linux-kernel - Re: [PATCH] [RFC] ntfs: disable for 64KB because of stack overflow risk

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Message-ID: <202109270923.7E71FE4F9@keescook>
Date:   Mon, 27 Sep 2021 09:23:20 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Anton Altaparmakov <anton@...era.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        linux-ntfs-dev@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] ntfs: disable for 64KB because of stack overflow
 risk

On Mon, Sep 27, 2021 at 04:18:03PM +0200, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@...db.de>
> 
> On ARM64 randconfig builds, we occasionally get warnings for NTFS:
> 
> fs/ntfs/aops.c: In function 'ntfs_write_mst_block':
> fs/ntfs/aops.c:1328:1: error: the frame size of 2224 bytes is larger than 2048 bytes [-Werror=frame-larger-than=]
> 
> The problem here is that with 64KB pages, we get two arrays on the
> stack that each have 128 pointers, for a total of 2KB. Before
> the VLA change, this could already happen with 512-byte blocks,
> however in practice NTFS should usually have 4KB blocks and not
> be affected by this (see link).
> 
> Now the stack usage is always > 2KB on any architecture with 64KB
> pages. Since both NTFS and 64KB page support are fairly rare,
> we may get away with just marking the combination as disallowed
> in Kconfig and see if anyone complains before we find a different
> way to address it.
> 
> Fixes: ac4ecf968acb ("ntfs: aops: remove VLA usage")
> Link: https://support.microsoft.com/en-us/help/140365/default-cluster-size-for-ntfs-fat-and-exfat
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

That seems reasonable.

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

-- 
Kees Cook