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SubjectRe: [PATCH] ELF: fix overflow in total mapping size calculation
On Thu, Oct 07, 2021 at 08:20:03PM +0300, Alexey Dobriyan wrote:
> On Tue, Oct 05, 2021 at 07:31:09PM -0700, Kees Cook wrote:
> > On Sun, Oct 03, 2021 at 03:11:24PM +0300, Alexey Dobriyan wrote:
> > > Kernel assumes that ELF program headers are ordered by mapping address,
> > > but doesn't enforce it. It is possible to make mapping size extremely huge
> > > by simply shuffling first and last PT_LOAD segments.
> > >
> > > As long as PT_LOAD segments do not overlap, it is silly to require
> > > sorting by v_addr anyway because mmap() doesn't care.
> > >
> > > Don't assume PT_LOAD segments are sorted and calculate min and max
> > > addresses correctly.
> >
> > Nice! Yes, this all make sense.
> >
> > >
> > > Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
> > > ---
> > >
> > > fs/binfmt_elf.c | 23 +++++++++++------------
> > > 1 file changed, 11 insertions(+), 12 deletions(-)
> > >
> > > --- a/fs/binfmt_elf.c
> > > +++ b/fs/binfmt_elf.c
> > > @@ -93,7 +93,7 @@ static int elf_core_dump(struct coredump_params *cprm);
> > > #define ELF_CORE_EFLAGS 0
> > > #endif
> > >
> > > -#define ELF_PAGESTART(_v) ((_v) & ~(unsigned long)(ELF_MIN_ALIGN-1))
> > > +#define ELF_PAGESTART(_v) ((_v) & ~(int)(ELF_MIN_ALIGN-1))
> >
> > Errr, this I don't like. I assume this is because of the min() use
> > below?
>
> Yes, this is to shut up the warning.
>
> The macro is slightly incorrect because "_v" can be either uint32_t or
> uint64_t. But standard ALIGN macros are slightly incorrect too.

Right, but "int" is neither 64-sized nor unsigned. :P I would just leave
this macro as-is.

>
> I don't want to clean this particular mess right now. Those are separate stables.
>
> > > #define ELF_PAGEOFFSET(_v) ((_v) & (ELF_MIN_ALIGN-1))
> > > #define ELF_PAGEALIGN(_v) (((_v) + ELF_MIN_ALIGN - 1) & ~(ELF_MIN_ALIGN - 1))
> > >
> > > @@ -399,22 +399,21 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
> > > return(map_addr);
> > > }
> > >
> > > -static unsigned long total_mapping_size(const struct elf_phdr *cmds, int nr)
> > > +static unsigned long total_mapping_size(const struct elf_phdr *phdr, int nr)
> > > {
> > > - int i, first_idx = -1, last_idx = -1;
> > > + elf_addr_t min_addr = -1;
> > > + elf_addr_t max_addr = 0;
> > > + bool pt_load = false;
> > > + int i;
> > >
> > > for (i = 0; i < nr; i++) {
> > > - if (cmds[i].p_type == PT_LOAD) {
> > > - last_idx = i;
> > > - if (first_idx == -1)
> > > - first_idx = i;
> > > + if (phdr[i].p_type == PT_LOAD) {
> > > + min_addr = min(min_addr, ELF_PAGESTART(phdr[i].p_vaddr));
> > > + max_addr = max(max_addr, phdr[i].p_vaddr + phdr[i].p_memsz);
> >
> > How about:
> > min_addr = min_t(elf_addr_t, min_addr, ELF_PAGESTART(phdr[i].p_vaddr));
> > max_addr = max_t(elf_addr_t, max_addr, phdr[i].p_vaddr + phdr[i].p_memsz);
>
> No! The proper fix is to fix ELF_PAGESTART().

Why? The warning from min() is about making sure there isn't an
unexpected type conversion. min_t() uses an explicit type, so why not
the above?

--
Kees Cook

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