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From: Andy Lutomirski <luto@amacapital.net>
To: x86@kernel.org
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>
Subject: [PATCH] x86_64: Add a comment explaining the TASK_SIZE_MAX guard page
Date: Tue,  4 Nov 2014 15:46:21 -0800	[thread overview]
Message-ID: <23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net> (raw)

That guard page is absolutely necessary; explain why for posterity.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/include/asm/processor.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index eb71ec794732..82d93ea13c0c 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -893,7 +893,13 @@ extern unsigned long thread_saved_pc(struct task_struct *tsk);
 
 #else
 /*
- * User space process size. 47bits minus one guard page.
+ * User space process size. 47bits minus one guard page.  The guard
+ * page is necessary on Intel CPUs: if a SYSCALL instruction is at
+ * the highest possible canonical userspace address, then that
+ * syscall will enter the kernel with a non-canonical return
+ * address, and SYSRET will explode dangerously.  We avoid this
+ * particular problem by preventing anything from being mapped
+ * at the maximum canonical address.
  */
 #define TASK_SIZE_MAX	((1UL << 47) - PAGE_SIZE)
 
-- 
1.9.3


             reply	other threads:[~2014-11-04 23:46 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-11-04 23:46 Andy Lutomirski [this message]
2014-11-10  9:45 ` [tip:x86/vdso] x86_64: Add a comment explaining the TASK_SIZE_MAX guard page tip-bot for Andy Lutomirski

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