In case of failure to get file, the uobj is overwritten and causes to
supply bad pointer as an input to uverbs_uobject_put().

  BUG: KASAN: null-ptr-deref in atomic_fetch_sub include/asm-generic/atomic-instrumented.h:199 [inline]
  BUG: KASAN: null-ptr-deref in refcount_sub_and_test include/linux/refcount.h:253 [inline]
  BUG: KASAN: null-ptr-deref in refcount_dec_and_test include/linux/refcount.h:281 [inline]
  BUG: KASAN: null-ptr-deref in kref_put include/linux/kref.h:64 [inline]
  BUG: KASAN: null-ptr-deref in uverbs_uobject_put+0x22/0x90 drivers/infiniband/core/rdma_core.c:57
  Write of size 4 at addr 0000000000000030 by task syz-executor.4/1691

  CPU: 1 PID: 1691 Comm: syz-executor.4 Not tainted 5.6.0 #17
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
  Call Trace:
   __dump_stack lib/dump_stack.c:77 [inline]
   dump_stack+0x94/0xce lib/dump_stack.c:118
   __kasan_report+0x10c/0x190 mm/kasan/report.c:515
   kasan_report+0x32/0x50 mm/kasan/common.c:625
   check_memory_region_inline mm/kasan/generic.c:187 [inline]
   check_memory_region+0x16d/0x1c0 mm/kasan/generic.c:193
   atomic_fetch_sub include/asm-generic/atomic-instrumented.h:199 [inline]
   refcount_sub_and_test include/linux/refcount.h:253 [inline]
   refcount_dec_and_test include/linux/refcount.h:281 [inline]
   kref_put include/linux/kref.h:64 [inline]
   uverbs_uobject_put+0x22/0x90 drivers/infiniband/core/rdma_core.c:57
   alloc_begin_fd_uobject+0x1d0/0x250 drivers/infiniband/core/rdma_core.c:486
   rdma_alloc_begin_uobject+0xa8/0xf0 drivers/infiniband/core/rdma_core.c:509
   __uobj_alloc include/rdma/uverbs_std_types.h:117 [inline]
   ib_uverbs_create_comp_channel+0x16d/0x230 drivers/infiniband/core/uverbs_cmd.c:982
   ib_uverbs_write+0xaa5/0xdf0 drivers/infiniband/core/uverbs_main.c:665
   __vfs_write+0x7c/0x100 fs/read_write.c:494
   vfs_write+0x168/0x4a0 fs/read_write.c:558
   ksys_write+0xc8/0x200 fs/read_write.c:611
   do_syscall_64+0x9c/0x390 arch/x86/entry/common.c:295
   entry_SYSCALL_64_after_hwframe+0x44/0xa9
  RIP: 0033:0x466479
  Code: f7 d8 64 89 02 b8 ff ff ff ff c3 66 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
  RSP: 002b:00007efe9f6a7c48 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
  RAX: ffffffffffffffda RBX: 000000000073bf00 RCX: 0000000000466479
  RDX: 0000000000000018 RSI: 0000000020000040 RDI: 0000000000000003
  RBP: 00007efe9f6a86bc R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000005
  R13: 0000000000000bf2 R14: 00000000004cb80a R15: 00000000006fefc0

Fixes: 849e1490 ("RDMA/core: Do not allow alloc_commit to fail")
Link: https://lore.kernel.org/r/20200421082929.311931-3-leon@kernel.orgSigned-off-by: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>