x86/ldt: Plug memory leak in error path

The error path in write_ldt() tries to free 'old_ldt' instead of the newly
allocated 'new_ldt', resulting in a memory leak. It also misses to clean up a
half populated LDT pagetable, which is not a leak as it gets cleaned up
when the process exits.

Free both the potentially half populated LDT pagetable and the newly
allocated LDT struct. This can be done unconditionally because once an LDT
is mapped subsequent maps will succeed, because the PTE page is already
populated and the two LDTs fit into that single page.

Reported-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Dominik Brodowski <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Fixes: f55f050 ("x86/pti: Put the LDT in its own PGD if PTI is on")
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1712311121340.1899@nanos
Signed-off-by: Ingo Molnar <[email protected]>

arch/x86/kernel/ldt.c

@@ -421,7 +421,13 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
 	 */
 	error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
 	if (error) {
-		free_ldt_struct(old_ldt);
+		/*
+		 * This only can fail for the first LDT setup. If an LDT is
+		 * already installed then the PTE page is already
+		 * populated. Mop up a half populated page table.
+		 */
+		free_ldt_pgtables(mm);
+		free_ldt_struct(new_ldt);
 		goto out_unlock;
 	}