linux-kernel - [patch V2 1/3] x86/ldt: Plug memory leak in error path

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Date:   Sun, 31 Dec 2017 11:24:34 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
cc:     Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [patch V2 1/3] 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 a LDT
is mapped subsequent maps will succeed because the PTE page is already
populated and the two LDTs fit into that single page.

Fixes: f55f0501cbf6 ("x86/pti: Put the LDT in its own PGD if PTI is on")
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/kernel/ldt.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -421,7 +421,13 @@ static int write_ldt(void __user *ptr, u
 	 */
 	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 a 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;
 	}