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Date:   Wed, 14 Feb 2018 19:12:03 +0200
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Marc-Andre Lureau <mlureau@redhat.com>
Cc:     =?iso-8859-1?Q?Marc-Andr=E9?= Lureau 
        <marcandre.lureau@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Baoquan He <bhe@redhat.com>,
        Sergio Lopez Pascual <slp@redhat.com>,
        "Somlo, Gabriel" <somlo@cmu.edu>, xiaolong.ye@intel.com
Subject: Re: [PATCH v14 9/9] RFC: fw_cfg: do DMA read operation
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On Wed, Feb 14, 2018 at 06:08:48PM +0100, Marc-Andre Lureau wrote:
> Hi
> 
> On Wed, Feb 14, 2018 at 5:59 PM, Michael S. Tsirkin <mst@redhat.com> wrote:
> > On Wed, Feb 14, 2018 at 05:52:10PM +0100, Marc-Andre Lureau wrote:
> >> >> @@ -282,8 +320,9 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
> >> >>  #endif
> >> >>
> >> >>       /* 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, false) < 0 ||
> >> >> +             memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
> >> >>               fw_cfg_io_cleanup();
> >> >>               return -ENODEV;
> >> >>       }
> >> >
> >> > Rather than add dead code, how about a promise not to
> >> > fail if dma is disabled? Patch will be smaller then.
> >>
> >> Even with dma disabled, you could have a locking bug which was
> >> silently ignored before and is now taken into account.
> >
> > I see. I'd start with a patch reporting errors to users then.
> > That would be a bugfix and can be merged for this version.
> 
> silently is the wrong word for it, there is already a WARN().

it's not visible to userspace though.

> However,
> the memcmp was done on uninitialzed sig[] (which likely resulted in
> returning -ENODEV in the function)

Oh, we have an access to an uninitialized memory too. Fun!

> Now it can check if the function failed to read before doing the memcmp.
> 
> Hope that clarifies it.

I think I get it. I'd make it a bugfix patch as patch 7 in the series.
(patch 1-6 are applied in my tree, will merge in this window if they do not blow up).

> >
> >> >
> >> >> @@ -466,8 +505,8 @@ 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;
> >> >> +     /* do not use DMA, virt_to_phys(buf) might not be ok */
> >> >> +     return fw_cfg_read_blob(entry->select, buf, pos, count, false);
> >> >>  }
> >> >>
> >> >>  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> >> >> @@ -632,7 +671,12 @@ 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), false);
> >> >> +     if (ret < 0) {
> >> >> +             return ret;
> >> >> +     }
> >> >> +
> >> >>       count = be32_to_cpu(files.count);
> >> >>       dir_size = count * sizeof(struct fw_cfg_file);
> >> >>
> >> >> @@ -640,7 +684,11 @@ 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, false);
> >> >> +     if (ret < 0) {
> >> >> +             goto end;
> >> >> +     }
> >> >>
> >> >>       for (i = 0; i < count; i++) {
> >> >>               ret = fw_cfg_register_file(&dir[i]);
> >> >> @@ -648,6 +696,7 @@ static int fw_cfg_register_dir_entries(void)
> >> >>                       break;
> >> >>       }
> >> >>
> >> >> +end:
> >> >>       kfree(dir);
> >> >>       return ret;
> >> >>  }
> >> >> @@ -688,7 +737,10 @@ 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), false);
> >> >> +     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)
> >> >> --
> >> >> 2.16.1.73.g5832b7e9f2