drm/nouveau: bail out of nouveau_channel_new if channel init fails

Unprivileged user can crash kernel by using DRM_IOCTL_NOUVEAU_CHANNEL_ALLOC
ioctl. This was reported by trinity[1] fuzzer.

[   71.073906] nouveau 0000:01:00.0: crashme[1329]: channel failed to initialise, -17
[   71.081730] BUG: kernel NULL pointer dereference, address: 00000000000000a0
[   71.088928] #PF: supervisor read access in kernel mode
[   71.094059] #PF: error_code(0x0000) - not-present page
[   71.099189] PGD 119590067 P4D 119590067 PUD 1054f5067 PMD 0
[   71.104842] Oops: 0000 [#1] SMP NOPTI
[   71.108498] CPU: 2 PID: 1329 Comm: crashme Not tainted 5.8.0-rc6+ #2
[   71.114993] Hardware name: AMD Pike/Pike, BIOS RPK1506A 09/03/2014
[   71.121213] RIP: 0010:nouveau_abi16_ioctl_channel_alloc+0x108/0x380 [nouveau]
[   71.128339] Code: 48 89 9d f0 00 00 00 41 8b 4c 24 04 41 8b 14 24 45 31 c0 4c 8d 4b 10 48 89 ee 4c 89 f7 e8 10 11 00 00 85 c0 75 78 48 8b 43 10 <8b> 90 a0 00 00 00 41 89 54 24 08 80 7d 3d 05 0f 86 bb 01 00 00 41
[   71.147074] RSP: 0018:ffffb4a1809cfd38 EFLAGS: 00010246
[   71.152526] RAX: 0000000000000000 RBX: ffff98cedbaa1d20 RCX: 00000000000003bf
[   71.159651] RDX: 00000000000003be RSI: 0000000000000000 RDI: 0000000000030160
[   71.166774] RBP: ffff98cee776de00 R08: ffffdc0144198a08 R09: ffff98ceeefd4000
[   71.173901] R10: ffff98cee7e81780 R11: 0000000000000001 R12: ffffb4a1809cfe08
[   71.181214] R13: ffff98cee776d000 R14: ffff98cec519e000 R15: ffff98cee776def0
[   71.188339] FS:  00007fd926250500(0000) GS:ffff98ceeac80000(0000) knlGS:0000000000000000
[   71.196418] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   71.202155] CR2: 00000000000000a0 CR3: 0000000106622000 CR4: 00000000000406e0
[   71.209297] Call Trace:
[   71.211777]  ? nouveau_abi16_ioctl_getparam+0x1f0/0x1f0 [nouveau]
[   71.218053]  drm_ioctl_kernel+0xac/0xf0 [drm]
[   71.222421]  drm_ioctl+0x211/0x3c0 [drm]
[   71.226379]  ? nouveau_abi16_ioctl_getparam+0x1f0/0x1f0 [nouveau]
[   71.232500]  nouveau_drm_ioctl+0x57/0xb0 [nouveau]
[   71.237285]  ksys_ioctl+0x86/0xc0
[   71.240595]  __x64_sys_ioctl+0x16/0x20
[   71.244340]  do_syscall_64+0x4c/0x90
[   71.248110]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[   71.253162] RIP: 0033:0x7fd925d4b88b
[   71.256731] Code: Bad RIP value.
[   71.259955] RSP: 002b:00007ffc743592d8 EFLAGS: 00000206 ORIG_RAX: 0000000000000010
[   71.267514] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fd925d4b88b
[   71.274637] RDX: 0000000000601080 RSI: 00000000c0586442 RDI: 0000000000000003
[   71.281986] RBP: 00007ffc74359340 R08: 00007fd926016ce0 R09: 00007fd926016ce0
[   71.289111] R10: 0000000000000003 R11: 0000000000000206 R12: 0000000000400620
[   71.296235] R13: 00007ffc74359420 R14: 0000000000000000 R15: 0000000000000000
[   71.303361] Modules linked in: rfkill sunrpc snd_hda_codec_realtek snd_hda_codec_generic ledtrig_audio snd_hda_intel snd_intel_dspcfg snd_hda_codec snd_hda_core edac_mce_amd snd_hwdep kvm_amd snd_seq ccp snd_seq_device snd_pcm kvm snd_timer snd irqbypass soundcore sp5100_tco pcspkr crct10dif_pclmul crc32_pclmul ghash_clmulni_intel wmi_bmof joydev i2c_piix4 fam15h_power k10temp acpi_cpufreq ip_tables xfs libcrc32c sd_mod t10_pi sg nouveau video mxm_wmi i2c_algo_bit drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops ttm broadcom bcm_phy_lib ata_generic ahci drm e1000 crc32c_intel libahci serio_raw tg3 libata firewire_ohci firewire_core wmi crc_itu_t dm_mirror dm_region_hash dm_log dm_mod
[   71.365269] CR2: 00000000000000a0

simplified reproducer
---------------------------------8<----------------------------------------
/*
 * gcc -o crashme crashme.c
 * ./crashme /dev/dri/renderD128
 */

struct drm_nouveau_channel_alloc {
	uint32_t     fb_ctxdma_handle;
	uint32_t     tt_ctxdma_handle;

	int          channel;
	uint32_t     pushbuf_domains;

	/* Notifier memory */
	uint32_t     notifier_handle;

	/* DRM-enforced subchannel assignments */
	struct {
		uint32_t handle;
		uint32_t grclass;
	} subchan[8];
	uint32_t nr_subchan;
};

static struct drm_nouveau_channel_alloc channel;

int main(int argc, char *argv[]) {
	int fd;
	int rv;

	if (argc != 2)
		die("usage: %s <dev>", 0, argv[0]);

	if ((fd = open(argv[1], O_RDONLY)) == -1)
		die("open %s", errno, argv[1]);

	if (ioctl(fd, DRM_IOCTL_NOUVEAU_CHANNEL_ALLOC, &channel) == -1 &&
			errno == EACCES)
		die("ioctl %s", errno, argv[1]);

	close(fd);

	printf("PASS\n");

	return 0;
}
---------------------------------8<----------------------------------------

[1] https://github.com/kernelslacker/trinity

Fixes: eeaf06ac1a55 ("drm/nouveau/svm: initial support for shared virtual memory")
Signed-off-by: Frantisek Hrbata <frantisek@hrbata.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
