[PATCH] fs: hidepid: Fixes hidepid non dumpable behavior

[nicolas.bouchinet@oss.cyber.gouv.fr]

From: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>

The hidepid mount option documentation defines the following modes:

- "noaccess": user may not access any `/proc/<pid>/ directories but
  their own.
- "invisible": all `/proc/<pid>/` will be fully invisible to other users.
- "ptraceable": means that procfs should only contain `/proc/<pid>/`
  directories that the caller can ptrace.

We thus expect that with "noaccess" and "invisible" users would be able to
see their own processes in `/proc/<pid>/`.

The implementation of hidepid however control accesses using the
`ptrace_may_access()` function in any cases. Thus, if a process set
itself as non-dumpable using the `prctl(PR_SET_DUMPABLE,
SUID_DUMP_DISABLE)` it becomes invisible to the user.

This patch fixes the `has_pid_permissions()` function in order to make
its behavior to match the documentation.

Note that since `ptrace_may_access()` is not called anymore with
"noaccess" and "invisible", the `security_ptrace_access_check()` will no
longer be called either.

Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
---
 fs/proc/base.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index c667702dc69b8ca2531e88e12ed7a18533f294dd..fb128cb5f95fe65016fce96c75aee18c762a30f2 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -746,9 +746,12 @@ static bool has_pid_permissions(struct proc_fs_info *fs_info,
 				 struct task_struct *task,
 				 enum proc_hidepid hide_pid_min)
 {
+	const struct cred *cred = current_cred(), *tcred;
+	kuid_t caller_uid;
+	kgid_t caller_gid;
 	/*
-	 * If 'hidpid' mount option is set force a ptrace check,
-	 * we indicate that we are using a filesystem syscall
+	 * If 'hidepid=ptraceable' mount option is set, force a ptrace check.
+	 * We indicate that we are using a filesystem syscall
 	 * by passing PTRACE_MODE_READ_FSCREDS
 	 */
 	if (fs_info->hide_pid == HIDEPID_NOT_PTRACEABLE)
@@ -758,7 +761,25 @@ static bool has_pid_permissions(struct proc_fs_info *fs_info,
 		return true;
 	if (in_group_p(fs_info->pid_gid))
 		return true;
-	return ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+
+	task_lock(task);
+	rcu_read_lock();
+	caller_uid = cred->fsuid;
+	caller_gid = cred->fsgid;
+	tcred = __task_cred(task);
+	if (uid_eq(caller_uid, tcred->euid) &&
+	    uid_eq(caller_uid, tcred->suid) &&
+	    uid_eq(caller_uid, tcred->uid)  &&
+	    gid_eq(caller_gid, tcred->egid) &&
+	    gid_eq(caller_gid, tcred->sgid) &&
+	    gid_eq(caller_gid, tcred->gid)) {
+		rcu_read_unlock();
+		task_unlock(task);
+		return true;
+	}
+	rcu_read_unlock();
+	task_unlock(task);
+	return false;
 }
 
 

---
base-commit: 884a80cc9208ce75831b2376f2b0464018d7f2c4
change-id: 20250718-hidepid_fix-d0743d0540e7

Best regards,
-- 
Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>