net/atm: fix slab-out-of-bounds read in vcc_setsockopt()

net/atm: fix slab-out-of-bounds read in vcc_setsockopt()

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>
Link: https://patch.msgid.link/20260805131508.3227331-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>