Messages in this thread | | | Date | Tue, 20 Nov 2018 09:15:54 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 01/13] x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock |
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On Mon, Nov 19, 2018 at 02:45:25PM -0800, Andy Lutomirski wrote: > The fault-handling code that takes mmap_sem needs to avoid a > deadlock that could occur if the kernel took a bad (OOPS-worthy) > page fault on a user address while holding mmap_sem. This can only > happen if the faulting instruction was in the kernel > (i.e. user_mode(regs)). Rather than checking the sw_error_code
!user_mode(regs), surely, as the patch actually does.
> (which will have the USER bit set if the fault was a USER-permission > access *or* if user_mode(regs)), just check user_mode(regs) > directly. > > The old code would have malfunctioned if the kernel executed a bogus > WRUSS instruction while holding mmap_sem. Fortunately, that is > extremely unlikely in current kernels, which don't use WRUSS. > > Signed-off-by: Andy Lutomirski <luto@kernel.org> > --- > arch/x86/mm/fault.c | 7 ++----- > 1 file changed, 2 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c > index 71d4b9d4d43f..91d4d2722f2e 100644 > --- a/arch/x86/mm/fault.c > +++ b/arch/x86/mm/fault.c > @@ -1344,13 +1344,10 @@ void do_user_addr_fault(struct pt_regs *regs, > * 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. > + * 2. The access did not originate in userspace. > */ > if (unlikely(!down_read_trylock(&mm->mmap_sem))) { > - if (!(sw_error_code & X86_PF_USER) && > - !search_exception_tables(regs->ip)) { > + if (!user_mode(regs) && !search_exception_tables(regs->ip)) { > /* > * Fault from code in kernel from > * which we do not expect faults. > -- > 2.17.2 >
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