linux-kernel - [PATCH] x86/mm: Switch from TASK_SIZE to TASK_SIZE

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Date:	Tue, 10 May 2016 14:10:29 -0700
From:	Andy Lutomirski <luto@...nel.org>
To:	x86@...nel.org
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	Dmitry Safonov <0x7f454c46@...il.com>,
	Ruslan Kabatsayev <b7.10110111@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Pavel Emelyanov <xemul@...allels.com>,
	Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...nel.org>
Subject: [PATCH] x86/mm: Switch from TASK_SIZE to TASK_SIZE_MAX in the page fault code

x86's page fault handlers had two TASK_SIZE uses that should have
been TASK_SIZE_MAX.  I don't think that either one had a visible
effect, but this makes the code clearer and should save a few bytes
of text.

(And I eventually want to eradicate TASK_SIZE.  This will help.)

Reported-by: Cyrill Gorcunov <gorcunov@...il.com>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/fault.c | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 1d75b98a8414..45940239b983 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -292,7 +292,7 @@ void vmalloc_sync_all(void)
 		return;
 
 	for (address = VMALLOC_START & PMD_MASK;
-	     address >= TASK_SIZE && address < FIXADDR_TOP;
+	     address >= TASK_SIZE_MAX && address < FIXADDR_TOP;
 	     address += PMD_SIZE) {
 		struct page *page;
 
@@ -854,8 +854,13 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
 				return;
 		}
 #endif
-		/* Kernel addresses are always protection faults: */
-		if (address >= TASK_SIZE)
+
+		/*
+		 * To avoid leaking information about the kernel page table
+		 * layout, pretend that user-mode accesses to kernel addresses
+		 * are always protection faults.
+		 */
+		if (address >= TASK_SIZE_MAX)
 			error_code |= PF_PROT;
 
 		if (likely(show_unhandled_signals))
-- 
2.5.5