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SubjectRe: usercopy whitelist woe in scsi_sense_cache
Date
Hi.

Cc'ing linux-block people (mainly, Christoph) too because of 17cb960f29c2.
Also, duplicating the initial statement for them.

With v4.16 (and now with v4.16.1) it is possible to trigger usercopy whitelist
warning and/or bug while doing smartctl on a SATA disk having blk-mq and BFQ
enabled. The warning looks like this:

===
[ 574.997022] Bad or missing usercopy whitelist? Kernel memory exposure
attempt detected from SLUB object 'scsi_sense_cache' (offset 76, size 22)!
[ 575.017332] WARNING: CPU: 0 PID: 32436 at mm/usercopy.c:81 usercopy_warn
+0x7d/0xa0
[ 575.025262] Modules linked in: nls_iso8859_1 nls_cp437 vfat fat kvm_intel
kvm bochs_drm iTCO_wdt ttm irqbypass iTCO_vendor_support ppdev drm_kms_helper
psmouse parport_pc i2c_i801 joydev pcspkr drm parport rtc_cmos mousedev
input_leds led_class intel_agp evdev syscopyarea qemu_fw_cfg intel_gtt
sysfillrect mac_hid lpc_ich sysimgblt agpgart fb_sys_fops ip_tables x_tables
xfs dm_thin_pool dm_persistent_data dm_bio_prison dm_bufio libcrc32c
crc32c_generic dm_crypt algif_skcipher af_alg hid_generic usbhid hid dm_mod
raid10 md_mod sr_mod sd_mod cdrom uhci_hcd crct10dif_pclmul crc32_pclmul
crc32c_intel ghash_clmulni_intel pcbc serio_raw xhci_pci ahci atkbd libps2
ehci_pci xhci_hcd aesni_intel libahci aes_x86_64 ehci_hcd crypto_simd
glue_helper cryptd libata usbcore usb_common i8042 serio virtio_scsi scsi_mod
[ 575.068775] virtio_blk virtio_net virtio_pci virtio_ring virtio
[ 575.073935] CPU: 0 PID: 32436 Comm: smartctl Not tainted 4.16.0-pf2 #1
[ 575.078078] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0
02/06/2015
[ 575.082451] RIP: 0010:usercopy_warn+0x7d/0xa0
[ 575.086223] RSP: 0018:ffff9ca84aee7c40 EFLAGS: 00010286
[ 575.097637] RAX: 0000000000000000 RBX: ffff95199d68304c RCX:
0000000000000001
[ 575.101471] RDX: 0000000000000001 RSI: ffffffffaeeb050a RDI:
00000000ffffffff
[ 575.105939] RBP: 0000000000000016 R08: 0000000000000000 R09:
000000000000028b
[ 575.110370] R10: ffffffffaee854e9 R11: 0000000000000001 R12:
0000000000000001
[ 575.113269] R13: ffff95199d683062 R14: ffff95199d68304c R15:
0000000000000016
[ 575.116132] FS: 00007f993d405040(0000) GS:ffff95199f600000(0000) knlGS:
0000000000000000
[ 575.119285] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 575.129619] CR2: 00007ffe2390f0a8 CR3: 000000001d774004 CR4:
0000000000160ef0
[ 575.133976] Call Trace:
[ 575.136311] __check_object_size+0x12f/0x1a0
[ 575.139576] sg_io+0x269/0x3f0
[ 575.142000] ? path_lookupat+0xaa/0x1f0
[ 575.144521] ? current_time+0x18/0x70
[ 575.147006] scsi_cmd_ioctl+0x257/0x410
[ 575.149782] ? xfs_bmapi_read+0x1c3/0x340 [xfs]
[ 575.161441] sd_ioctl+0xbf/0x1a0 [sd_mod]
[ 575.165036] blkdev_ioctl+0x8ca/0x990
[ 575.168291] ? read_null+0x10/0x10
[ 575.171638] block_ioctl+0x39/0x40
[ 575.174998] do_vfs_ioctl+0xa4/0x630
[ 575.178261] ? vfs_write+0x164/0x1a0
[ 575.181410] SyS_ioctl+0x74/0x80
[ 575.190904] do_syscall_64+0x74/0x190
[ 575.195200] entry_SYSCALL_64_after_hwframe+0x3d/0xa2
[ 575.199267] RIP: 0033:0x7f993c984d87
[ 575.201350] RSP: 002b:00007ffe238aeed8 EFLAGS: 00000246 ORIG_RAX:
0000000000000010
[ 575.204386] RAX: ffffffffffffffda RBX: 00007ffe238af180 RCX:
00007f993c984d87
[ 575.208349] RDX: 00007ffe238aeef0 RSI: 0000000000002285 RDI:
0000000000000003
[ 575.211254] RBP: 00007ffe238af1d0 R08: 0000000000000010 R09:
0000000000000000
[ 575.220511] R10: 0000000000000000 R11: 0000000000000246 R12:
00005637ec8e9ce0
[ 575.225238] R13: 0000000000000000 R14: 00005637ec8e3550 R15:
00000000000000da
[ 575.230056] Code: 6c e4 ae 41 51 48 c7 c0 19 6e e5 ae 49 89 f1 48 0f 44 c2
48 89 f9 4d 89 d8 4c 89 d2 48 c7 c7 70 6e e5 ae 48 89 c6 e8 c3 5c e5 ff <0f>
0b 48 83 c4 18 c3 48 c7 c6 04 cb e4 ae 49 89 f1 49 89 f3 eb
[ 575.239027] ---[ end trace 6e3293933bdd4761 ]---
===

Usually, the warning is triggered first, and all the subsequent printouts are
bugs because offset gets too big so that it doesn't fit into a real SLAB
object size:

[ 1687.609889] usercopy: Kernel memory exposure attempt detected from SLUB
object 'scsi_sense_cache' (offset 107, size 22)!
[ 1687.614197] ------------[ cut here ]------------
[ 1687.615993] kernel BUG at mm/usercopy.c:100!

To give you an idea regarding variety of offsets, I've summarised the kernel
log from my server:

$ sudo journalctl -kb | grep "Kernel memory exposure attempt detected" |
grep -oE 'offset [0-9]+, size [0-9]+' | sort | uniq -c
9 offset 107, size 22
6 offset 108, size 22
8 offset 109, size 22
7 offset 110, size 22
5 offset 111, size 22
5 offset 112, size 22
2 offset 113, size 22
2 offset 114, size 22
1 offset 115, size 22
1 offset 116, size 22
1 offset 119, size 22
1 offset 85, size 22

So far, I wasn't able to trigger this with mq-deadline (or without blk-mq).
Maybe, this has something to do with blk-mq+BFQ re-queuing, or it's just me
not being persistent enough.

It looks like this code path was re-written completely with 17cb960f29c2, but
it went merged for the upcoming v4.17 only, and thus I haven't tried it yet.

Kees took a brief look at it already: [1]. This is what smartctl does [2]
(just a usual strace capture when the bug is not triggered).

Christoph, do you have some idea on why this can happen?

Thanks.

Regards,
Oleksandr

[1] https://marc.info/?l=linux-scsi&m=152287333013845&w=2
[2] https://gist.github.com/pfactum/6f58f8891468aeba1ab2cc9f45668735


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