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

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Date:   Fri, 07 Sep 2018 12:49:00 -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
Subject: [RFC][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>
---

 b/arch/x86/mm/fault.c |   27 ++++++++++++++-------------
 1 file changed, 14 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	2018-09-07 11:21:47.696751898 -0700
+++ b/arch/x86/mm/fault.c	2018-09-07 11:21:47.700751898 -0700
@@ -1349,24 +1349,25 @@ void do_user_addr_space_fault(struct pt_
 		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
+	 * inside well-defined areas of code 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:
+	 * 1. We failed to acquire mmap_sem, and
+	 * 2. The access was an explicit kernel-mode access
+	 *    (X86_PF_USER=0).
 	 */
 	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;
 		}
_