Re: [PATCH v4 1/6] binfmt_elf: Support segments with 0 filesz and misaligned starts

[Pedro Falcato]

On Fri, Sep 29, 2023 at 4:24 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
>
> Implement a helper elf_load() that wraps elf_map() and performs all
> of the necessary work to ensure that when "memsz > filesz" the bytes
> described by "memsz > filesz" are zeroed.
>
> An outstanding issue is if the first segment has filesz 0, and has a
> randomized location. But that is the same as today.
>
> In this change I replaced an open coded padzero() that did not clear
> all of the way to the end of the page, with padzero() that does.
>
> I also stopped checking the return of padzero() as there is at least
> one known case where testing for failure is the wrong thing to do.
> It looks like binfmt_elf_fdpic may have the proper set of tests
> for when error handling can be safely completed.
>
> I found a couple of commits in the old history
> https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git,
> that look very interesting in understanding this code.
>
> commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail")
> commit c6e2227e4a3e ("[SPARC64]: Missing user access return value checks in fs/binfmt_elf.c and fs/compat.c")
> commit 5bf3be033f50 ("v2.4.10.1 -> v2.4.10.2")
>
> Looking at commit 39b56d902bf3 ("[PATCH] binfmt_elf: clearing bss may fail"):
> >  commit 39b56d902bf35241e7cba6cc30b828ed937175ad
> >  Author: Pavel Machek <pavel@xxxxxx>
> >  Date:   Wed Feb 9 22:40:30 2005 -0800
> >
> >     [PATCH] binfmt_elf: clearing bss may fail
> >
> >     So we discover that Borland's Kylix application builder emits weird elf
> >     files which describe a non-writeable bss segment.
> >
> >     So remove the clear_user() check at the place where we zero out the bss.  I
> >     don't _think_ there are any security implications here (plus we've never
> >     checked that clear_user() return value, so whoops if it is a problem).
> >
> >     Signed-off-by: Pavel Machek <pavel@xxxxxxx>
> >     Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
> >     Signed-off-by: Linus Torvalds <torvalds@xxxxxxxx>
>
> It seems pretty clear that binfmt_elf_fdpic with skipping clear_user() for
> non-writable segments and otherwise calling clear_user(), aka padzero(),
> and checking it's return code is the right thing to do.
>
> I just skipped the error checking as that avoids breaking things.
>
> And notably, it looks like Borland's Kylix died in 2005 so it might be
> safe to just consider read-only segments with memsz > filesz an error.
>
> Reported-by: Sebastian Ott <sebott@xxxxxxxxxx>
> Reported-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>
> Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@xxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Link: https://lore.kernel.org/r/87sf71f123.fsf@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  fs/binfmt_elf.c | 111 +++++++++++++++++++++---------------------------
>  1 file changed, 48 insertions(+), 63 deletions(-)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 7b3d2d491407..2a615f476e44 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -110,25 +110,6 @@ static struct linux_binfmt elf_format = {
>
>  #define BAD_ADDR(x) (unlikely((unsigned long)(x) >= TASK_SIZE))
>
> -static int set_brk(unsigned long start, unsigned long end, int prot)
> -{
> -       start = ELF_PAGEALIGN(start);
> -       end = ELF_PAGEALIGN(end);
> -       if (end > start) {
> -               /*
> -                * Map the last of the bss segment.
> -                * If the header is requesting these pages to be
> -                * executable, honour that (ppc32 needs this).
> -                */
> -               int error = vm_brk_flags(start, end - start,
> -                               prot & PROT_EXEC ? VM_EXEC : 0);
> -               if (error)
> -                       return error;
> -       }
> -       current->mm->start_brk = current->mm->brk = end;
> -       return 0;
> -}
> -
>  /* We need to explicitly zero any fractional pages
>     after the data section (i.e. bss).  This would
>     contain the junk from the file that should not
> @@ -406,6 +387,51 @@ static unsigned long elf_map(struct file *filep, unsigned long addr,
>         return(map_addr);
>  }
>
> +static unsigned long elf_load(struct file *filep, unsigned long addr,
> +               const struct elf_phdr *eppnt, int prot, int type,
> +               unsigned long total_size)
> +{
> +       unsigned long zero_start, zero_end;
> +       unsigned long map_addr;
> +
> +       if (eppnt->p_filesz) {
> +               map_addr = elf_map(filep, addr, eppnt, prot, type, total_size);
> +               if (BAD_ADDR(map_addr))
> +                       return map_addr;
> +               if (eppnt->p_memsz > eppnt->p_filesz) {
> +                       zero_start = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +                               eppnt->p_filesz;
> +                       zero_end = map_addr + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +                               eppnt->p_memsz;
> +
> +                       /* Zero the end of the last mapped page */
> +                       padzero(zero_start);
> +               }
> +       } else {
> +               map_addr = zero_start = ELF_PAGESTART(addr);
> +               zero_end = zero_start + ELF_PAGEOFFSET(eppnt->p_vaddr) +
> +                       eppnt->p_memsz;

What happens if a previous segment has mapped ELF_PAGESTART(addr)?
Don't we risk mapping over that?
Whereas AFAIK old logic would just padzero the bss bytes.

-- 
Pedro