On Fri, 2015-01-30 at 19:08 +0700, Arseny Solokha wrote: > MMU_NO_CONTEXT is conditionally defined as 0 or (unsigned int)-1. However, > in __flush_tlb_page() a corresponding variable is only tested for open > coded 0, which can cause NULL pointer dereference if `mm' argument was > legitimately passed as such. > > Bail out early in case the first argument is NULL, thus eliminate confusion > between different values of MMU_NO_CONTEXT and avoid disabling and then > re-enabling preemption unnecessarily.
So the comment above isn't quite right... we don't *test* it for open coded 0, we test it for MMU_NO_CONTEXT, however we *set* it to 0 for NULL mm. This is actually correct... on all except 8xx :-) 0 *is* the PID of the kernel context, and NULL mm usually means kernel context. However, it's correct that this function will not deal properly with a NULL mm for other reasons. It must only be called for user contexts. Instead of just returning, I would WARN_ON, because if it's ever called for a kernel page, then it will not do what's expected and that will need fixing. Just a silent return isn't right. This is different from returning on MMU_NO_CONTEXT, in this case, we know there's no active TLB entries for the process, and thus nothing to flush. Cheers, Ben. > Signed-off-by: Arseny Solokha <asolo...@kb.kras.ru> > --- > arch/powerpc/mm/tlb_nohash.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/mm/tlb_nohash.c b/arch/powerpc/mm/tlb_nohash.c > index f38ea4d..ab0616b 100644 > --- a/arch/powerpc/mm/tlb_nohash.c > +++ b/arch/powerpc/mm/tlb_nohash.c > @@ -284,8 +284,11 @@ void __flush_tlb_page(struct mm_struct *mm, unsigned > long vmaddr, > struct cpumask *cpu_mask; > unsigned int pid; > > + if (unlikely(!mm)) > + return; > + > preempt_disable(); > - pid = mm ? mm->context.id : 0 Here we test mm, if we pass NULL, that means the kernel mm which has PID 0, which is not MMU_NO_CONTEXT > ; > + pid = mm->context.id; > if (unlikely(pid == MMU_NO_CONTEXT)) > goto bail; > cpu_mask = mm_cpumask(mm); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
