Re: [PATCH] xen/hypfs: fix writing of custom parameter

[Jan Beulich <jbeulich@xxxxxxxx>]

On 11.09.2020 07:30, Juergen Gross wrote:
> Today the maximum allowed data length for writing a hypfs node is
> tested in the generic hypfs_write() function. For custom runtime
> parameters this might be wrong, as the maximum allowed size is derived
> from the buffer holding the current setting, while there might be ways
> to set the parameter needing more characters than the minimal
> representation of that value.
> 
> One example for this is the "ept" parameter. Its value buffer is sized
> to be able to hold the string "exec-sp=0" or "exec-sp=1", while it is
> allowed to use e.g. "no-exec-sp" or "exec-sp=yes" for setting it.
> 
> Fix that by moving the length check one level down to the type
> specific write function.
> 
> In order to avoid allocation of arbitrary sized buffers use a new
> MAX_PARAM_SIZE macro as an upper limit for custom writes. The value
> of MAX_PARAM_SIZE is the same as the limit in parse_params() for a
> single parameter.
> 
> Fixes: 5b5ccafb0c42 ("xen: add basic hypervisor filesystem support")

Perhaps rather a659d7cab9af ("xen: add runtime parameter access
support to hypfs"), as that where hypfs_write_custom() got introduced?

> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan