Messages in this thread | | | Subject | Re: [RFC][PATCH 5/8] x86/mm: fix exception table comments | From | Sean Christopherson <> | Date | Fri, 07 Sep 2018 14:04:40 -0700 |
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On Fri, 2018-09-07 at 12:49 -0700, Dave Hansen wrote: > 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).
Might be worth reminding the reader that X86_PF_USER will be set in sw_error_code for implicit accesses. I saw "explicit" and my mind immediately jumped to hw_error_code for whatever reason. E.g.:
* 2. The access was an explicit kernel-mode access (we set X86_PF_USER * in sw_error_code for implicit kernel-mode accesses).
> */ > 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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