proc: protect ptrace_may_access() with exec_update_lock (part 1)

Fix the easy cases where procfs currently calls ptrace_may_access() without
exec_update_lock protection, where the fix is to simply add the extra lock
or use mm_access():

 - do_task_stat(): grab exec_update_lock
 - proc_pid_wchan(): grab exec_update_lock
 - proc_map_files_lookup(): use mm_access() instead of get_task_mm()
 - proc_map_files_readdir(): use mm_access() instead of get_task_mm()
 - proc_ns_get_link(): grab exec_update_lock
 - proc_ns_readlink(): grab exec_update_lock

Fixes: f83ce3e6b0 ("proc: avoid information leaks to non-privileged processes")
Cc: stable@vger.kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Link: https://patch.msgid.link/20260518-procfs-lockfix-part1-v1-1-5c3d20e0ac33@google.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/proc/array.c

@@ -482,6 +482,11 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	unsigned long flags;
 	int exit_code = task->exit_code;
 	struct signal_struct *sig = task->signal;
+	int ret;
+
+	ret = down_read_killable(&task->signal->exec_update_lock);
+	if (ret)
+		return ret;
 
 	state = *get_task_state(task);
 	vsize = eip = esp = 0;
@@ -657,6 +662,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 		seq_puts(m, " 0");
 
 	seq_putc(m, '\n');
+	up_read(&task->signal->exec_update_lock);
 	if (mm)
 		mmput(mm);
 	return 0;

fs/proc/base.c

@@ -423,18 +423,24 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
 {
 	unsigned long wchan;
 	char symname[KSYM_NAME_LEN];
+	int err;
 
+	err = down_read_killable(&task->signal->exec_update_lock);
+	if (err)
+		return err;
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
 		goto print0;
 
 	wchan = get_wchan(task);
 	if (wchan && !lookup_symbol_name(wchan, symname)) {
 		seq_puts(m, symname);
+		up_read(&task->signal->exec_update_lock);
 		return 0;
 	}
 
 print0:
 	seq_putc(m, '0');
+	up_read(&task->signal->exec_update_lock);
 	return 0;
 }
 #endif /* CONFIG_KALLSYMS */
@@ -2360,17 +2366,15 @@ static struct dentry *proc_map_files_lookup(struct inode *dir,
 	if (!task)
 		goto out;
 
-	result = ERR_PTR(-EACCES);
-	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
-		goto out_put_task;
-
 	result = ERR_PTR(-ENOENT);
 	if (dname_to_vma_addr(dentry, &vm_start, &vm_end))
 		goto out_put_task;
 
-	mm = get_task_mm(task);
+	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
-	if (!mm)
+	if (IS_ERR(mm)) {
+		result = ERR_CAST(mm);
 		goto out_put_task;
+	}
 
 	result = ERR_PTR(-EINTR);
 	if (mmap_read_lock_killable(mm))
@@ -2420,24 +2424,23 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 	if (!task)
 		goto out;
 
-	ret = -EACCES;
-	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
-		goto out_put_task;
-
 	ret = 0;
 	if (!dir_emit_dots(file, ctx))
 		goto out_put_task;
 
-	mm = get_task_mm(task);
+	mm = mm_access(task, PTRACE_MODE_READ_FSCREDS);
-	if (!mm)
+	if (IS_ERR(mm)) {
-		goto out_put_task;
+		ret = PTR_ERR(mm);
-
+		/* if the task has no mm, the directory should just be empty */
-	ret = mmap_read_lock_killable(mm);
+		if (ret == -ESRCH)
-	if (ret) {
+			ret = 0;
-		mmput(mm);
 		goto out_put_task;
 	}
 
+	ret = mmap_read_lock_killable(mm);
+	if (ret)
+		goto out_put_mm;
+
 	nr_files = 0;
 
 	/*
@@ -2462,8 +2465,7 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 		if (!p) {
 			ret = -ENOMEM;
 			mmap_read_unlock(mm);
-			mmput(mm);
+			goto out_put_mm;
-			goto out_put_task;
 		}
 
 		p->start = vma->vm_start;
@@ -2471,7 +2473,6 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 		p->mode = vma->vm_file->f_mode;
 	}
 	mmap_read_unlock(mm);
-	mmput(mm);
 
 	for (i = 0; i < nr_files; i++) {
 		char buf[4 * sizeof(long) + 2];	/* max: %lx-%lx\0 */
@@ -2488,6 +2489,8 @@ proc_map_files_readdir(struct file *file, struct dir_context *ctx)
 		ctx->pos++;
 	}
 
+out_put_mm:
+	mmput(mm);
 out_put_task:
 	put_task_struct(task);
 out:

fs/proc/namespaces.c

@@ -55,6 +55,10 @@ static const char *proc_ns_get_link(struct dentry *dentry,
 	if (!task)
 		return ERR_PTR(-EACCES);
 
+	error = down_read_killable(&task->signal->exec_update_lock);
+	if (error)
+		goto out_put_task;
+
 	if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS))
 		goto out;
 
@@ -64,6 +68,8 @@ static const char *proc_ns_get_link(struct dentry *dentry,
 
 	error = nd_jump_link(&ns_path);
 out:
+	up_read(&task->signal->exec_update_lock);
+out_put_task:
 	put_task_struct(task);
 	return ERR_PTR(error);
 }
@@ -80,11 +86,17 @@ static int proc_ns_readlink(struct dentry *dentry, char __user *buffer, int bufl
 	if (!task)
 		return res;
 
+	res = down_read_killable(&task->signal->exec_update_lock);
+	if (res)
+		goto out_put_task;
+
 	if (ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
 		res = ns_get_name(name, sizeof(name), task, ns_ops);
 		if (res >= 0)
 			res = readlink_copy(buffer, buflen, name, strlen(name));
 	}
+	up_read(&task->signal->exec_update_lock);
+out_put_task:
 	put_task_struct(task);
 	return res;
 }