Re: [PATCH] binder: prevent UAF read in print_binder_transaction_log_entry()

[Joel Fernandes]

On Tue, Oct 08, 2019 at 03:01:59PM +0200, Christian Brauner wrote:
> When a binder transaction is initiated on a binder device coming from a
> binderfs instance, a pointer to the name of the binder device is stashed
> in the binder_transaction_log_entry's context_name member. Later on it
> is used to print the name in print_binder_transaction_log_entry(). By
> the time print_binder_transaction_log_entry() accesses context_name
> binderfs_evict_inode() might have already freed the associated memory
> thereby causing a UAF. Do the simple thing and prevent this by copying
> the name of the binder device instead of stashing a pointer to it.
> 
> Reported-by: Jann Horn <jannh@xxxxxxxxxx>
> Fixes: 03e2e07e3814 ("binder: Make transaction_log available in binderfs")
> Link: https://lore.kernel.org/r/CAG48ez14Q0-F8LqsvcNbyR2o6gPW8SHXsm4u5jmD9MpsteM2Tw@xxxxxxxxxxxxxx
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Todd Kjos <tkjos@xxxxxxxxxxx>
> Cc: Hridya Valsaraju <hridya@xxxxxxxxxx>
> Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
> ---
>  drivers/android/binder.c          | 4 +++-
>  drivers/android/binder_internal.h | 2 +-
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/android/binder.c b/drivers/android/binder.c
> index c0a491277aca..5b9ac2122e89 100644
> --- a/drivers/android/binder.c
> +++ b/drivers/android/binder.c
> @@ -57,6 +57,7 @@
>  #include <linux/sched/signal.h>
>  #include <linux/sched/mm.h>
>  #include <linux/seq_file.h>
> +#include <linux/string.h>
>  #include <linux/uaccess.h>
>  #include <linux/pid_namespace.h>
>  #include <linux/security.h>
> @@ -66,6 +67,7 @@
>  #include <linux/task_work.h>
>  
>  #include <uapi/linux/android/binder.h>
> +#include <uapi/linux/android/binderfs.h>
>  
>  #include <asm/cacheflush.h>
>  
> @@ -2876,7 +2878,7 @@ static void binder_transaction(struct binder_proc *proc,
>  	e->target_handle = tr->target.handle;
>  	e->data_size = tr->data_size;
>  	e->offsets_size = tr->offsets_size;
> -	e->context_name = proc->context->name;
> +	strscpy(e->context_name, proc->context->name, BINDERFS_MAX_NAME);

Strictly speaking, proc-context->name can also be initialized for !BINDERFS
so the BINDERFS in the MAX_NAME macro is misleading. So probably there should
be a BINDER_MAX_NAME (and associated checks for whether non BINDERFS names
fit within the MAX.

>  	if (reply) {

>  		binder_inner_proc_lock(proc);
> diff --git a/drivers/android/binder_internal.h b/drivers/android/binder_internal.h
> index bd47f7f72075..ae991097d14d 100644
> --- a/drivers/android/binder_internal.h
> +++ b/drivers/android/binder_internal.h
> @@ -130,7 +130,7 @@ struct binder_transaction_log_entry {
>  	int return_error_line;
>  	uint32_t return_error;
>  	uint32_t return_error_param;
> -	const char *context_name;
> +	char context_name[BINDERFS_MAX_NAME + 1];

Same comment here, context_name can be used for non-BINDERFS transactions as
well such as default binder devices.

One more thought, this can be made dependent on CONFIG_BINDERFS since regular
binder devices cannot be unregistered AFAICS and as Jann said, the problem is
BINDERFS specific. That way we avoid the memcpy for _every_ transaction.
These can be thundering when Android starts up.

(I secretly wish C strings could be refcounted to avoid exactly this issue,
that should not be hard to develop but I am not sure if it is worth it for
this problem :) - For one, it will avoid having to do the strcpy for _every_
transaction).

Other than these nits, please add my tag on whichever is the final solution:

Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

thanks,

 - Joel


>  };
>  
>  struct binder_transaction_log {
> -- 
> 2.23.0
>