On Fri, 2018-09-07 at 12:49 -0700, Dave Hansen wrote:
> From: Dave Hansen <dave.han...@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.han...@linux.intel.com>
> Cc: Sean Christopherson <sean.j.christopher...@intel.com>
> Cc: "Peter Zijlstra (Intel)" <pet...@infradead.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: x...@kernel.org
> Cc: Andy Lutomirski <l...@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;
> }
> _
