x86/fsgsbase/64: Fix NULL deref in 86_fsgsbase_read_task

syzbot found its way in 86_fsgsbase_read_task() and triggered this oops:

   KASAN: null-ptr-deref in range [0x0000000000000008-0x000000000000000f]
   CPU: 0 PID: 6866 Comm: syz-executor262 Not tainted 5.8.0-syzkaller #0
   RIP: 0010:x86_fsgsbase_read_task+0x16d/0x310 arch/x86/kernel/process_64.c:393
   Call Trace:
     putreg32+0x3ab/0x530 arch/x86/kernel/ptrace.c:876
     genregs32_set arch/x86/kernel/ptrace.c:1026 [inline]
     genregs32_set+0xa4/0x100 arch/x86/kernel/ptrace.c:1006
     copy_regset_from_user include/linux/regset.h:326 [inline]
     ia32_arch_ptrace arch/x86/kernel/ptrace.c:1061 [inline]
     compat_arch_ptrace+0x36c/0xd90 arch/x86/kernel/ptrace.c:1198
     __do_compat_sys_ptrace kernel/ptrace.c:1420 [inline]
     __se_compat_sys_ptrace kernel/ptrace.c:1389 [inline]
     __ia32_compat_sys_ptrace+0x220/0x2f0 kernel/ptrace.c:1389
     do_syscall_32_irqs_on arch/x86/entry/common.c:84 [inline]
     __do_fast_syscall_32+0x57/0x80 arch/x86/entry/common.c:126
     do_fast_syscall_32+0x2f/0x70 arch/x86/entry/common.c:149
     entry_SYSENTER_compat_after_hwframe+0x4d/0x5c

This can happen if ptrace() or sigreturn() pokes an LDT selector into FS
or GS for a task with no LDT and something tries to read the base before
a return to usermode notices the bad selector and fixes it.

The fix is to make sure ldt pointer is not NULL.

Fixes: 07e1d88a ("x86/fsgsbase/64: Fix ptrace() to read the FS/GS base accurately")
Co-developed-by: Jann Horn <jannh@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Markus T Metzger <markus.t.metzger@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Shankar <ravi.v.shankar@intel.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/x86/kernel/process_64.c

@@ -390,7 +390,7 @@ unsigned long x86_fsgsbase_read_task(struct task_struct *task,
 		 */
 		mutex_lock(&task->mm->context.lock);
 		ldt = task->mm->context.ldt;
-		if (unlikely(idx >= ldt->nr_entries))
+		if (unlikely(!ldt || idx >= ldt->nr_entries))
 			base = 0;
 		else
 			base = get_desc_base(ldt->entries + idx);