Messages in this thread Patch in this message | | | Subject | [RFC][PATCH 5/8] x86/mm: fix exception table comments | From | Dave Hansen <> | Date | Fri, 07 Sep 2018 12:49:00 -0700 |
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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> ---
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; } _
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