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Message-ID: <CAKbZUD3E2if8Sncy+M2YKncc_Zh08-86W6U5wR0ZMazShxbHHA@mail.gmail.com>
Date: Fri, 29 Sep 2023 13:06:38 +0100
From: Pedro Falcato <pedro.falcato@...il.com>
To: Kees Cook <keescook@...omium.org>
Cc: Eric Biederman <ebiederm@...ssion.com>,
Sebastian Ott <sebott@...hat.com>,
Thomas Weißschuh <linux@...ssschuh.net>,
Al Viro <viro@...iv.linux.org.uk>,
Christian Brauner <brauner@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
linux-mm@...ck.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 1/6] binfmt_elf: Support segments with 0 filesz and
misaligned starts
On Fri, Sep 29, 2023 at 4:24 AM Kees Cook <keescook@...omium.org> wrote:
>
> From: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> 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@....cz>
> > 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@...e.cz>
> > Signed-off-by: Andrew Morton <akpm@...l.org>
> > Signed-off-by: Linus Torvalds <torvalds@...l.org>
>
> 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@...hat.com>
> Reported-by: Thomas Weißschuh <linux@...ssschuh.net>
> Closes: https://lkml.kernel.org/r/20230914-bss-alloc-v1-1-78de67d2c6dd@weissschuh.net
> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> Link: https://lore.kernel.org/r/87sf71f123.fsf@email.froward.int.ebiederm.org
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
> 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
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