Re: linux 5.14.3: free_user_ns causes NULL pointer dereference

Hillf Danton
2021-10-11T13:09:36Z

On Sun, 10 Oct 2021 10:59:10 +0200 Rune Kleveland wrote:
> Just wanted to let you know that I still get these on stock Fedora
> kernel 5.14.10 on the IBM blades. But it took 10 hours before the first

Thanks for your report.

> server crashed. The other 4 still runs fine since 15 hours ago. So for
> me it seems more stable now, but that could just be a coincidence.
>
> Best regards,
>
> Rune
>
> ------------[ cut here ]------------
> kernel BUG at mm/slub.c:321!

We did not see this before.

> invalid opcode: 0000 [#1] SMP PTI
> CPU: 22 PID: 1838853 Comm: python3 Not tainted 5.14.10-200.fc34.x86_64 #1
> Hardware name: IBM BladeCenter HS22 -[7870TKN]-/68Y8161, BIOS
> -[P9E164CUS-1.28]- 04/17/2018
> RIP: 0010:__slab_free+0x245/0x4a0
> Code: 0f b6 5c 24 1b 44 8b 44 24 1c 48 89 44 24 08 48 8b 54 24 20 4c 8b
> 4c 24 28 e9 8a fe ff ff 41 f7 45 08 00 0d 21 00 75 98 eb 8d <0f> 0b 49
> 3b 54 24 28 0f 85 53 ff ff ff 49 8b 44 24 08 4>
> RSP: 0018:ffffb71dcfd6fda0 EFLAGS: 00010246
> RAX: ffff9c5480d35860 RBX: ffff9c5480d35800 RCX: ffff9c5480d35800
> RDX: 00000000802a0029 RSI: ffffeb41da034d00 RDI: ffff9c4f00042800
> RBP: ffffb71dcfd6fe50 R08: 0000000000000001 R09: ffffffff9210b6a5
> R10: ffff9c548102e000 R11: 0000000062667658 R12: ffffeb41da034d00
> R13: ffff9c4f00042800 R14: ffff9c5480d35800 R15: ffff9c5480d35800
> FS:  00007f7a89765740(0000) GS:ffff9c6c1fc80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7a7ad7f4b0 CR3: 0000000564af4002 CR4: 00000000000206e0
> Call Trace:
>  ? filename_lookup+0x135/0x1b0
>  ? put_ucounts+0x65/0x70
>  kfree+0x369/0x3c0
>  put_ucounts+0x65/0x70
>  put_cred_rcu+0x70/0xd0
>  do_faccessat+0x113/0x240
>  do_syscall_64+0x3b/0x90
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7f7a899cc44b
> Code: 77 05 c3 0f 1f 40 00 48 8b 15 29 1a 0d 00 f7 d8 64 89 02 48 c7 c0
> ff ff ff ff c3 0f 1f 40 00 f3 0f 1e fa b8 15 00 00 00 0f 05 <48> 3d 00
> f0 ff ff 77 05 c3 0f 1f 40 00 48 8b 15 f9 19 0>
> RSP: 002b:00007ffd01fa9ce8 EFLAGS: 00000202 ORIG_RAX: 0000000000000015
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f7a899cc44b
> RDX: 0000000000000000 RSI: 0000000000000001 RDI: 00007f79a96d6e10
> RBP: 0000000000000001 R08: 0000000000000000 R09: 00007f7a7c1fb930
> R10: 00007f79a96d6000 R11: 0000000000000202 R12: 00007ffd01fa9d00
> R13: 0000000000000001 R14: 0000556841045c90 R15: 00000000ffffff9c
> Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver nfs
> lockd grace sunrpc fscache netfs nft_fib_inet nft_fib_ipv4 nft_fib_ipv6
> nft_fib nft_reject_inet nf_reject_ipv4 nf_rejec>
> ---[ end trace 0a81b150eacde1d5 ]---
> RIP: 0010:__slab_free+0x245/0x4a0
> Code: 0f b6 5c 24 1b 44 8b 44 24 1c 48 89 44 24 08 48 8b 54 24 20 4c 8b
> 4c 24 28 e9 8a fe ff ff 41 f7 45 08 00 0d 21 00 75 98 eb 8d <0f> 0b 49
> 3b 54 24 28 0f 85 53 ff ff ff 49 8b 44 24 08 4>
> RSP: 0018:ffffb71dcfd6fda0 EFLAGS: 00010246
> RAX: ffff9c5480d35860 RBX: ffff9c5480d35800 RCX: ffff9c5480d35800
> RDX: 00000000802a0029 RSI: ffffeb41da034d00 RDI: ffff9c4f00042800
> RBP: ffffb71dcfd6fe50 R08: 0000000000000001 R09: ffffffff9210b6a5
> R10: ffff9c548102e000 R11: 0000000062667658 R12: ffffeb41da034d00
> R13: ffff9c4f00042800 R14: ffff9c5480d35800 R15: ffff9c5480d35800
> FS:  00007f7a89765740(0000) GS:ffff9c6c1fc80000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007f7a7ad7f4b0 CR3: 0000000564af4002 CR4: 00000000000206e0
> ------------[ cut here ]------------

Although the chance for double free is not zero, corruption can not be
ruled out today yet because reports [1,2] in this thread caught no count
underflow.

Want to know what this DF looks like with kasan enabled.

Hillf

CPU 1 CPU 2
----- -----
put_ucounts

atomic_dec_and_lock_irqsave

get_ucounts

hlist_del_init(&ucounts->node);
spin_unlock_irqrestore(&ucounts_lock, flags);

put_ucounts
atomic_dec_and_lock_irqsave
hlist_del_init(&ucounts->node);
spin_unlock_irqrestore(&ucounts_lock, flags);
kfree(ucounts); kfree(ucounts);

[1] https://lore.kernel.org/linux-mm/CAOUHufaaPoTOCML+v8FomeWtN_PncmaHT9d6-XJG8RPzTcFZug@mail.gmail.com/

[2] https://lore.kernel.org/linux-mm/BCdQdQjRHZ9CJwIhmvKGfnFtMBJuatrZUX5In1gLlPswtrs4HwNONAbJleaeSfOOenMIdc_FPi0wwPIdW5Vodm4Aj-0iCqQY57288M5WFuY=@protonmail.ch/