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Message-ID: <8a87957e-4d33-9351-ae74-243441cb03cd@opteya.com>
Date: Thu, 7 Apr 2022 12:56:05 +0200
From: Yann Droneaud <ydroneaud@...eya.com>
To: Stephen Hemminger <stephen@...workplumber.org>
Cc: "David S . Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] af_unix: Escape abstract unix socket address
Hi,
Le 06/04/2022 à 23:59, Stephen Hemminger a écrit :
> On Wed, 6 Apr 2022 12:22:13 +0200
> Yann Droneaud <ydroneaud@...eya.com> wrote:
>
>> Abstract unix socket address are bytes sequences up to
>> 108 bytes (UNIX_PATH_MAX == sizeof(struct sockaddr_un) -
>> offsetof(struct sockaddr_un, sun_path)).
>>
>> As with any random string of bytes, printing them in
>> /proc/net/unix should be done with caution to prevent
>> misbehavior.
>>
>> It would have been great to use seq_escape_mem() to escape
>> the control characters in a reversible way.
>>
>> Unfortunately userspace might expect that NUL bytes are
>> replaced with '@' characters as it's done currently.
>>
>> So this patch implements the following scheme: any control
>> characters, including NUL, in the abstract unix socket
>> addresses is replaced by '@' characters.
>>
>> Sadly, with such non reversible escape scheme, abstract
>> addresses such as "\0\0", "\0\a", "\0\b", "\0\t", etc.
>> will have the same representation: "@@".
>>
>> But will prevent "cat /proc/net/unix" from messing with
>> terminal, and will prevent "\n" in abstract address from
>> messing with parsing the list of Unix sockets.
>>
>> Signed-off-by: Yann Droneaud <ydroneaud@...eya.com>
>> ---
>> net/unix/af_unix.c | 3 ++-
>> 1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>> index e71a312faa1e..8021efd92301 100644
>> --- a/net/unix/af_unix.c
>> +++ b/net/unix/af_unix.c
>> @@ -3340,7 +3340,8 @@ static int unix_seq_show(struct seq_file *seq, void *v)
>> i++;
>> }
>> for ( ; i < len; i++)
>> - seq_putc(seq, u->addr->name->sun_path[i] ?:
>> + seq_putc(seq, !iscntrl(u->addr->name->sun_path[i]) ?
>> + u->addr->name->sun_path[i] :
>> '@');
>> }
>> unix_state_unlock(s);
> Unfortunately, you will break userspace ABI with this.
It's a wanted side effect.
Consider the following program
#include <stddef.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#define ADDRESS "\0\n0000000000000000: 00000003 00000000 00000000 0001 03 1234567890 /bin/true"
int main(void)
{
static const struct sockaddr_un un = {
.sun_family = AF_UNIX,
.sun_path = ADDRESS,
};
int s;
s = socket(AF_UNIX, SOCK_STREAM, 0);
if (s < 0) {
perror("socket");
return 1;
}
if (bind(s, (const struct sockaddr *)&un, offsetof(struct sockaddr_un,sun_path) + sizeof(ADDRESS) - 1) < 0) {
perror("bind");
return 1;
}
while (1)
pause();
return 0;
}
This confuses
- cat /proc/net/unix
- netstat -x
Only ss -xl doesn't take /bin/true as a Unix socket (but ss output is broken because it doesn't escape \n in unix addresses)
Regards.
--
Yann Droneaud
OPTEYA
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