[Xen-devel] [PATCH 28/29] drivers: convert sbd_duart.map_guard from atomic_t to refcount_t

[Elena Reshetova <elena.reshetova@xxxxxxxxx>]

refcount_t type and corresponding API should be
used instead of atomic_t when the variable is used as
a reference counter. This allows to avoid accidental
refcounter overflows that might lead to use-after-free
situations.

Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
Signed-off-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Signed-off-by: David Windsor <dwindsor@xxxxxxxxx>
---
 drivers/tty/serial/sb1250-duart.c | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/drivers/tty/serial/sb1250-duart.c 
b/drivers/tty/serial/sb1250-duart.c
index 771f361..041625c 100644
--- a/drivers/tty/serial/sb1250-duart.c
+++ b/drivers/tty/serial/sb1250-duart.c
@@ -41,7 +41,7 @@
 #include <linux/tty_flip.h>
 #include <linux/types.h>
 
-#include <linux/atomic.h>
+#include <linux/refcount.h>
 #include <asm/io.h>
 #include <asm/war.h>
 
@@ -103,7 +103,7 @@ struct sbd_port {
 struct sbd_duart {
        struct sbd_port         sport[2];
        unsigned long           mapctrl;
-       atomic_t                map_guard;
+       refcount_t              map_guard;
 };
 
 #define to_sport(uport) container_of(uport, struct sbd_port, port)
@@ -654,15 +654,13 @@ static void sbd_release_port(struct uart_port *uport)
 {
        struct sbd_port *sport = to_sport(uport);
        struct sbd_duart *duart = sport->duart;
-       int map_guard;
 
        iounmap(sport->memctrl);
        sport->memctrl = NULL;
        iounmap(uport->membase);
        uport->membase = NULL;
 
-       map_guard = atomic_add_return(-1, &duart->map_guard);
-       if (!map_guard)
+       if(refcount_dec_and_test(&duart->map_guard))
                release_mem_region(duart->mapctrl, DUART_CHANREG_SPACING);
        release_mem_region(uport->mapbase, DUART_CHANREG_SPACING);
 }
@@ -698,7 +696,6 @@ static int sbd_request_port(struct uart_port *uport)
 {
        const char *err = KERN_ERR "sbd: Unable to reserve MMIO resource\n";
        struct sbd_duart *duart = to_sport(uport)->duart;
-       int map_guard;
        int ret = 0;
 
        if (!request_mem_region(uport->mapbase, DUART_CHANREG_SPACING,
@@ -706,11 +703,11 @@ static int sbd_request_port(struct uart_port *uport)
                printk(err);
                return -EBUSY;
        }
-       map_guard = atomic_add_return(1, &duart->map_guard);
-       if (map_guard == 1) {
+       refcount_inc(&duart->map_guard);
+       if (refcount_read(&duart->map_guard) == 1) {
                if (!request_mem_region(duart->mapctrl, DUART_CHANREG_SPACING,
                                        "sb1250-duart")) {
-                       atomic_add(-1, &duart->map_guard);
+                       refcount_dec(&duart->map_guard);
                        printk(err);
                        ret = -EBUSY;
                }
@@ -718,8 +715,7 @@ static int sbd_request_port(struct uart_port *uport)
        if (!ret) {
                ret = sbd_map_port(uport);
                if (ret) {
-                       map_guard = atomic_add_return(-1, &duart->map_guard);
-                       if (!map_guard)
+                       if (refcount_dec_and_test(&duart->map_guard))
                                release_mem_region(duart->mapctrl,
                                                   DUART_CHANREG_SPACING);
                }
-- 
2.7.4


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