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Subjectvirtio-rng scatterlist null pointer dereference with VMAP_STACK=y
With VMAP_STACK=y and HW_RANDOM_VIRTIO=y, I get the following crash:

[ 1.470437] BUG: unable to handle kernel NULL pointer dereference at (null)
[ 1.473350] IP: [<ffffffff812f7c35>] sg_init_one+0x65/0x90
[ 1.474658] PGD 0
[ 1.475169] Oops: 0000 [#1] SMP

Entering kdb (current=0xffff880069b0c980, pid 1) on processor 1 Oops: (null)
due to oops @ 0xffffffff812f7c35
CPU: 1 PID: 1 Comm: swapper/0 Not tainted 4.8.0-08682-g41844e3 #246
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
task: ffff880069b0c980 task.stack: ffffc90000c54000
RIP: 0010:[<ffffffff812f7c35>] [<ffffffff812f7c35>] sg_init_one+0x65/0x90
RSP: 0000:ffffc90000c57d00 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffffc90000c57d20 RCX: 0000000000082000
RDX: 0000000000000d68 RSI: 0000000410000c57 RDI: ffffc90000c57d40
RBP: 0000000000000820 R08: 0000000000000000 R09: ffffc90000c57d20
R10: 0000000000019228 R11: ffff88017fff8500 R12: 0000000000000010
R13: 0000000000000010 R14: 0000000000000000 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88017fc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000001806000 CR4: 00000000001406a0
Stack:
ffff8800694bf500 0000000000000001 ffffc90000c57d68 ffffffff8138286f
0000000000000002 0000000000000000 0000000000000000 0000000000000000
ffff8800694bf500 ffffc90000c57d68 ffff8800694bf500 0000000000000000
Call Trace:
[<ffffffff8138286f>] ? virtio_read+0x9f/0xe0
[<ffffffff81381ade>] ? add_early_randomness+0x3e/0xa0
[<ffffffff813820af>] ? set_current_rng+0x3f/0x160
[<ffffffff81382416>] ? hwrng_register+0x186/0x1d0
[<ffffffff81382620>] ? virtrng_scan+0x10/0x20
[<ffffffff8135790e>] ? virtio_dev_probe+0x21e/0x2b0
[<ffffffff8138789e>] ? really_probe+0x14e/0x250
[<ffffffff81387a22>] ? __driver_attach+0x82/0xa0
[<ffffffff813879a0>] ? really_probe+0x250/0x250
[<ffffffff81385d22>] ? bus_for_each_dev+0x52/0x80
[<ffffffff81387093>] ? bus_add_driver+0x1a3/0x220
[<ffffffff818b0634>] ? hwrng_modinit+0xc/0xc
[<ffffffff81388046>] ? driver_register+0x56/0xd0
[<ffffffff818b0634>] ? hwrng_modinit+0xc/0xc
[<ffffffff810003c2>] ? do_one_initcall+0x32/0x150
[<ffffffff81095900>] ? parse_args+0x170/0x390
[<ffffffff8188a188>] ? kernel_init_freeable+0x11e/0x1a3
[<ffffffff818898f4>] ? set_debug_rodata+0xc/0xc
[<ffffffff8157d930>] ? rest_init+0x70/0x70
[<ffffffff8157d935>] ? kernel_init+0x5/0x100
[<ffffffff81582625>] ? ret_from_fork+0x25/0x30
Code: 01 c5 48 89 ee 48 89 e9 48 c1 ed 23 48 8b 04 ed c0 49 9c 81 48 c1 ee 0c
48 c1 e9 1b 48 85 c0 74 0a 0f b6 c9 48 c1 e1 04 48 01 c8 <48> 8b 08 48 89 f0 89
53 08 48 c1 e0 06 44 89 63 0c 48 83 e1 fc


It looks like add_early_randomness() uses a buffer on the stack, which (with
VMAP_STACK) fails to be added to a scatterlist due to use of virt_to_page in
sg_set_buf().

None of this looks obviously wrong to me, but in combination it is not
functional. Hence, I don't have a particular fix in mind, and I've CC'ed folks
from both the hw_random and scatterlist history.

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