Re: [PATCH v2 2/2] block/null: Enable 'read-zeroes' mode by default

[Stefan Hajnoczi]

On Thu, Feb 11, 2021 at 03:26:56PM +0100, Philippe Mathieu-Daudé wrote:
> The null-co driver is meant for (performance) testing.
> By default, read operation does nothing, the provided buffer
> is not filled with zero values and its content is unchanged.
> 
> This performance 'feature' becomes an issue from a security
> perspective.  For example, using the default null-co driver,
> buf[] is uninitialized, the blk_pread() call succeeds and we
> then access uninitialized memory:
> 
>   static int guess_disk_lchs(BlockBackend *blk,
>                              int *pcylinders, int *pheads,
>                              int *psectors)
>   {
>       uint8_t buf[BDRV_SECTOR_SIZE];
>       ...
> 
>       if (blk_pread(blk, 0, buf, BDRV_SECTOR_SIZE) < 0) {
>           return -1;
>       }
>       /* test msdos magic */
>       if (buf[510] != 0x55 || buf[511] != 0xaa) {
>           return -1;
>       }
> 
> We could audit all the uninitialized buffers and the
> bdrv_co_preadv() handlers, but it is simpler to change the
> default of this testing driver. Performance tests will have
> to adapt and use 'null-co,read-zeroes=off'.
> 
> Suggested-by: Max Reitz <mreitz@redhat.com>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  block/null.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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