Re: [PATCH v8 6/7] xen/serial: harden serial_tx_buffer checks

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 27 Jul 2026, dmukhin@xxxxxxxx wrote:
> From: Denis Mukhin <dmukhin@xxxxxxxx> 
> 
> Ensure the user-defined value never crosses 2GB boundary and always
> rounded to the next power of 2 to align logic with console driver
> conring buffer management code.
> 
> Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
> ---
> Changes since v7:
> - addressed Jan's feedback:
>   
> https://lore.kernel.org/xen-devel/89029dbd-df1f-45d4-8a02-720cd6a42cab@xxxxxxxx/
> - kept only check for large buffer in serial_async_transmit()
>   and a doc update.
> ---
>  docs/misc/xen-command-line.pandoc | 2 ++
>  xen/drivers/char/serial.c         | 2 ++
>  2 files changed, 4 insertions(+)
> 
> diff --git a/docs/misc/xen-command-line.pandoc 
> b/docs/misc/xen-command-line.pandoc
> index 1c711fa98086..2be8772b329a 100644
> --- a/docs/misc/xen-command-line.pandoc
> +++ b/docs/misc/xen-command-line.pandoc
> @@ -2396,6 +2396,8 @@ accidentally leaking secrets by releasing pages without 
> proper sanitization.
>  
>  Set the serial transmit buffer size.
>  
> +The value provided will be rounded down to the nearest power of 2.
> +
>  ### serrors (ARM)
>  > `= diverse | panic`
>  
> diff --git a/xen/drivers/char/serial.c b/xen/drivers/char/serial.c
> index cf0abf1893e5..ba1647309ab8 100644
> --- a/xen/drivers/char/serial.c
> +++ b/xen/drivers/char/serial.c
> @@ -523,6 +523,8 @@ void __init serial_async_transmit(struct serial_port 
> *port)
>          return;
>      if ( serial_txbufsz < PAGE_SIZE )
>          serial_txbufsz = PAGE_SIZE;
> +    if ( serial_txbufsz > GB(2) )
> +        serial_txbufsz = CONFIG_SERIAL_TX_BUFSIZE;
>      while ( serial_txbufsz & (serial_txbufsz - 1) )
>          serial_txbufsz &= serial_txbufsz - 1;

My understanding of this loop is that, given that serial_txbufsz is
unsigned int, it is already clamping it to 2GB max


>      port->txbuf = xvmalloc_array(char, serial_txbufsz);
> -- 
> 2.54.0
>