6000 CR4:[PII_REDACTED]0006f0
DR0: [PII_REDACTED] DR1: [PII_REDACTED] DR2: [PII_REDACTED]
DR3: [PII_REDACTED] DR6: 00000000fffe0ff0 DR7: [PII_REDACTED]
Call Trace:
 <TASK>
 ? show_regs+0xa3/0xc0
 ? __warn+0xa5/0x1c0
 ? refcount_warn_saturate+0xdf/0x1d0
 ? report_bug+0x1fc/0x2d0
 ? refcount_warn_saturate+0xdf/0x1d0
 ? handle_bug+0xa1/0x110
 ? exc_invalid_op+0x3c/0xb0
 ? asm_exc_invalid_op+0x1f/0x30
 ? __warn_printk+0xcc/0x140
 ? __warn_printk+0xd5/0x140
 ? refcount_warn_saturate+0xdf/0x1d0
 get_net_ns+0xa4/0xc0
 ? __pfx_get_net_ns+0x10/0x10
 open_related_ns+0x5a/0x130
 __tun_chr_ioctl+0x1616/0x2370
 ? __sanitizer_cov_trace_switch+0x58/0xa0
 ? __sanitizer_cov_trace_const_cmp2+0x1c/0x30
 ? __pfx_tun_chr_ioctl+0x10/0x10
 tun_chr_ioctl+0x2f/0x40
 __x64_sys_ioctl+0x11b/0x160
 x64_sys_call+0x1211/0x20d0
 do_syscall_64+0x9e/0x1d0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f5b28f165d7
Code: b3 66 90 48 8b 05 b1 48 2d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 81 48 2d 00 8
RSP: 002b:00007ffc2b59c5e8 EFLAGS: 00000246 ORIG_RAX: [PII_REDACTED]
RAX: ffffffffffffffda RBX: [PII_REDACTED] RCX: 00007f5b28f165d7
RDX: [PII_REDACTED] RSI:[PII_REDACTED]0054e3 RDI: [PII_REDACTED]
RBP: 00007ffc2b59c650 R08: 00007f5b291ed8c0 R09: 00007f5b2961a4c0
R10: [PII_REDACTED] R11: [PII_REDACTED] R12: [PII_REDACTED]
R13: 00007ffc2b59cf40 R14: [PII_REDACTED] R15: [PII_REDACTED]
 </TASK>
Kernel panic - not syncing: kernel: panic_on_warn set ...

This is trigger as below:
          ns0                                    ns1
tun_set_iff() //dev is tun0
   tun->dev = dev
//ip link set tun0 netns ns1
                                       put_net() //ref is 0
__tun_chr_ioctl() //TUNGETDEVNETNS
   net = dev_net(tun->dev);
   open_related_ns(&net->ns, get_net_ns); //ns1
     get_net_ns()
        get_net() //addition on 0

Use maybe_get_net() in get_net_ns in case net's ref is zero to fix this
CVSS Score: 7.8 (HIGH)
CWE: CWE-416
References: https://git.kernel.org/stable/c/1b631bffcb2c09551888f3c723f4365c91fe05ef; https://git.kernel.org/stable/c/2b82028a1f5ee3a8e04090776b10c534144ae77b; https://git.kernel.org/stable/c/3a6cd326ead7c8bb1f64486789a01974a9f1ad55