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

Dave Hansen
2018-09-28T16:06:16Z

From: Dave Hansen <dave.hansen@linux.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@linux.intel.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.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;
}
_