On Thu, Sep 16, 2021 at 12:41:56PM +0200, Peter Zijlstra wrote: > > The vmalloc guard pages are added on top of each allocation, thereby > isolating any two allocations from one another. The top guard of the > lower allocation is the bottom guard guard of the higher allocation > etc. > > Therefore VM_NO_GUARD is dangerous; it breaks the basic premise of > isolating separate allocations. > > There are only two in-tree users of this flag, neither of which use it > through the exported interface. Ensure it stays this way. > > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > --- > include/linux/vmalloc.h | 2 +- > mm/vmalloc.c | 7 +++++++ > 2 files changed, 8 insertions(+), 1 deletion(-) > > diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h > index 671d402c3778..10e9571ff0b2 100644 > --- a/include/linux/vmalloc.h > +++ b/include/linux/vmalloc.h > @@ -22,7 +22,7 @@ struct notifier_block; /* in notifier.h */ > #define VM_USERMAP 0x00000008 /* suitable for remap_vmalloc_range */ > #define VM_DMA_COHERENT 0x00000010 /* dma_alloc_coherent */ > #define VM_UNINITIALIZED 0x00000020 /* vm_struct is not fully initialized */ > -#define VM_NO_GUARD 0x00000040 /* don't add guard page */ > +#define VM_NO_GUARD 0x00000040 /* ***DANGEROUS*** don't add guard page */ > #define VM_KASAN 0x00000080 /* has allocated kasan shadow memory */ > #define VM_FLUSH_RESET_PERMS 0x00000100 /* reset direct map and flush TLB on unmap, can't be freed in atomic context */ > #define VM_MAP_PUT_PAGES 0x00000200 /* put pages and free array in vfree */ > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index d77830ff604c..01927ebea267 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -2743,6 +2743,13 @@ void *vmap(struct page **pages, unsigned int count, > > might_sleep(); > > + /* > + * Your top guard is someone else's bottom guard. Not having a top > + * guard compromises someone else's mappings too. > + */ > + if (WARN_ON_ONCE(flags & VM_NO_GUARD)) > + flags &= ~VM_NO_GUARD; > + > if (count > totalram_pages()) > return NULL; Acked-by: Will Deacon <will@xxxxxxxxxx> Thanks! Will
