linux-kernel - Re: Review of KPTI patchset

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Date:   Sat, 30 Dec 2017 23:45:04 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Hugh Dickins <hughd@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Review of KPTI patchset

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;
 	}