[v2,1/2] test_firmware: prevent race conditions by a correct implementation of locking

[ Upstream commit 4acfe3dfde685a5a9eaec5555351918e2d7266a1 ]

Dan Carpenter spotted a race condition in a couple of situations like
these in the test_firmware driver:

static int test_dev_config_update_u8(const char *buf, size_t size, u8 *cfg)
{
        u8 val;
        int ret;

        ret = kstrtou8(buf, 10, &val);
        if (ret)
                return ret;

        mutex_lock(&test_fw_mutex);
        *(u8 *)cfg = val;
        mutex_unlock(&test_fw_mutex);

        /* Always return full write size even if we didn't consume all */
        return size;
}

static ssize_t config_num_requests_store(struct device *dev,
                                         struct device_attribute *attr,
                                         const char *buf, size_t count)
{
        int rc;

        mutex_lock(&test_fw_mutex);
        if (test_fw_config->reqs) {
                pr_err("Must call release_all_firmware prior to changing config\n");
                rc = -EINVAL;
                mutex_unlock(&test_fw_mutex);
                goto out;
        }
        mutex_unlock(&test_fw_mutex);

        // NOTE: HERE is the race!!! Function can be preempted!

        // test_fw_config->reqs can change between the release of
        // the lock about and acquire of the lock in the
        // test_dev_config_update_u8()

        rc = test_dev_config_update_u8(buf, count,
                                       &test_fw_config->num_requests);

out:
        return rc;
}

static ssize_t config_read_fw_idx_store(struct device *dev,
                                        struct device_attribute *attr,
                                        const char *buf, size_t count)
{
        return test_dev_config_update_u8(buf, count,
                                         &test_fw_config->read_fw_idx);
}

The function test_dev_config_update_u8() is called from both the locked
and the unlocked context, function config_num_requests_store() and
config_read_fw_idx_store() which can both be called asynchronously as
they are driver's methods, while test_dev_config_update_u8() and siblings
change their argument pointed to by u8 *cfg or similar pointer.

To avoid deadlock on test_fw_mutex, the lock is dropped before calling
test_dev_config_update_u8() and re-acquired within test_dev_config_update_u8()
itself, but alas this creates a race condition.

Having two locks wouldn't assure a race-proof mutual exclusion.

This situation is best avoided by the introduction of a new, unlocked
function __test_dev_config_update_u8() which can be called from the locked
context and reducing test_dev_config_update_u8() to:

static int test_dev_config_update_u8(const char *buf, size_t size, u8 *cfg)
{
        int ret;

        mutex_lock(&test_fw_mutex);
        ret = __test_dev_config_update_u8(buf, size, cfg);
        mutex_unlock(&test_fw_mutex);

        return ret;
}

doing the locking and calling the unlocked primitive, which enables both
locked and unlocked versions without duplication of code.

Fixes: c92316bf8e948 ("test_firmware: add batched firmware tests")
Cc: Luis R. Rodriguez <mcgrof@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Russ Weight <russell.h.weight@intel.com>
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Tianfei Zhang <tianfei.zhang@intel.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: linux-kselftest@vger.kernel.org
Cc: stable@vger.kernel.org # v5.4, 4.19
Suggested-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/20230509084746.48259-1-mirsad.todorovac@alu.unizg.hr
Signed-off-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>

[ This is the patch to fix the racing condition in locking for the 5.4, ]
[ 4.19 and 4.4 stable branches. Not all the fixes from the upstream     ]
[ commit apply, but those which do are verbatim equal to those in the   ]
[ upstream commit.                                                      ]
---
v2:
 bundled locking and ENOSPC patches together.
 tested on 5.4 and 4.19 stable.

 lib/test_firmware.c | 37 ++++++++++++++++++++++++++++---------
 1 file changed, 28 insertions(+), 9 deletions(-)