drm: Prevent drm_copy_field() to attempt copying a NULL pointer

There are some struct drm_driver fields that are required by drivers since
drm_copy_field() attempts to copy them to user-space via DRM_IOCTL_VERSION.

But it can be possible that a driver has a bug and did not set some of the
fields, which leads to drm_copy_field() attempting to copy a NULL pointer:

[ +10.395966] Unable to handle kernel access to user memory outside uaccess routines at virtual address 0000000000000000
[  +0.010955] Mem abort info:
[  +0.002835]   ESR = 0x0000000096000004
[  +0.003872]   EC = 0x25: DABT (current EL), IL = 32 bits
[  +0.005395]   SET = 0, FnV = 0
[  +0.003113]   EA = 0, S1PTW = 0
[  +0.003182]   FSC = 0x04: level 0 translation fault
[  +0.004964] Data abort info:
[  +0.002919]   ISV = 0, ISS = 0x00000004
[  +0.003886]   CM = 0, WnR = 0
[  +0.003040] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000115dad000
[  +0.006536] [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
[  +0.006925] Internal error: Oops: 96000004 [#1] SMP
...
[  +0.011113] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[  +0.007061] pc : __pi_strlen+0x14/0x150
[  +0.003895] lr : drm_copy_field+0x30/0x1a4
[  +0.004156] sp : ffff8000094b3a50
[  +0.003355] x29: ffff8000094b3a50 x28: ffff8000094b3b70 x27: 0000000000000040
[  +0.007242] x26: ffff443743c2ba00 x25: 0000000000000000 x24: 0000000000000040
[  +0.007243] x23: ffff443743c2ba00 x22: ffff8000094b3b70 x21: 0000000000000000
[  +0.007241] x20: 0000000000000000 x19: ffff8000094b3b90 x18: 0000000000000000
[  +0.007241] x17: 0000000000000000 x16: 0000000000000000 x15: 0000aaab14b9af40
[  +0.007241] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
[  +0.007239] x11: 0000000000000000 x10: 0000000000000000 x9 : ffffa524ad67d4d8
[  +0.007242] x8 : 0101010101010101 x7 : 7f7f7f7f7f7f7f7f x6 : 6c6e6263606e7141
[  +0.007239] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
[  +0.007241] x2 : 0000000000000000 x1 : ffff8000094b3b90 x0 : 0000000000000000
[  +0.007240] Call trace:
[  +0.002475]  __pi_strlen+0x14/0x150
[  +0.003537]  drm_version+0x84/0xac
[  +0.003448]  drm_ioctl_kernel+0xa8/0x16c
[  +0.003975]  drm_ioctl+0x270/0x580
[  +0.003448]  __arm64_sys_ioctl+0xb8/0xfc
[  +0.003978]  invoke_syscall+0x78/0x100
[  +0.003799]  el0_svc_common.constprop.0+0x4c/0xf4
[  +0.004767]  do_el0_svc+0x38/0x4c
[  +0.003357]  el0_svc+0x34/0x100
[  +0.003185]  el0t_64_sync_handler+0x11c/0x150
[  +0.004418]  el0t_64_sync+0x190/0x194
[  +0.003716] Code: 92402c04 b200c3e8 f13fc09f 5400088c (a9400c02)
[  +0.006180] ---[ end trace 0000000000000000 ]---

Reported-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220705100215.572498-3-javierm@redhat.com

drivers/gpu/drm/drm_ioctl.c

@@ -474,6 +474,12 @@ static int drm_copy_field(char __user *buf, size_t *buf_len, const char *value)
 {
 	size_t len;
 
+	/* don't attempt to copy a NULL pointer */
+	if (WARN_ONCE(!value, "BUG: the value to copy was not set!")) {
+		*buf_len = 0;
+		return 0;
+	}
+
 	/* don't overflow userbuf */
 	len = strlen(value);
 	if (len > *buf_len)