Re: [RFC PATCH v1 1/1] binfmt_elf: mseal address zero

Jeff Xu
2024-08-05T21:34:14Z

On Mon, Aug 5, 2024 at 2:05 PM Kees Cook <kees@kernel.org> wrote:
>
> On Thu, Aug 01, 2024 at 05:08:33PM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@chromium.org>
> >
> > Some legacy SVr4 apps might depend on page on address zero
> > to be readable, however I can't find a reason that the page
> > ever becomes writeable, so seal it.
> >
> > If there is a compain, we can make this configurable.
> >
> > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> > ---
> > fs/binfmt_elf.c | 4 ++++
> > include/linux/mm.h | 4 ++++
> > mm/mseal.c | 2 +-
> > 3 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index 19fa49cd9907..e4d35d6f5d65 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1314,6 +1314,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
> > emulate the SVr4 behavior. Sigh. */
> > error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
> > MAP_FIXED | MAP_PRIVATE, 0);
> > +
> > +#ifdef CONFIG_64BIT
> > + do_mseal(0, PAGE_SIZE, 0);
> > +#endif
>
> Instead of wrapping this in #ifdefs, does it make more sense to adjust
> the mm.h declaration instead, like this below...
>
Sure.

> > }
> >
> > regs = current_pt_regs();
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index c4b238a20b76..b5fed60ddcd9 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4201,4 +4201,8 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
> >
> > int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
> >
> > +#ifdef CONFIG_64BIT
> > +int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
>
> #else
> static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> {
> return -ENOTSUPP;
> }
>
OK.

> > +#endif
> > +
> > #endif /* _LINUX_MM_H */
> > diff --git a/mm/mseal.c b/mm/mseal.c
> > index bf783bba8ed0..7a40a84569c8 100644
> > --- a/mm/mseal.c
> > +++ b/mm/mseal.c
> > @@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
> > *
> > * unseal() is not supported.
> > */
> > -static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> > +int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> > {
> > size_t len;
> > int ret = 0;
> > --
> > 2.46.0.rc1.232.g9752f9e123-goog
> >
>
> And if it returns an error code, should we check it when used in
> load_elf_binary()? (And if so, should the mm.h return 0 for non-64bit?)
>
It shouldn't fail. I can add pr_warning to handle the error case:
pr_warning("pid=%d, couldn't seal the page on address 0.\n",
task_pid_nr(current));

Thanks!
Best regards,
-Jeff

> --
> Kees Cook