Re: [PATCH v2 4/8] qemu: Validate firmware blob configuration
On 6/9/20 11:52 AM, Daniel P. Berrangé wrote:
On Thu, Jun 04, 2020 at 08:44:05PM +0200, Michal Privoznik wrote:
> There are recommendations and limitations to the name of the
> config blobs we need to follow [1].
>
> Firstly, we don't want users to change any value only add new
> blobs. This means, that the name must have "opt/" prefix and at
> the same time must not begin with "opt/ovmf" nor "opt/org.qemu"
> as these are reserved for OVMF or QEMU respectively.
>
> Secondly, there is a limit (FW_CFG_MAX_FILE_PATH in qemu.git) of
> 56 characters for filename.
Ewww, that is horrible. I'm have inclined to say we should leave the
limit unchecked in libvirt, and file a BZ against QEMU. It should be
using g_strdup_printf() with filenames and not allocating on the
stack. We already see peoiple exceeding the 100 charater limit of UNIX
sockets, so a 56 character limit is going to be trivially exceeded
without even trying hard.
Ah, I got it wrong when reading the documentation. The limit is the name
of the entry, not file name associated. For instance the following works:
-fw_cfg
name=opt/com.example/blah,file=/tmp/some_very_long_path_that_is_more_than_fifty_six_characters_long_to_see_what_happens_if_I_do_that
but if @name would be too long QEMU would fail:
qemu-system-x86_64: -fw_cfg
name=opt/com.example/some_very_long_...,file=...: name too long (max. 55
char)
And this comes from kernel's implementation of qemu_fw_cfg. However, the
name can be up to PAGE_SIZE long (minus 2 for terminating \n and NUL),
according to sysfs documentation (Documentation/filesystems/sysfs.rst:241):
- The buffer will always be PAGE_SIZE bytes in length. On i386, this
is 4096.
Given this new realization, I think I'll just remove the check and not
fill bug. I don't think we need longer names, do we?
Michal