linux-kernel - [PATCH] x86/entry/64: Further improve paranoid

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Message-Id: <c47daa1888dc2298e7e1d3f82bd76b776ea33393.1539542111.git.luto@kernel.org>
Date:   Sun, 14 Oct 2018 11:38:18 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>
Subject: [PATCH] x86/entry/64: Further improve paranoid_entry comments

Commit 16561f27f94e ("x86/entry: Add some paranoid entry/exit CR3
handling comments") added some comments.  This improves them a bit:

 - When I first read the new comments, it was unclear to me whether
   they were referring to the case where paranoid_entry interrupted
   other entry code or where paranoid_entry was itself interrupted.
   Clarify it.

 - Remove the EBX comment.  We no longer use EBX as a SWAPGS
   indicator.

Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: "H. Peter Anvin" <hpa@...or.come
Cc: Borislav Petkov <bp@...en8.de>
Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/entry/entry_64.S | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1d9b4a300c8c..f95dcb209fdf 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1189,15 +1189,13 @@ ENTRY(paranoid_entry)
 1:
 	/*
 	 * Always stash CR3 in %r14.  This value will be restored,
-	 * verbatim, at exit.  Needed if kernel is interrupted
-	 * after switching to the user CR3 value but before
-	 * returning to userspace.
+	 * verbatim, at exit.  Needed if paranoid_entry interrupted
+	 * another entry that already switched to the user CR3 value
+	 * but has not yet returned to userspace.
 	 *
 	 * This is also why CS (stashed in the "iret frame" by the
 	 * hardware at entry) can not be used: this may be a return
-	 * to kernel code, but with a user CR3 value.  The %ebx flag
-	 * for SWAPGS is also unusable for CR3 because there is a
-	 * window with a user GS and a kernel CR3.
+	 * to kernel code, but with a user CR3 value.
 	 */
 	SAVE_AND_SWITCH_TO_KERNEL_CR3 scratch_reg=%rax save_reg=%r14
 
-- 
2.17.1