[PATCH v4] net/af_unix: don't create a path for a bound socket

[PATCH v4] net/af_unix: don't create a path for a bound socket

[Denis Kirjanov]

in the case of a socket which is already bound to an adress
there is no sense to create a path in the next attempts

here is a program that shows the issue:

int main()
{
    int s;
    struct sockaddr_un a;

    s = socket(AF_UNIX, SOCK_STREAM, 0);
    if (s<0)
        perror("socket() failed\n");

    printf("First bind()\n");

    memset(&a, 0, sizeof(a));
    a.sun_family = AF_UNIX;
    strncpy(a.sun_path, "/tmp/.first_bind", sizeof(a.sun_path));

    if ((bind(s, (const struct sockaddr*) &a, sizeof(a))) == -1)
        perror("bind() failed\n");

    printf("Second bind()\n");

    memset(&a, 0, sizeof(a));
    a.sun_family = AF_UNIX;
    strncpy(a.sun_path, "/tmp/.first_bind_failed", sizeof(a.sun_path));

    if ((bind(s, (const struct sockaddr*) &a, sizeof(a))) == -1)
        perror("bind() failed\n");
}

kda@SLES15-SP2:~> ./test
First bind()
Second bind()
bind() failed
: Invalid argument

kda@SLES15-SP2:~> ls -la /tmp/.first_bind
.first_bind         .first_bind_failed

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
v2: move a new path creation after the address assignment check
v3: fixed goto labels on the error path
v4: check the assigned address with bindlock held

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
---
 net/unix/af_unix.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 41c3303c3357..489d49a1739c 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1029,6 +1029,16 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 		goto out;
 	}
 
+	/* check if we're already bound to a path */
+	err = mutex_lock_interruptible(&u->bindlock);
+	if (err)
+		goto out;
+	if (u->addr)
+		err = -EINVAL;
+	mutex_unlock(&u->bindlock);
+	if (err)
+		goto out;
+
 	err = unix_mkname(sunaddr, addr_len, &hash);
 	if (err < 0)
 		goto out;
@@ -1049,10 +1059,6 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	if (err)
 		goto out_put;
 
-	err = -EINVAL;
-	if (u->addr)
-		goto out_up;
-
 	err = -ENOMEM;
 	addr = kmalloc(sizeof(*addr)+addr_len, GFP_KERNEL);
 	if (!addr)
-- 
2.16.4

Re: [PATCH v4] net/af_unix: don't create a path for a bound socket

[Jakub Kicinski]

On Thu,  3 Dec 2020 11:18:44 +0300 Denis Kirjanov wrote:
> in the case of a socket which is already bound to an adress
> there is no sense to create a path in the next attempts

> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 41c3303c3357..489d49a1739c 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1029,6 +1029,16 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>  		goto out;
>  	}
>  
> +	/* check if we're already bound to a path */
> +	err = mutex_lock_interruptible(&u->bindlock);
> +	if (err)
> +		goto out;
> +	if (u->addr)
> +		err = -EINVAL;
> +	mutex_unlock(&u->bindlock);

This, like v1, is not atomic with the creation, two threads can pass
this check and then try to assign to u->addr.

Naive question - can't we add the removal of the unnecessary node in
the error path, in the kernel?

> +	if (err)
> +		goto out;
> +
>  	err = unix_mkname(sunaddr, addr_len, &hash);
>  	if (err < 0)
>  		goto out;
> @@ -1049,10 +1059,6 @@ static int unix_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
>  	if (err)
>  		goto out_put;
>  
> -	err = -EINVAL;
> -	if (u->addr)
> -		goto out_up;
> -
>  	err = -ENOMEM;
>  	addr = kmalloc(sizeof(*addr)+addr_len, GFP_KERNEL);
>  	if (!addr)