linux-kernel - [PATCH 5/8] x86/mm: fix exception table comments

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Message-Id: <20180928160227.077DDD7A@viggo.jf.intel.com>
Date:   Fri, 28 Sep 2018 09:02:27 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     linux-kernel@...r.kernel.org
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        sean.j.christopherson@...el.com, peterz@...radead.org,
        tglx@...utronix.de, x86@...nel.org, luto@...nel.org,
        jannh@...gle.com
Subject: [PATCH 5/8] x86/mm: fix exception table comments


From: Dave Hansen <dave.hansen@...ux.intel.com>

The comments here are wrong.  They are too absolute about where
faults can occur when running in the kernel.  The comments are
also a bit hard to match up with the code.

Trim down the comments, and make them more precise.

Also add a comment explaining why we are doing the
bad_area_nosemaphore() path here.

Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Sean Christopherson <sean.j.christopherson@...el.com>
Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: x86@...nel.org
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Jann Horn <jannh@...gle.com>
Cc: Sean Christopherson <sean.j.christopherson@...el.com>
---

 b/arch/x86/mm/fault.c |   28 +++++++++++++++-------------
 1 file changed, 15 insertions(+), 13 deletions(-)

diff -puN arch/x86/mm/fault.c~pkeys-fault-warnings-03 arch/x86/mm/fault.c
--- a/arch/x86/mm/fault.c~pkeys-fault-warnings-03	2018-09-27 10:17:23.489343567 -0700
+++ b/arch/x86/mm/fault.c	2018-09-27 10:17:23.493343567 -0700
@@ -1353,24 +1353,26 @@ void do_user_addr_fault(struct pt_regs *
 		flags |= FAULT_FLAG_INSTRUCTION;
 
 	/*
-	 * When running in the kernel we expect faults to occur only to
-	 * addresses in user space.  All other faults represent errors in
-	 * the kernel and should generate an OOPS.  Unfortunately, in the
-	 * case of an erroneous fault occurring in a code path which already
-	 * holds mmap_sem we will deadlock attempting to validate the fault
-	 * against the address space.  Luckily the kernel only validly
-	 * references user space from well defined areas of code, which are
-	 * listed in the exceptions table.
+	 * Kernel-mode access to the user address space should only occur
+	 * on well-defined single instructions listed in the exception
+	 * tables.  But, an erroneous kernel fault occurring outside one of
+	 * those areas which also holds mmap_sem might deadlock attempting
+	 * to validate the fault against the address space.
 	 *
-	 * As the vast majority of faults will be valid we will only perform
-	 * the source reference check when there is a possibility of a
-	 * deadlock. Attempt to lock the address space, if we cannot we then
-	 * validate the source. If this is invalid we can skip the address
-	 * space check, thus avoiding the deadlock:
+	 * Only do the expensive exception table search when we might be at
+	 * risk of a deadlock.  This happens if we
+	 * 1. Failed to acquire mmap_sem, and
+	 * 2. The access did not originate in userspace.  Note: either the
+	 *    hardware or earlier page fault code may set X86_PF_USER
+	 *    in sw_error_code.
 	 */
 	if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
 		if (!(sw_error_code & X86_PF_USER) &&
 		    !search_exception_tables(regs->ip)) {
+			/*
+			 * Fault from code in kernel from
+			 * which we do not expect faults.
+			 */
 			bad_area_nosemaphore(regs, sw_error_code, address, NULL);
 			return;
 		}
_