Re: [PATCH] ELF: fix overflow in total mapping size calculation

[Alexey Dobriyan <adobriyan@gmail.com>]

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.

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().