Jann Horn <jannh@xxxxxxxxxx> writes: > On Tue, May 25, 2021 at 9:37 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> 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> >> --- >> fs/proc/base.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/fs/proc/base.c b/fs/proc/base.c >> index 3851bfcdba56..58bbf334265b 100644 >> --- a/fs/proc/base.c >> +++ b/fs/proc/base.c >> @@ -2703,6 +2703,10 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf, >> void *page; >> int rv; >> >> + /* A task may only write when it was the opener. */ >> + if (file->f_cred != current_real_cred()) >> + return -EPERM; > > With this, if a task forks, the child will then still be able to open > its parent's /proc/$pid/attr/current and trick the parent into writing > to that, right? Is that acceptable? If not, the ->open handler should > probably also check for "current->thread_pid == proc_pid(inode)", or > something like that? Currently exec always allocates a new cred. So you can only ``trick'' another process that was forked from you. I don't think it counts as tricking or any kind of danger if you are simply confusing yourself. Eric
