From: tip-bot for Andy Lutomirski <tipbot@zytor.com>
To: linux-tip-commits@vger.kernel.org
Cc: hpa@zytor.com, mingo@kernel.org, linux-kernel@vger.kernel.org,
tglx@linutronix.de, luto@amacapital.net
Subject: [tip:x86/vdso] x86_64: Add a comment explaining the TASK_SIZE_MAX guard page
Date: Mon, 10 Nov 2014 01:45:33 -0800 [thread overview]
Message-ID: <tip-07114f0f1cda8b2ef6e884d0c7b268a32cce7903@git.kernel.org> (raw)
In-Reply-To: <23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net>
Commit-ID: 07114f0f1cda8b2ef6e884d0c7b268a32cce7903
Gitweb: http://git.kernel.org/tip/07114f0f1cda8b2ef6e884d0c7b268a32cce7903
Author: Andy Lutomirski <luto@amacapital.net>
AuthorDate: Tue, 4 Nov 2014 15:46:21 -0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Mon, 10 Nov 2014 10:43:13 +0100
x86_64: Add a comment explaining the TASK_SIZE_MAX guard page
That guard page is absolutely necessary; explain why for
posterity.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@amacapital.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
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 eb71ec7..82d93ea 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)
prev parent reply other threads:[~2014-11-10 9:45 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-04 23:46 [PATCH] " Andy Lutomirski
2014-11-10 9:45 ` tip-bot for Andy Lutomirski [this message]
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