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x86/mm: Fix exception table comments 

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.

Cc: x86@kernel.org
Cc: Jann Horn <jannh@google.com>
Cc: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180928160227.077DDD7A@viggo.jf.intel.com
This commit is contained in:
Dave Hansen 2018-09-28 09:02:27 -07:00 committed by Peter Zijlstra
parent 5b0c2cac54
commit 88259744e2
1 changed files with 15 additions and 13 deletions
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28
arch/x86/mm/fault.c
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@ -1351,24 +1351,26 @@ void do_user_addr_fault(struct pt_regs *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;
}