[RFC PATCH 3/4] net: pass a kernel pointer via 'optlen_t' to proto[ops].getsockopt() hooks

[Stefan Metzmacher <metze@samba.org>]

The motivation for this is to remove the SOL_SOCKET limitation
from io_uring_cmd_getsockopt().

The reason for this limitation is that io_uring_cmd_getsockopt()
passes a kernel pointer.

The first idea would be to change the optval and optlen arguments
to the protocol specific hooks also to sockptr_t, as that
is already used for setsockopt() and also by do_sock_getsockopt()
sk_getsockopt() and BPF_CGROUP_RUN_PROG_GETSOCKOPT().

But as Linus don't like 'sockptr_t' I used a different approach.

Instead of passing the optlen as user or kernel pointer,
we only ever pass a kernel pointer and do the
translation from/to userspace in do_sock_getsockopt().

The simple solution would be to just remove the
'__user' from the int *optlen argument, but it
seems the compiler doesn't complain about
'__user' vs. without it, so instead I used
a helper struct in order to make sure everything
compiles with a typesafe change.

That together with get_optlen() and put_optlen() helper
macros make it relatively easy to review and check the
behaviour is most likely unchanged.

In order to avoid uapi changes regarding different error
code orders regarding -EFAULT, the real -EFAULT handling
is deferred to get_optlen() and put_optlen().

This allows io_uring_cmd_getsockopt() to remove the
SOL_SOCKET limitation.

Removing 'sockptr_t optlen' from existing code
is for patch for another day.

Link: https://lore.kernel.org/io-uring/86b1dce5-4bb4-4a0b-9cff-e72f488bf57d@samba.org/T/#t
Cc: Jens Axboe <axboe@kernel.dk>
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CC: Stefan Metzmacher <metze@samba.org>
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Signed-off-by: Stefan Metzmacher <metze@samba.org>
---
 include/linux/sockptr.h | 20 +++++++++++++++-----
 net/socket.c            | 31 +++++++++++++++++++++++++++++--
 2 files changed, 44 insertions(+), 7 deletions(-)

diff --git a/include/linux/sockptr.h b/include/linux/sockptr.h
index 1baf66f26f4f..06ec7fd73028 100644
--- a/include/linux/sockptr.h
+++ b/include/linux/sockptr.h
@@ -170,20 +170,25 @@ static inline int check_zeroed_sockptr(sockptr_t src, size_t offset,
 }
 
 typedef struct {
-	int __user *up;
+	int *kp;
 } optlen_t;
 
 #define __check_optlen_t(__optlen)				\
 ({								\
 	optlen_t *__ptr __maybe_unused = &__optlen; \
-	BUILD_BUG_ON(sizeof(*((__ptr)->up)) != sizeof(int));	\
+	BUILD_BUG_ON(sizeof(*((__ptr)->kp)) != sizeof(int));	\
 })
 
 #define get_optlen(__val, __optlen)				\
 ({								\
 	long __err;						\
 	__check_optlen_t(__optlen);				\
-	__err = get_user(__val, __optlen.up);			\
+	if ((__optlen).kp != NULL) {				\
+		(__val) = *((__optlen).kp);			\
+		__err = 0;					\
+	} else {						\
+		__err = -EFAULT;				\
+	}							\
 	__err;							\
 })
 
@@ -191,13 +196,18 @@ typedef struct {
 ({								\
 	long __err;						\
 	__check_optlen_t(__optlen);				\
-	__err = put_user(__val, __optlen.up);			\
+	if ((__optlen).kp != NULL) {				\
+		*((__optlen).kp) = (__val);			\
+		__err = 0;					\
+	} else {						\
+		__err = -EFAULT;				\
+	}							\
 	__err;							\
 })
 
 static inline sockptr_t OPTLEN_SOCKPTR(optlen_t optlen)
 {
-	return (sockptr_t) { .user = optlen.up, };
+	return (sockptr_t) { .kernel = optlen.kp, .is_kernel = true };
 }
 
 #endif /* _LINUX_SOCKPTR_H */
diff --git a/net/socket.c b/net/socket.c
index fa2de12c10e6..81e5c9767bbc 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2350,15 +2350,42 @@ int do_sock_getsockopt(struct socket *sock, bool compat, int level,
 	} else if (unlikely(!ops->getsockopt)) {
 		err = -EOPNOTSUPP;
 	} else {
-		optlen_t _optlen = { .up = NULL, };
+		optlen_t _optlen = { .kp = NULL, };
+		int koptlen;
 
 		if (WARN_ONCE(optval.is_kernel,
 			      "Invalid argument type"))
 			return -EOPNOTSUPP;
 
-		_optlen.up = optlen.user;
+		if (optlen.is_kernel) {
+			_optlen.kp = optlen.kernel;
+		} else if (optlen.user != NULL) {
+			/*
+			 * If optlen.user is NULL,
+			 * we pass _optlen.kp = NULL
+			 * in order to avoid breaking
+			 * any uapi for getsockopt()
+			 * implementations that ignore
+			 * the optlen pointer completely
+			 * or do any level and optname
+			 * checking before hitting a
+			 * potential -EFAULT condition.
+			 *
+			 * Also when optlen.user is not NULL,
+			 * but copy_from_sockptr() causes -EFAULT,
+			 * we'll pass optlen.kp = NULL in order
+			 * to defer a possible -EFAULT return
+			 * to the caller to get_optlen() and put_optlen().
+			 */
+			if (copy_from_sockptr(&koptlen, optlen, sizeof(koptlen)) == 0)
+				_optlen.kp = &koptlen;
+		}
 		err = ops->getsockopt(sock, level, optname, optval.user,
 				      _optlen);
+		if (err != -EFAULT && _optlen.kp == &koptlen) {
+			if (copy_to_sockptr(optlen, &koptlen, sizeof(koptlen)))
+				return -EFAULT;
+		}
 	}
 
 	if (!compat)
-- 
2.34.1