Re: [PATCH 3/3] firmware: add mutex fw_lock_fallback for race condition

[Luis Chamberlain <mcgrof@kernel.org>]

On Thu, Aug 15, 2019 at 05:09:45PM -0700, Scott Branden wrote:
> A race condition exists between _request_firmware_prepare checking
> if firmware is assigned and firmware_fallback_sysfs creating a sysfs
> entry (kernel trace below).  To avoid such condition add a mutex
> fw_lock_fallback to protect against such condition.

I am not buying this fix, and it seems sloppy. More below.

> misc test_firmware: Falling back to sysfs fallback for: nope-test-firmware.bin

So the fallback kicks in with the file that is not there.

> sysfs: cannot create duplicate filename '/devices/virtual/misc/test_firmware/nope-test-firmware.bin'

And we have a duplicate entry, for the *device* created to allow us to
create a file entry to allow us to copy the file. Your tests had a loop,
so there is actually a race between two entries being created while
one one failed.

> CPU: 4 PID: 2059 Comm: test_firmware-3 Not tainted 5.3.0-rc4 #1
> Hardware name: Dell Inc. OptiPlex 7010/0KRC95, BIOS A13 03/25/2013
> Call Trace:
>  dump_stack+0x67/0x90
>  sysfs_warn_dup.cold+0x17/0x24
>  sysfs_create_dir_ns+0xb3/0xd0
>  kobject_add_internal+0xa6/0x2a0
>  kobject_add+0x7e/0xb0

Note: kobject_add().

>  ? _cond_resched+0x15/0x30
>  device_add+0x121/0x670
>  firmware_fallback_sysfs+0x15c/0x3c9
>  _request_firmware+0x432/0x5a0
>  ? devres_find+0x63/0xc0
>  request_firmware_into_buf+0x63/0x80
>  test_fw_run_batch_request+0x96/0xe0
>  kthread+0xfb/0x130
>  ? reset_store+0x30/0x30
>  ? kthread_park+0x80/0x80
>  ret_from_fork+0x3a/0x50
> kobject_add_internal failed for nope-test-firmware.bin with -EEXIST, don't try to register things with the same name in the same directory.

So above it makes it even clearer, two kobjets with the same name.

> Signed-off-by: Scott Branden <scott.branden@broadcom.com>
> ---
>  drivers/base/firmware_loader/main.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/drivers/base/firmware_loader/main.c b/drivers/base/firmware_loader/main.c
> index bf44c79beae9..ce9896e3b782 100644
> --- a/drivers/base/firmware_loader/main.c
> +++ b/drivers/base/firmware_loader/main.c
> @@ -88,6 +88,7 @@ static inline struct fw_priv *to_fw_priv(struct kref *ref)
>  /* fw_lock could be moved to 'struct fw_sysfs' but since it is just
>   * guarding for corner cases a global lock should be OK */
>  DEFINE_MUTEX(fw_lock);
> +DEFINE_MUTEX(fw_lock_fallback);

The reason I don't like this fix is that this mutex is named after ther
fallback interface... but...

>  
>  static struct firmware_cache fw_cache;
>  
> @@ -758,6 +759,17 @@ _request_firmware(const struct firmware **firmware_p, const char *name,
>  	if (!firmware_p)
>  		return -EINVAL;
>  
> +	/*
> +	 * There is a race condition between _request_firmware_prepare checking
> +	 * if firmware is assigned and firmware_fallback_sysfs creating sysfs
> +	 * entries with duplicate names.
> +	 * Yet, with this lock the firmware_test locks up with cache enabled
> +	 * and no event used during firmware test.
> +	 * This points to some very racy code I don't know how to entirely fix.
> +	 */
> +	if (opt_flags & FW_OPT_NOCACHE)
> +		mutex_lock(&fw_lock_fallback);

Whoa.. What does no-cache have anything to do with the fallback interface
other than the fact we enable this feature for the fallback interface?
We don't need to penalize non-fallback users who *also* may want to
enable the no-cache feature.

So, the fix should be within the boundaries of the creation / deletion
of the kobject, not this nocache feature. Can you please re-evaluate
this code and look for a more compartamentalized solution to the
fallback code only?

  Luis