[PATCH v2] qemu-sockets: fix unix socket path copy (again)

[PATCH v2] qemu-sockets: fix unix socket path copy (again)

[Michael Tokarev]

Commit 4cfd970ec188558daa6214f26203fe553fb1e01f added an
assert which ensures the path within an address of a unix
socket returned from the kernel is at least one byte and
does not exceed sun_path buffer. Both of this constraints
are wrong:

A unix socket can be unnamed, in this case the path is
completely empty (not even \0)

And some implementations (notable linux) can add extra
trailing byte (\0) _after_ the sun_path buffer if we
passed buffer larger than it (and we do).

So remove the assertion (since it causes real-life breakage)
but at the same time fix the usage of sun_path. Namely,
we should not access sun_path[0] if kernel did not return
it at all (this is the case for unnamed sockets),
and use the returned salen when copyig actual path as an
upper constraint for the amount of bytes to copy - this
will ensure we wont exceed the information provided by
the kernel, regardless whenever there is a trailing \0
or not. This also helps with unnamed sockets.

Note the case of abstract socket, the sun_path is actually
a blob and can contain \0 characters, - it should not be
passed to g_strndup and the like, it should be accessed by
memcpy-like functions.

Fixes: 4cfd970ec188558daa6214f26203fe553fb1e01f
Fixes: http://bugs.debian.org/993145
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
CC: qemu-stable@nongnu.org
---
 util/qemu-sockets.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
index f2f3676d1f..842bd707ac 100644
--- a/util/qemu-sockets.c
+++ b/util/qemu-sockets.c
@@ -1345,13 +1345,11 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
     SocketAddress *addr;
     struct sockaddr_un *su = (struct sockaddr_un *)sa;
 
-    assert(salen >= sizeof(su->sun_family) + 1 &&
-           salen <= sizeof(struct sockaddr_un));
-
     addr = g_new0(SocketAddress, 1);
     addr->type = SOCKET_ADDRESS_TYPE_UNIX;
+    salen -= offsetof(struct sockaddr_un, sun_path);
 #ifdef CONFIG_LINUX
-    if (!su->sun_path[0]) {
+    if (salen > 0 && !su->sun_path[0]) {
         /* Linux abstract socket */
         addr->u.q_unix.path = g_strndup(su->sun_path + 1,
                                         salen - sizeof(su->sun_family) - 1);
@@ -1363,7 +1361,7 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
     }
 #endif
 
-    addr->u.q_unix.path = g_strndup(su->sun_path, sizeof(su->sun_path));
+    addr->u.q_unix.path = g_strndup(su->sun_path, salen);
     return addr;
 }
 #endif /* WIN32 */
-- 
2.30.2

Re: [PATCH v2] qemu-sockets: fix unix socket path copy (again)

[Daniel P. Berrangé]

On Wed, Sep 01, 2021 at 03:50:55PM +0300, Michael Tokarev wrote:
> Commit 4cfd970ec188558daa6214f26203fe553fb1e01f added an
> assert which ensures the path within an address of a unix
> socket returned from the kernel is at least one byte and
> does not exceed sun_path buffer. Both of this constraints
> are wrong:
> 
> A unix socket can be unnamed, in this case the path is
> completely empty (not even \0)
> 
> And some implementations (notable linux) can add extra
> trailing byte (\0) _after_ the sun_path buffer if we
> passed buffer larger than it (and we do).
> 
> So remove the assertion (since it causes real-life breakage)
> but at the same time fix the usage of sun_path. Namely,
> we should not access sun_path[0] if kernel did not return
> it at all (this is the case for unnamed sockets),
> and use the returned salen when copyig actual path as an
> upper constraint for the amount of bytes to copy - this
> will ensure we wont exceed the information provided by
> the kernel, regardless whenever there is a trailing \0
> or not. This also helps with unnamed sockets.
> 
> Note the case of abstract socket, the sun_path is actually
> a blob and can contain \0 characters, - it should not be
> passed to g_strndup and the like, it should be accessed by
> memcpy-like functions.
> 
> Fixes: 4cfd970ec188558daa6214f26203fe553fb1e01f
> Fixes: http://bugs.debian.org/993145
> Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
> CC: qemu-stable@nongnu.org
> ---
>  util/qemu-sockets.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/util/qemu-sockets.c b/util/qemu-sockets.c
> index f2f3676d1f..842bd707ac 100644
> --- a/util/qemu-sockets.c
> +++ b/util/qemu-sockets.c
> @@ -1345,13 +1345,11 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
>      SocketAddress *addr;
>      struct sockaddr_un *su = (struct sockaddr_un *)sa;
>  
> -    assert(salen >= sizeof(su->sun_family) + 1 &&
> -           salen <= sizeof(struct sockaddr_un));
> -
>      addr = g_new0(SocketAddress, 1);
>      addr->type = SOCKET_ADDRESS_TYPE_UNIX;
> +    salen -= offsetof(struct sockaddr_un, sun_path);

Adjusted salen value....

>  #ifdef CONFIG_LINUX
> -    if (!su->sun_path[0]) {
> +    if (salen > 0 && !su->sun_path[0]) {
>          /* Linux abstract socket */
>          addr->u.q_unix.path = g_strndup(su->sun_path + 1,
>                                          salen - sizeof(su->sun_family) - 1);

....still assuming the original salen value.

> @@ -1363,7 +1361,7 @@ socket_sockaddr_to_address_unix(struct sockaddr_storage *sa,
>      }
>  #endif
>  
> -    addr->u.q_unix.path = g_strndup(su->sun_path, sizeof(su->sun_path));
> +    addr->u.q_unix.path = g_strndup(su->sun_path, salen);
>      return addr;
>  }
>  #endif /* WIN32 */

Regards,
Daniel
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