[PATCH v7 6/7] xen/serial: harden serial_tx_buffer checks

[dmukhin@xxxxxxxx]

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 with console driver conring
buffer management code.

Signed-off-by: Denis Mukhin <dmukhin@xxxxxxxx>
---
Changes since v6:
- new patch
---
 docs/misc/xen-command-line.pandoc |  2 ++
 xen/drivers/char/serial.c         | 21 ++++++++++++++++++++-
 2 files changed, 22 insertions(+), 1 deletion(-)

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 e3c356408987..25d0dbf91ae9 100644
--- a/xen/drivers/char/serial.c
+++ b/xen/drivers/char/serial.c
@@ -16,7 +16,26 @@
 #include <asm/processor.h>
 
 unsigned int __ro_after_init serial_txbufsz = CONFIG_SERIAL_TX_BUFSIZE;
-size_param("serial_tx_buffer", serial_txbufsz);
+
+static int __init cf_check parse_serial_txbufsz(const char *s)
+{
+    long long val;
+
+    if ( parse_signed_integer("serial_tx_buffer", s, NULL, &val) )
+        return 0;
+
+    if ( val < KB(16) || val > GB(2) )
+    {
+        printk(XENLOG_WARNING "'serial_tx_buffer=%lld' value out of range, "
+               "falling back to default\n", val);
+        val = CONFIG_SERIAL_TX_BUFSIZE;
+    }
+
+    serial_txbufsz = PAGE_SIZE << get_order_from_bytes(val);
+
+    return 0;
+}
+custom_param("serial_tx_buffer", parse_serial_txbufsz);
 
 #define mask_serial_rxbuf_idx(_i) ((_i)&(serial_rxbufsz-1))
 #define mask_serial_txbuf_idx(_i) ((_i)&(serial_txbufsz-1))
-- 
2.54.0