Re: request_module DoS

From: Luis Chamberlain
Date: Sat May 07 2022 - 04:02:31 EST

On Sat, May 07, 2022 at 07:10:23AM +0000, Christophe Leroy wrote:
> > There are some code paths in the kernel where you can reliably
> > trigger a request_module of a non-existant module. For example,
> > if you attempt to load a non-existent crypto algorithm, or create
> > a socket of a non-existent network family, it will result in a
> > request_module call that is guaranteed to fail.
> >
> > As user-space can do this repeatedly, it can quickly overwhelm
> > the concurrency limit in kmod. This in itself is expected,
> > however, at least on some platforms this appears to result in
> > a live-lock. Here is an example triggered by stress-ng on ppc64:
> >
> > [ 579.845320] request_module: modprobe crypto-aegis256 cannot be processed, kmod busy with 50 threads for more than 5 seconds now
> > [ 580.414590] __request_module: 25 callbacks suppressed
> > [ 580.414597] request_module: kmod_concurrent_max (0) close to 0 (max_modprobes: 50), for module crypto-aegis256-all, throttling...
> > [ 580.423082] watchdog: CPU 784 self-detected hard LOCKUP @ plpar_hcall_norets_notrace+0x18/0x2c
> > [ 580.423097] watchdog: CPU 784 TB:1297691958559475, last heartbeat TB:1297686321743840 (11009ms ago)
> > [ 580.423099] Modules linked in: cast6_generic cast5_generic cast_common camellia_generic blowfish_generic blowfish_common tun nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 rfkill bonding tls ip_set nf_tables nfnetlink pseries_rng binfmt_misc drm drm_panel_orientation_quirks xfs libcrc32c sd_mod t10_pi sg ibmvscsi ibmveth scsi_transport_srp vmx_crypto dm_mirror dm_region_hash dm_log dm_mod fuse
> > [ 580.423136] CPU: 784 PID: 77071 Comm: stress-ng Kdump: loaded Not tainted 5.14.0-55.el9.ppc64le #1
> > [ 580.423139] NIP: c0000000000f8ff4 LR: c0000000001f7c38 CTR: 0000000000000000
> > [ 580.423140] REGS: c0000043fdd7bd60 TRAP: 0900 Not tainted (5.14.0-55.el9.ppc64le)
> > [ 580.423142] MSR: 800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE> CR: 28008202 XER: 20040000
> > [ 580.423148] CFAR: 0000000000000c00 IRQMASK: 1
> > GPR00: 0000000028008202 c0000044c46b3850 c000000002a46f00 0000000000000000
> > GPR04: ffffffffffffffff 0000000000000000 0000000000000010 c000000002a83060
> > GPR08: 0000000000000000 0000000000000001 0000000000000001 0000000000000000
> > GPR12: c0000000001b9530 c0000043ffe16700 0000000200000117 0000000010185ea8
> > GPR16: 0000000010212150 0000000010186198 00000000101863a0 000000001021b3c0
> > GPR20: 0000000000000001 0000000000000000 0000000000000001 00000000000000ff
> > GPR24: c0000043f4a00e14 c0000043fafe0e00 000000000c440000 0000000000000000
> > GPR28: c0000043f4a00e00 c0000043f4a00e00 c0000000021e0e00 c000000002561aa0
> > [ 580.423166] NIP [c0000000000f8ff4] plpar_hcall_norets_notrace+0x18/0x2c
> > [ 580.423168] LR [c0000000001f7c38] __pv_queued_spin_lock_slowpath+0x528/0x530
> > [ 580.423173] Call Trace:
> > [ 580.423174] [c0000044c46b3850] [0000000100006b60] 0x100006b60 (unreliable)
> > [ 580.423177] [c0000044c46b3910] [c000000000ea6948] _raw_spin_lock_irqsave+0xa8/0xc0
> > [ 580.423182] [c0000044c46b3940] [c0000000001dd7c0] prepare_to_wait_event+0x40/0x200
> > [ 580.423185] [c0000044c46b39a0] [c00000000019e9e0] __request_module+0x320/0x510
> > [ 580.423188] [c0000044c46b3ac0] [c0000000006f1a14] crypto_alg_mod_lookup+0x1e4/0x2e0
> > [ 580.423192] [c0000044c46b3b60] [c0000000006f2178] crypto_alloc_tfm_node+0xa8/0x1a0
> > [ 580.423194] [c0000044c46b3be0] [c0000000006f84f8] crypto_alloc_aead+0x38/0x50
> > [ 580.423196] [c0000044c46b3c00] [c00000000072cba0] aead_bind+0x70/0x140
> > [ 580.423199] [c0000044c46b3c40] [c000000000727824] alg_bind+0xb4/0x210
> > [ 580.423201] [c0000044c46b3cc0] [c000000000bc2ad4] __sys_bind+0x114/0x160
> > [ 580.423205] [c0000044c46b3d90] [c000000000bc2b48] sys_bind+0x28/0x40
> > [ 580.423207] [c0000044c46b3db0] [c000000000030880] system_call_exception+0x160/0x300
> > [ 580.423209] [c0000044c46b3e10] [c00000000000c168] system_call_vectored_common+0xe8/0x278
> > [ 580.423213] --- interrupt: 3000 at 0x7fff9b824464
> > [ 580.423214] NIP: 00007fff9b824464 LR: 0000000000000000 CTR: 0000000000000000
> > [ 580.423215] REGS: c0000044c46b3e80 TRAP: 3000 Not tainted (5.14.0-55.el9.ppc64le)
> > [ 580.423216] MSR: 800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE> CR: 42004802 XER: 00000000
> > [ 580.423221] IRQMASK: 0
> > GPR00: 0000000000000147 00007fffdcff2780 00007fff9b917100 0000000000000004
> > GPR04: 00007fffdcff27e0 0000000000000058 0000000000000000 0000000000000000
> > GPR08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> > GPR12: 0000000000000000 00007fff9bc9efe0 0000000200000117 0000000010185ea8
> > GPR16: 0000000010212150 0000000010186198 00000000101863a0 000000001021b3c0
> > GPR20: 0000000000000004 00007fffdcff2a00 0000000300000117 00000000101862b8
> > GPR24: 0000000000000004 0000000046401570 0000000046401120 0000000046404650
> > GPR28: 0000000000000020 0000000000000020 0000000000000060 0000000046404bf0
> > [ 580.423236] NIP [00007fff9b824464] 0x7fff9b824464
> > [ 580.423237] LR [0000000000000000] 0x0
> > [ 580.423238] --- interrupt: 3000
> > [ 580.423239] Instruction dump:
> > [ 580.423241] e8690000 7c0803a6 3884fff8 78630100 78840020 4bfffeb8 3c4c0295 3842df24
> > [ 580.423244] 7c421378 7c000026 90010008 44000022 <38800000> 988d0931 80010008 7c0ff120
> >
> > Would it be possible to modify kmod so that in such cases that
> > request_module calls fail more quickly rather than repeatedly
> > obtaining a spinlock that appears to be under high contention?

kmod count limit is lockless but prepare_to_wait_event() does hold a
lock...

You can try to reproduce by using adding a new test type for crypto-aegis256
on lib/test_kmod.c. These tests however can try something similar but other
modules.

/tools/testing/selftests/kmod/kmod.sh -t 0008
/tools/testing/selftests/kmod/kmod.sh -t 0009

I can't decipher this yet.

Luis