Re: [PATCH v2] fs: exec: apply CLOEXEC before changing dumpable task flags

[Eric W. Biederman]

Aleksa Sarai <asarai@xxxxxxx> writes:

> If you have a process that has set itself to be non-dumpable, and it
> then undergoes exec(2), any CLOEXEC file descriptors it has open are
> "exposed" during a race window between the dumpable flags of the process
> being reset for exec(2) and CLOEXEC being applied to the file
> descriptors. This can be exploited by a process by attempting to access
> /proc/<pid>/fd/... during this window, without requiring CAP_SYS_PTRACE.
>
> The race in question is after set_dumpable has been (for get_link,
> though the trace is basically the same for readlink):
>
> [vfs]
> -> proc_pid_link_inode_operations.get_link
>    -> proc_pid_get_link
>       -> proc_fd_access_allowed
>          -> ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
>
> Which will return 0, during the race window and CLOEXEC file descriptors
> will still be open during this window because do_close_on_exec has not
> been called yet. As a result, the ordering of these calls should be
> reversed to avoid this race window.
>
> This is of particular concern to container runtimes, where joining a
> PID namespace with file descriptors referring to the host filesystem
> can result in security issues (since PRCTL_SET_DUMPABLE doesn't protect
> against access of CLOEXEC file descriptors -- file descriptors which may
> reference filesystem objects the container shouldn't have access to).

That seems reasonable.  I was thinking cred_guard_mutex should handle
this case, but it obviously won't because only ptrace_attach takes that.
Sigh with enough cleanups the code might even become comprehensible and
correct in there.

I have dropped this onto my for-testing branch for now (so I don't
forget it) and after the chaos of the merge window ends I will forward
this along.

Eric


> Cc: dev@xxxxxxxxxxxxxxxxxx
> Cc: <stable@xxxxxxxxxxxxxxx> # v3.2+
> Reported-by: Michael Crosby <crosbymichael@xxxxxxxxx>
> Signed-off-by: Aleksa Sarai <asarai@xxxxxxx>
> ---
>  fs/exec.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index 4e497b9ee71e..b0a98ef03253 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -19,7 +19,7 @@
>   * current->executable is only used by the procfs.  This allows a dispatch
>   * table to check for several different types  of binary formats.  We keep
>   * trying until we recognize the file or we run out of supported binary
> - * formats. 
> + * formats.
>   */
>  
>  #include <linux/slab.h>
> @@ -1266,6 +1266,13 @@ int flush_old_exec(struct linux_binprm * bprm)
>  	flush_thread();
>  	current->personality &= ~bprm->per_clear;
>  
> +	/*
> +	 * We have to apply CLOEXEC before we change whether the process is
> +	 * dumpable (in setup_new_exec) to avoid a race with a process in userspace
> +	 * trying to access the should-be-closed file descriptors of a process
> +	 * undergoing exec(2).
> +	 */
> +	do_close_on_exec(current->files);
>  	return 0;
>  
>  out:
> @@ -1315,7 +1322,6 @@ void setup_new_exec(struct linux_binprm * bprm)
>  	   group */
>  	current->self_exec_id++;
>  	flush_signal_handlers(current, 0);
> -	do_close_on_exec(current->files);
>  }
>  EXPORT_SYMBOL(setup_new_exec);