Re: Review of KPTI patchset

[Thomas Gleixner <tglx@linutronix.de>]

On Sat, 30 Dec 2017, Thomas Gleixner wrote:
> On Sat, 30 Dec 2017, Mathieu Desnoyers wrote:
> The only asymetry is in the error path of write_ldt() which can leak a half
> allocated page table. But, that's a nasty one because if there is an
> existing LDT mapped, then the pagetable cannot be freed. So yes, it's not
> nice, but harmless and needs some thought to fix.

In fact it's not a leak. It's just memory waste because the pagetable gets
freed when the process exits.

The memory waste is rather simple to fix. Delta patch below.

Thanks,

	tglx

8<--------------
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -421,6 +421,14 @@ static int write_ldt(void __user *ptr, u
 	 */
 	error = map_ldt_struct(mm, new_ldt, old_ldt ? !old_ldt->slot : 0);
 	if (error) {
+		/*
+		 * Drop potentially half populated page table if the
+		 * mapping code failed and this was the first attempt to
+		 * install a LDT. If there is a LDT installed then the LDT
+		 * pagetable cannot be freed for obvious reasons.
+		 */
+		if (!old_ldt)
+			free_ldt_pgtables(mm);
 		free_ldt_struct(new_ldt);
 		goto out_unlock;
 	}