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

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Message-ID: <4baef4a5-6b02-4e2f-870a-810c615a59ce@t-8ch.de>
Date: Mon, 5 Aug 2024 13:51:55 +0200
From: Thomas Weißschuh <linux@...ssschuh.net>
To: Luis Chamberlain <mcgrof@...nel.org>, Kees Cook <kees@...nel.org>, 
	Joel Granados <j.granados@...sung.com>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] sysctl: avoid spurious permanent empty tables

(trimmed recipients to only sysctl maintainers)

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.

Unfortunately I forgot to include the following trailers in this patch:

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202408051453.f638857e-lkp@intel.com

They will be part of v3 if it comes to that, but if you end up applying
v2, please do also add those trailers.

> 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>
> ---
>  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);


Thanks,
Thomas