[tip:x86/vdso] x86_64: Add a comment explaining the TASK_SIZE_MAX guard page

[tip-bot for Andy Lutomirski]

Commit-ID:  07114f0f1cda8b2ef6e884d0c7b268a32cce7903
Gitweb:     http://git.kernel.org/tip/07114f0f1cda8b2ef6e884d0c7b268a32cce7903
Author:     Andy Lutomirski <luto@xxxxxxxxxxxxxx>
AuthorDate: Tue, 4 Nov 2014 15:46:21 -0800
Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
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@xxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/23320cb5017c2da8475ec20fcde8089d82aa2699.1415144745.git.luto@xxxxxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 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)
 
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