[09/11] fs: reorder capability check last

Commit Message

Christian Göttsche Nov. 25, 2024, 10:40 a.m. UTC

From: Christian Göttsche <[e-mail]>

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 <[e-mail]>
---
 fs/fhandle.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Christian Brauner Nov. 25, 2024, 12:17 p.m. UTC | #1

On Mon, Nov 25, 2024 at 11:40:01AM +0100, Christian Göttsche wrote:
> From: Christian Göttsche <[e-mail]>
> 
> 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 <[e-mail]>
> ---

Reviewed-by: Christian Brauner <[e-mail]>

Patch

diff --git a/fs/fhandle.c b/fs/fhandle.c
index 5f801139358e..01b3e14e07de 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -265,9 +265,9 @@  static inline bool may_decode_fh(struct handle_to_path_ctx *ctx,
 	if (ns_capable(root->mnt->mnt_sb->s_user_ns, CAP_SYS_ADMIN))
 		ctx->flags = HANDLE_CHECK_PERMS;
 	else if (is_mounted(root->mnt) &&
+		 !has_locked_children(real_mount(root->mnt), root->dentry) &&
 		 ns_capable(real_mount(root->mnt)->mnt_ns->user_ns,
-			    CAP_SYS_ADMIN) &&
-		 !has_locked_children(real_mount(root->mnt), root->dentry))
+			    CAP_SYS_ADMIN))
 		ctx->flags = HANDLE_CHECK_PERMS | HANDLE_CHECK_SUBTREE;
 	else
 		return false;