linux-kernel - Re: [PATCH v2 1/6] sysctl: avoid spurious permanent empty tables

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Message-ID: <ef0dd949-e8a3-4b61-9d2d-3593b139cc4f@t-8ch.de>
Date: Sat, 24 Aug 2024 20:05:08 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Joel Granados <j.granados@...sung.com>, 
	Luis Chamberlain <mcgrof@...nel.org>, Kees Cook <kees@...nel.org>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>, 
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/6] sysctl: avoid spurious permanent empty tables

Hi Joel,

On 2024-08-05 11:39:35+0000, Thomas Weißschuh wrote:
> The test if a table is a permanently empty one, inspects the address of
> the registered ctl_table argument.
> However as sysctl_mount_point is an empty array and does not occupy and
> space it can end up sharing an address with another object in memory.
> If that other object itself is a "struct ctl_table" then registering
> that table will fail as it's incorrectly recognized as permanently empty.
> 
> Avoid this issue by adding a dummy element to the array so that is not
> empty anymore.
> Explicitly register the table with zero elements as otherwise the dummy
> element would be recognized as a sentinel element which would lead to a
> runtime warning from the sysctl core.
> 
> While the issue seems not being encountered at this time, this seems
> mostly to be due to luck.
> Also a future change, constifying sysctl_mount_point and root_table, can
> reliably trigger this issue on clang 18.
> 
> Given that empty arrays are non-standard in the first place it seems
> prudent to avoid them if possible.
> 
> Fixes: 4a7b29f65094 ("sysctl: move sysctl type to ctl_table_header")
> Fixes: a35dd3a786f5 ("sysctl: drop now unnecessary out-of-bounds check")
> Cc: stable@...r.kernel.org
> Signed-off-by: Thomas Weißschuh <linux@...ssschuh.net>

Any updates on this?
I fear it can theoretically also happen on v6.11.

> ---
>  fs/proc/proc_sysctl.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 9553e77c9d31..d11ebc055ce0 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -29,8 +29,13 @@ static const struct inode_operations proc_sys_inode_operations;
>  static const struct file_operations proc_sys_dir_file_operations;
>  static const struct inode_operations proc_sys_dir_operations;
>  
> -/* Support for permanently empty directories */
> -static struct ctl_table sysctl_mount_point[] = { };
> +/*
> + * Support for permanently empty directories.
> + * Must be non-empty to avoid sharing an address with other tables.
> + */
> +static struct ctl_table sysctl_mount_point[] = {
> +	{ }
> +};
>  
>  /**
>   * register_sysctl_mount_point() - registers a sysctl mount point
> @@ -42,7 +47,7 @@ static struct ctl_table sysctl_mount_point[] = { };
>   */
>  struct ctl_table_header *register_sysctl_mount_point(const char *path)
>  {
> -	return register_sysctl(path, sysctl_mount_point);
> +	return register_sysctl_sz(path, sysctl_mount_point, 0);
>  }
>  EXPORT_SYMBOL(register_sysctl_mount_point);
>  
> 
> -- 
> 2.46.0
>