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

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Date:   Fri, 29 Sep 2023 10:45:35 -0500
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Sebastian Ott <sebott@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Weißschuh <linux@...ssschuh.net>,
        Pedro Falcato <pedro.falcato@...il.com>,
        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 0/6] binfmt_elf: Support segments with 0 filesz and
 misaligned starts

Sebastian Ott <sebott@...hat.com> writes:

> Hello Kees,
>
> On Thu, 28 Sep 2023, Kees Cook wrote:
>> This is the continuation of the work Eric started for handling
>> "p_memsz > p_filesz" in arbitrary segments (rather than just the last,
>> BSS, segment). I've added the suggested changes:
>>
>> - drop unused "elf_bss" variable
>> - refactor load_elf_interp() to use elf_load()
>> - refactor load_elf_library() to use elf_load()
>> - report padzero() errors when PROT_WRITE is present
>> - drop vm_brk()
>
> While I was debugging the initial issue I stumbled over the following
> - care to take it as part of this series?
>
> ----->8
> [PATCH] mm: vm_brk_flags don't bail out while holding lock
>
> Calling vm_brk_flags() with flags set other than VM_EXEC
> will exit the function without releasing the mmap_write_lock.
>
> Just do the sanity check before the lock is acquired. This
> doesn't fix an actual issue since no caller sets a flag other
> than VM_EXEC.

That seems like a sensible patch.

Have you by any chance read this code enough to understand what is
gained by calling vm_brk_flags rather than vm_mmap without a file?

Unless there is a real advantage it probably makes sense to replace
the call of vm_brk_flags with vm_mmap(NULL, ...) as binfmt_elf_fdpic
has already done.

That would allow removing vm_brk_flags and sys_brk would be the last
caller of do_brk_flags.

Eric


> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: linux-mm@...ck.org
> Signed-off-by: Sebastian Ott <sebott@...hat.com>
> ---
>   mm/mmap.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index b56a7f0c9f85..7ed286662839 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3143,13 +3143,13 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
>   	if (!len)
>   		return 0;
>
> -	if (mmap_write_lock_killable(mm))
> -		return -EINTR;
> -
>   	/* Until we need other flags, refuse anything except VM_EXEC. */
>   	if ((flags & (~VM_EXEC)) != 0)
>   		return -EINVAL;
>
> +	if (mmap_write_lock_killable(mm))
> +		return -EINTR;
> +
>   	ret = check_brk_limits(addr, len);
>   	if (ret)
>   		goto limits_failed;