Re: Patch "fs: exec: apply CLOEXEC before changing dumpable task flags" has been added to the 4.4-stable tree

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Jan 04, 2017 at 11:06:09AM +0100, gregkh@linuxfoundation.org wrote:
> 
> This is a note to let you know that I've just added the patch titled
> 
>     fs: exec: apply CLOEXEC before changing dumpable task flags
> 
> to the 4.4-stable tree which can be found at:
>     http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> 
> The filename of the patch is:
>      fs-exec-apply-cloexec-before-changing-dumpable-task-flags.patch
> and it can be found in the queue-4.4 subdirectory.
> 
> If you, or anyone else, feels it should not be added to the stable tree,
> please let <stable@vger.kernel.org> know about it.

Oops, no, this version breaks the build, I had to drop it...

greg k-h



> 
> 
> >From 613cc2b6f272c1a8ad33aefa21cad77af23139f7 Mon Sep 17 00:00:00 2001
> From: Aleksa Sarai <asarai@suse.de>
> Date: Wed, 21 Dec 2016 16:26:24 +1100
> Subject: fs: exec: apply CLOEXEC before changing dumpable task flags
> 
> From: Aleksa Sarai <asarai@suse.de>
> 
> commit 613cc2b6f272c1a8ad33aefa21cad77af23139f7 upstream.
> 
> 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).
> 
> Cc: dev@opencontainers.org
> Reported-by: Michael Crosby <crosbymichael@gmail.com>
> Signed-off-by: Aleksa Sarai <asarai@suse.de>
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> 
> ---
>  fs/exec.c |   10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> --- 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>
> @@ -1114,6 +1114,13 @@ int flush_old_exec(struct linux_binprm *
>  	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:
> @@ -1176,7 +1183,6 @@ void setup_new_exec(struct linux_binprm
>  	   group */
>  	current->self_exec_id++;
>  	flush_signal_handlers(current, 0);
> -	do_close_on_exec(current->files);
>  }
>  EXPORT_SYMBOL(setup_new_exec);
>  
> 
> 
> Patches currently in stable-queue which might be from asarai@suse.de are
> 
> queue-4.4/fs-exec-apply-cloexec-before-changing-dumpable-task-flags.patch
> --
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