linux-kernel - Re: [PATCH 06/11] ubifs: reorder capability check last

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Message-ID: <Z0RbO1lSXoUnAtxj@e110455-lin.cambridge.arm.com>
Date: Mon, 25 Nov 2024 11:10:51 +0000
From: Liviu Dudau <liviu.dudau@....com>
To: cgzones@...glemail.com
Cc: linux-security-module@...r.kernel.org,
	Boris Brezillon <boris.brezillon@...labora.com>,
	Steven Price <steven.price@....com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Richard Weinberger <richard@....at>,
	Zhihao Cheng <chengzhihao1@...wei.com>,
	Serge Hallyn <serge@...lyn.com>,
	Julia Lawall <Julia.Lawall@...ia.fr>,
	Nicolas Palix <nicolas.palix@...g.fr>, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, linux-mtd@...ts.infradead.org,
	cocci@...ia.fr
Subject: Re: [PATCH 06/11] ubifs: reorder capability check last

Hi Christian,

On Mon, Nov 25, 2024 at 11:39:58AM +0100, Christian Göttsche wrote:
> From: Christian Göttsche <cgzones@...glemail.com>
> 
> capable() calls refer to enabled LSMs whether to permit or deny the
> request.  This is relevant in connection with SELinux, where a
> capability check results in a policy decision and by default a denial
> message on insufficient permission is issued.
> It can lead to three undesired cases:
>   1. A denial message is generated, even in case the operation was an
>      unprivileged one and thus the syscall succeeded, creating noise.
>   2. To avoid the noise from 1. the policy writer adds a rule to ignore
>      those denial messages, hiding future syscalls, where the task
>      performs an actual privileged operation, leading to hidden limited
>      functionality of that task.
>   3. To avoid the noise from 1. the policy writer adds a rule to permit
>      the task the requested capability, while it does not need it,
>      violating the principle of least privilege.
> 
> Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
> ---
>  drivers/gpu/drm/panthor/panthor_drv.c | 2 +-
>  fs/ubifs/budget.c                     | 5 +++--
>  2 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/panthor/panthor_drv.c b/drivers/gpu/drm/panthor/panthor_drv.c
> index ac7e53f6e3f0..2de0c3627fbf 100644
> --- a/drivers/gpu/drm/panthor/panthor_drv.c
> +++ b/drivers/gpu/drm/panthor/panthor_drv.c
> @@ -791,7 +791,7 @@ static int group_priority_permit(struct drm_file *file,
>  		return 0;
>  
>  	/* Higher priorities require CAP_SYS_NICE or DRM_MASTER */
> -	if (capable(CAP_SYS_NICE) || drm_is_current_master(file))
> +	if (drm_is_current_master(file) || capable(CAP_SYS_NICE))
>  		return 0;
>  
>  	return -EACCES;

Can the patch above be split into a separate one? It's for a different subsystem than ubifs.

Otherwise, it looks good to me, so you can add my Reviewed-by to the new patch.

Best regards,
Liviu

> diff --git a/fs/ubifs/budget.c b/fs/ubifs/budget.c
> index d76eb7b39f56..6137aeadec3f 100644
> --- a/fs/ubifs/budget.c
> +++ b/fs/ubifs/budget.c
> @@ -256,8 +256,9 @@ long long ubifs_calc_available(const struct ubifs_info *c, int min_idx_lebs)
>   */
>  static int can_use_rp(struct ubifs_info *c)
>  {
> -	if (uid_eq(current_fsuid(), c->rp_uid) || capable(CAP_SYS_RESOURCE) ||
> -	    (!gid_eq(c->rp_gid, GLOBAL_ROOT_GID) && in_group_p(c->rp_gid)))
> +	if (uid_eq(current_fsuid(), c->rp_uid) ||
> +	    (!gid_eq(c->rp_gid, GLOBAL_ROOT_GID) && in_group_p(c->rp_gid)) ||
> +	    capable(CAP_SYS_RESOURCE))
>  		return 1;
>  	return 0;
>  }
> -- 
> 2.45.2
> 

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