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


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