Messages in this thread | | | Date | Wed, 4 Oct 2017 19:12:30 +0200 | From | Michal Hocko <> | Subject | Re: MAP_FIXED for ELF mappings |
| |
On Wed 04-10-17 23:37:41, Baoquan He wrote: > On 10/04/17 at 05:17pm, Michal Hocko wrote: > > On Wed 04-10-17 23:12:38, Baoquan He wrote: > > > I made a clean up patch according to Oleg's suggestion. It's trying to > > > get an map area to cover total_size, then do mmap for for the 1st > > > program segment only. Not sure if this way is correct. > > > > > > >From 40f231bb78a74caebcb4a898089a9fa5323be05f Mon Sep 17 00:00:00 2001 > > > From: Baoquan He <bhe@redhat.com> > > > Date: Fri, 29 Sep 2017 21:35:30 +0800 > > > Subject: [PATCH] binfmt_elf: Clean up the elf_map > > > > > > Oleg pointed out that it's really ugly to do mmap of the total_size, then > > > unmap the region excluding the 1st segment. The right way should be search > > > an unmapped area which can cover region of total_size, then map the 1st > > > segment only. > > > > > > And also update the code comment accordingly. In below commit, the relevant > > > code comment is not changed to cover the ELF binary image case. > > > commit a87938b2e2 ("fs/binfmt_elf.c: fix bug in loading of PIE binaries") > > > > > > Signed-off-by: Baoquan He <bhe@redhat.com> > > > --- > > > fs/binfmt_elf.c | 27 +++++++++++++++------------ > > > 1 file changed, 15 insertions(+), 12 deletions(-) > > > > > > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c > > > index 72b7ecba7ead..43a47b2aa3f6 100644 > > > --- a/fs/binfmt_elf.c > > > +++ b/fs/binfmt_elf.c > > > @@ -357,22 +357,25 @@ static unsigned long elf_map(struct file *filep, unsigned long addr, > > > return addr; > > > > > > /* > > > - * total_size is the size of the ELF (interpreter) image. > > > - * The _first_ mmap needs to know the full size, otherwise > > > - * randomization might put this image into an overlapping > > > - * position with the ELF binary image. (since size < total_size) > > > - * So we first map the 'big' image - and unmap the remainder at > > > - * the end. (which unmap is needed for ELF images with holes.) > > > + * total_size is the size of the ELF binary image or the ELF loader > > > + * image. For loader image, the _first_ mmap needs to know the full > > > + * size, otherwise randomization might put image into an overlapping > > > + * position with the ELF binary image.(since size < total_size) > > > + * So we use total_size to get an area to cover the whole loader image, > > > + * then map the 1st progment segment only with its own size. For binary > > > + * image, similarly, the _first_ mmap also needs to know the full size, > > > + * otherwise randomization might put image above mm->mmap_base. > > Oh, no, here, it won't include the PIE binary case. Since it must be > from ELF_ET_DYN_BASE. Here should be the "ld.so program" case. > > > > */ > > > if (total_size) { > > > total_size = ELF_PAGEALIGN(total_size); > > > - map_addr = vm_mmap(filep, addr, total_size, prot, flags, off); > > > - if (!BAD_ADDR(map_addr)) > > > - vm_munmap(map_addr+size, total_size-size); > > > - } else > > > - map_addr = vm_mmap(filep, addr, size, prot, flags, off); > > > + addr = get_unmapped_area(file, addr, total_size, off, flags); > > > > So how does this prevent clobbering an existing VMA when flags contains > > MAP_FIXED? > > Earlier flush_old_exec() is called to clean all old VMAs.
Yes, but we already have a new stack mapped and that was the point of the referenced CVE where the binary segments got mapped over the stack AFAIU. -- Michal Hocko SUSE Labs
| |