linux-kernel - Re: [PATCH v2 3/3] elf: Don't write past end of notes for regset gap

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Date:   Thu, 17 Mar 2022 14:26:33 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     dave.hansen@...el.com, len.brown@...el.com, tony.luck@...el.com,
        rafael.j.wysocki@...el.com, reinette.chatre@...el.com,
        dan.j.williams@...el.com, viro@...iv.linux.org.uk,
        ebiederm@...ssion.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH v2 3/3] elf: Don't write past end of notes for regset gap

On Thu, Mar 17, 2022 at 12:20:13PM -0700, Rick Edgecombe wrote:
> In fill_thread_core_info() the ptrace accessible registers are collected
> to be written out as notes in a core file. The note array is allocated
> from a size calculated by iterating the user regset view, and counting the
> regsets that have a non-zero core_note_type. However, this only allows for
> there to be non-zero core_note_type at the end of the regset view. If
> there are any gaps in the middle, fill_thread_core_info() will overflow the
> note allocation, as it iterates over the size of the view and the
> allocation would be smaller than that.
> 
> There doesn't appear to be any arch that has gaps such that they exceed
> the notes allocation, but the code is brittle and tries to support
> something it doesn't. It could be fixed by increasing the allocation size,
> but instead just have the note collecting code utilize the array better.
> This way the allocation can stay smaller.
> 
> Even in the case of no arch's that have gaps in their regset views, this
> introduces a change in the resulting indicies of t->notes. It does not
> introduce any changes to the core file itself, because any blank notes are
> skipped in write_note_info().
> 
> In case, the allocation logic between fill_note_info() and
> fill_thread_core_info() ever diverges from the usage logic, warn and skip
> writing any notes that would overflow the array.
> 
> This fix is derrived from an earlier one[0] by Yu-cheng Yu.
> 
> [0] https://lore.kernel.org/lkml/20180717162502.32274-1-yu-cheng.yu@intel.com/
> 
> Co-developed-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@...el.com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> 
> ---
> 
> v2:
>  - Warn and break out of the note collecting loop if the allocation would
>    overflow. Note: I tweaked it slightly to do break instead of continue
>    and to do it before SET_PR_FPVALID(). (Kees)

This looks great; thank you for the tweak. :)

Acked-by: Kees Cook <keescook@...omium.org>

Shall I take this separately into the for-next/execve tree, or would you
rather is stay in this series?

-Kees

-- 
Kees Cook