[PATCH v2 net] net/atm: fix slab-out-of-bounds read in vcc_setsockopt()

[Eric Dumazet <edumazet@google.com>]

vcc_setsockopt() contained an ineffective optlen check:
  if (__SO_LEVEL_MATCH(optname, level) && optlen != __SO_SIZE(optname))
      return -EINVAL;

If __SO_LEVEL_MATCH(optname, level) evaluated to false (e.g. if the caller
passed a mismatched level), the length check optlen != __SO_SIZE(optname)
was short-circuited and bypassed. Execution then fell through to switch(optname),
calling copy_from_sockptr() assuming optval contained sufficient space.

Furthermore, even if level matched, a cgroup BPF setsockopt filter could shrink
optlen after entry. Because copy_from_sockptr() on kernel pointers uses memcpy(),
this leads to a KASAN slab-out-of-bounds read when optlen is smaller than the
expected structure size.

Fix this by using copy_safe_from_sockptr(), which unconditionally validates
that optlen is at least the expected size before copying. Also change the local
'value' variable type from 'unsigned long' to 'int' so that SO_SETCLP matches
its sizeof(int) ABI encoding on 64-bit systems.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+53ecc09fb81df10ef4de@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=53ecc09fb81df10ef4de
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
v2: change @value type to "int"
v1: https://lore.kernel.org/netdev/20260804145806.2112004-1-edumazet@google.com/T/#u

 net/atm/common.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/net/atm/common.c b/net/atm/common.c
index c7f92405daf050ecd2a507e41efdd622ab8dc6a8..81195727fa18ba4e6146c0c02cd3898d17fc436f 100644
--- a/net/atm/common.c
+++ b/net/atm/common.c
@@ -710,7 +710,7 @@ int vcc_setsockopt(struct socket *sock, int level, int optname,
 		   sockptr_t optval, unsigned int optlen)
 {
 	struct atm_vcc *vcc;
-	unsigned long value;
+	int value;
 	int error;
 
 	if (__SO_LEVEL_MATCH(optname, level) && optlen != __SO_SIZE(optname))
@@ -722,8 +722,10 @@ int vcc_setsockopt(struct socket *sock, int level, int optname,
 	{
 		struct atm_qos qos;
 
-		if (copy_from_sockptr(&qos, optval, sizeof(qos)))
-			return -EFAULT;
+		error = copy_safe_from_sockptr(&qos, sizeof(qos), optval,
+					       optlen);
+		if (error)
+			return error;
 		error = check_qos(&qos);
 		if (error)
 			return error;
@@ -737,8 +739,10 @@ int vcc_setsockopt(struct socket *sock, int level, int optname,
 		return 0;
 	}
 	case SO_SETCLP:
-		if (copy_from_sockptr(&value, optval, sizeof(value)))
-			return -EFAULT;
+		error = copy_safe_from_sockptr(&value, sizeof(value), optval,
+					       optlen);
+		if (error)
+			return error;
 		if (value)
 			vcc->atm_options |= ATM_ATMOPT_CLP;
 		else
-- 
2.55.0.571.g244d577d93-goog