linux-kernel - Re: [PATCH v15 08/11] fw_cfg: handle fw_cfg_read

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Message-ID: <CAJ+F1CKwBjRDTxToDwOrPnnaiMHvF6EgK=dgSC-FW7n6UMDeKA@mail.gmail.com>
Date:   Wed, 28 Feb 2018 12:49:35 +0100
From:   Marc-André Lureau <marcandre.lureau@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     open list <linux-kernel@...r.kernel.org>, slp@...hat.com,
        bhe@...hat.com, somlo@....edu, xiaolong.ye@...el.com
Subject: Re: [PATCH v15 08/11] fw_cfg: handle fw_cfg_read_blob() error

Hi

On Tue, Feb 27, 2018 at 1:20 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Thu, Feb 15, 2018 at 10:33:09PM +0100, Marc-André Lureau wrote:
>> fw_cfg_read_blob() may fail, but does not return error. This may lead
>> to undefined behaviours, such as a memcmp(sig, "QEMU") on uninitilized
>> memory.
>
> I don't think that's true - there's a memset there that
> will initialize the memory. probe is likely the only
> case where it returns a slightly incorrect data.

Right, I'll update the commit message.

>> Return an error if ACPI locking failed. Also, the following
>> DMA read/write extension will add more error paths that should be
>> handled appropriately.
>>
>> Signed-off-by: Marc-André Lureau <marcandre.lureau@...hat.com>
>> ---
>>  drivers/firmware/qemu_fw_cfg.c | 32 ++++++++++++++++++++++----------
>>  1 file changed, 22 insertions(+), 10 deletions(-)
>>
>> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
>> index f6f90bef604c..5e6e5ac71dab 100644
>> --- a/drivers/firmware/qemu_fw_cfg.c
>> +++ b/drivers/firmware/qemu_fw_cfg.c
>> @@ -59,8 +59,8 @@ static void fw_cfg_sel_endianness(u16 key)
>>  }
>>
>>  /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
>> -static void fw_cfg_read_blob(u16 key,
>> -                     void *buf, loff_t pos, size_t count)
>> +static ssize_t fw_cfg_read_blob(u16 key,
>> +                             void *buf, loff_t pos, size_t count)
>>  {
>>       u32 glk = -1U;
>>       acpi_status status;
>> @@ -73,7 +73,7 @@ static void fw_cfg_read_blob(u16 key,
>>               /* Should never get here */
>>               WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
>>               memset(buf, 0, count);
>> -             return;
>> +             return -EINVAL;
>>       }
>>
>>       mutex_lock(&fw_cfg_dev_lock);
>
> Wouldn't something like -EBUSY be more appropriate?

In theory, it would be a general failure right? I don't think we want
the caller to retry. I think in EINVAL fits better, but I don't think
it matters much this or EBUSY.

>> @@ -84,6 +84,7 @@ static void fw_cfg_read_blob(u16 key,
>>       mutex_unlock(&fw_cfg_dev_lock);
>>
>>       acpi_release_global_lock(glk);
>> +     return count;
>>  }
>>
>>  /* clean up fw_cfg device i/o */
>> @@ -165,8 +166,9 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
>>       }
>>
>>       /* verify fw_cfg device signature */
>> -     fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
>> -     if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>> +     if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
>> +                             0, FW_CFG_SIG_SIZE) < 0 ||
>> +             memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>>               fw_cfg_io_cleanup();
>>               return -ENODEV;
>>       }
>> @@ -326,8 +328,7 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>>       if (count > entry->size - pos)
>>               count = entry->size - pos;
>>
>> -     fw_cfg_read_blob(entry->select, buf, pos, count);
>> -     return count;
>> +     return fw_cfg_read_blob(entry->select, buf, pos, count);
>>  }
>>
>>  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
>> @@ -483,7 +484,11 @@ static int fw_cfg_register_dir_entries(void)
>>       struct fw_cfg_file *dir;
>>       size_t dir_size;
>>
>> -     fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count, 0, sizeof(files_count));
>> +     ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count,
>> +                     0, sizeof(files_count));
>> +     if (ret < 0)
>> +             return ret;
>> +
>>       count = be32_to_cpu(files_count);
>>       dir_size = count * sizeof(struct fw_cfg_file);
>>
>> @@ -491,7 +496,10 @@ static int fw_cfg_register_dir_entries(void)
>>       if (!dir)
>>               return -ENOMEM;
>>
>> -     fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files_count), dir_size);
>> +     ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
>> +                     sizeof(files_count), dir_size);
>> +     if (ret < 0)
>> +             goto end;
>>
>>       for (i = 0; i < count; i++) {
>>               ret = fw_cfg_register_file(&dir[i]);
>> @@ -499,6 +507,7 @@ static int fw_cfg_register_dir_entries(void)
>>                       break;
>>       }
>>
>> +end:
>>       kfree(dir);
>>       return ret;
>>  }
>> @@ -539,7 +548,10 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
>>               goto err_probe;
>>
>>       /* get revision number, add matching top-level attribute */
>> -     fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
>> +     err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
>> +     if (err < 0)
>> +             goto err_probe;
>> +
>>       fw_cfg_rev = le32_to_cpu(rev);
>>       err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
>>       if (err)
>
> I think that this is the only case where it's not doing the right thing right now in
> that it shows 0 as the revision to the users.  Is it worth failing probe
> here?  We could just skip the attribute, could we not?

I think it's best to fail the probe if we have a read failure at that time.

-- 
Marc-André Lureau