udmabuf: Set ubuf->sg = NULL if the creation of sg table fails

In the Linux kernel, the following vulnerability has been resolved:

udmabuf: Set ubuf->sg = NULL if the creation of sg table fails

When userspace tries to map the dmabuf and if for some reason
(e.g. OOM) the creation of the sg table fails, ubuf->sg needs to be
set to NULL. Otherwise, when the userspace subsequently closes the
dmabuf fd, we'd try to erroneously free the invalid sg table from
release_udmabuf resulting in the following crash reported by syzbot:

general protection fault, probably for non-canonical address
0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 PID: 3609 Comm: syz-executor487 Not tainted
5.19.0-syzkaller-13930-g7ebfc85e2cd7 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 07/22/2022
RIP: 0010:dma_unmap_sgtable include/linux/dma-mapping.h:378 [inline]
RIP: 0010:put_sg_table drivers/dma-buf/udmabuf.c:89 [inline]
RIP: 0010:release_udmabuf+0xcb/0x4f0 drivers/dma-buf/udmabuf.c:114
Code: 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 2b 04 00 00 48 8d 7d 0c 4c
8b 63 30 48 b8 00 00 00 00 00 fc ff df 48 89 fa 48 c1 ea 03 <0f> b6 14
02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 08 84 d2 0f 85 e2
RSP: 0018:ffffc900037efd30 EFLAGS: 00010246
RAX: dffffc0000000000 RBX: ffffffff8cb67800 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff84ad27e0 RDI: 0000000000000000
RBP: fffffffffffffff4 R08: 0000000000000005 R09: 0000000000000000
R10: 0000000000000000 R11: 000000000008c07c R12: ffff88801fa05000
R13: ffff888073db07e8 R14: ffff888025c25440 R15: 0000000000000000
FS: 0000555555fc4300(0000) GS:ffff8880b9a00000(0000)
knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fc1c0ce06e4 CR3: 00000000715e6000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
dma_buf_release+0x157/0x2d0 drivers/dma-buf/dma-buf.c:78
__dentry_kill+0x42b/0x640 fs/dcache.c:612
dentry_kill fs/dcache.c:733 [inline]
dput+0x806/0xdb0 fs/dcache.c:913
__fput+0x39c/0x9d0 fs/file_table.c:333
task_work_run+0xdd/0x1a0 kernel/task_work.c:177
ptrace_notify+0x114/0x140 kernel/signal.c:2353
ptrace_report_syscall include/linux/ptrace.h:420 [inline]
ptrace_report_syscall_exit include/linux/ptrace.h:482 [inline]
syscall_exit_work kernel/entry/common.c:249 [inline]
syscall_exit_to_user_mode_prepare+0x129/0x280 kernel/entry/common.c:276
__syscall_exit_to_user_mode_work kernel/entry/common.c:281 [inline]
syscall_exit_to_user_mode+0x9/0x50 kernel/entry/common.c:294
do_syscall_64+0x42/0xb0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7fc1c0c35b6b
Code: 0f 05 48 3d 00 f0 ff ff 77 45 c3 0f 1f 40 00 48 83 ec 18 89 7c 24
0c e8 63 fc ff ff 8b 7c 24 0c 41 89 c0 b8 03 00 00 00 0f 05 <48> 3d 00
f0 ff ff 77 35 44 89 c7 89 44 24 0c e8 a1 fc ff ff 8b 44
RSP: 002b:00007ffd78a06090 EFLAGS: 00000293 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000007 RCX: 00007fc1c0c35b6b
RDX: 0000000020000280 RSI: 0000000040086200 RDI: 0000000000000006
RBP: 0000000000000007 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000293 R12: 000000000000000c
R13: 0000000000000003 R14: 00007fc1c0cfe4a0 R15: 00007ffd78a06140
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:dma_unmap_sgtable include/linux/dma-mapping.h:378 [inline]
RIP: 0010:put_sg_table drivers/dma-buf/udmabuf.c:89 [inline]
RIP: 0010:release_udmabuf+0xcb/0x4f0 drivers/dma-buf/udmabuf.c:114