[PATCH 4.4 04/54] proc: Check /proc/$pid/attr/ writes against file opener

[Greg Kroah-Hartman]

From: Kees Cook <keescook@xxxxxxxxxxxx>

commit bfb819ea20ce8bbeeba17e1a6418bf8bda91fc28 upstream.

Fix another "confused deputy" weakness[1]. Writes to /proc/$pid/attr/
files need to check the opener credentials, since these fds do not
transition state across execve(). Without this, it is possible to
trick another process (which may have different credentials) to write
to its own /proc/$pid/attr/ files, leading to unexpected and possibly
exploitable behaviors.

[1] https://www.kernel.org/doc/html/latest/security/credentials.html?highlight=confused#open-file-credentials

Fixes: 1da177e4c3f41 ("Linux-2.6.12-rc2")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/proc/base.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2413,6 +2413,10 @@ static ssize_t proc_pid_attr_write(struc
 	ssize_t length;
 	struct task_struct *task = get_proc_task(inode);
 
+	/* A task may only write when it was the opener. */
+	if (file->f_cred != current_real_cred())
+		return -EPERM;
+
 	length = -ESRCH;
 	if (!task)
 		goto out_no_task;