bpf, xdp: fix crash in xdp_umem_unaccount_pages

syzkaller was able to trigger the following panic for AF_XDP:

  BUG: KASAN: null-ptr-deref in atomic64_sub include/asm-generic/atomic-instrumented.h:144 [inline]
  BUG: KASAN: null-ptr-deref in atomic_long_sub include/asm-generic/atomic-long.h:199 [inline]
  BUG: KASAN: null-ptr-deref in xdp_umem_unaccount_pages.isra.4+0x3d/0x80 net/xdp/xdp_umem.c:135
  Write of size 8 at addr 0000000000000060 by task syz-executor246/4527

  CPU: 1 PID: 4527 Comm: syz-executor246 Not tainted 4.17.0+ #89
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
  Call Trace:
   __dump_stack lib/dump_stack.c:77 [inline]
   dump_stack+0x1b9/0x294 lib/dump_stack.c:113
   kasan_report_error mm/kasan/report.c:352 [inline]
   kasan_report.cold.7+0x6d/0x2fe mm/kasan/report.c:412
   check_memory_region_inline mm/kasan/kasan.c:260 [inline]
   check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
   kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278
   atomic64_sub include/asm-generic/atomic-instrumented.h:144 [inline]
   atomic_long_sub include/asm-generic/atomic-long.h:199 [inline]
   xdp_umem_unaccount_pages.isra.4+0x3d/0x80 net/xdp/xdp_umem.c:135
   xdp_umem_reg net/xdp/xdp_umem.c:334 [inline]
   xdp_umem_create+0xd6c/0x10f0 net/xdp/xdp_umem.c:349
   xsk_setsockopt+0x443/0x550 net/xdp/xsk.c:531
   __sys_setsockopt+0x1bd/0x390 net/socket.c:1935
   __do_sys_setsockopt net/socket.c:1946 [inline]
   __se_sys_setsockopt net/socket.c:1943 [inline]
   __x64_sys_setsockopt+0xbe/0x150 net/socket.c:1943
   do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
   entry_SYSCALL_64_after_hwframe+0x49/0xbe

In xdp_umem_reg() the call to xdp_umem_account_pages() passed
with CAP_IPC_LOCK where we didn't need to end up charging rlimit
on memlock for the current user and therefore umem->user continues
to be NULL. Later on through fault injection syzkaller triggered
a failure in either umem->pgs or umem->pages allocation such that
we bail out and undo accounting in xdp_umem_unaccount_pages()
where we eventually hit the panic since it tries to deref the
umem->user.

The code is pretty close to mm_account_pinned_pages() and
mm_unaccount_pinned_pages() pair and potentially could reuse
it even in a later cleanup, and it appears that the initial
commit c0c77d8f ("xsk: add user memory registration support
sockopt") got this right while later follow-up introduced the
bug via a49049ea ("xsk: simplified umem setup").

Fixes: a49049ea ("xsk: simplified umem setup")
Reported-by: syzbot+979217770b09ebf5c407@syzkaller.appspotmail.com
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/xdp/xdp_umem.c

@@ -132,8 +132,10 @@ static void xdp_umem_unpin_pages(struct xdp_umem *umem)
 
 static void xdp_umem_unaccount_pages(struct xdp_umem *umem)
 {
-	atomic_long_sub(umem->npgs, &umem->user->locked_vm);
-	free_uid(umem->user);
+	if (umem->user) {
+		atomic_long_sub(umem->npgs, &umem->user->locked_vm);
+		free_uid(umem->user);
+	}
 }
 
 static void xdp_umem_release(struct xdp_umem *umem)