On Thu, 27 May 2021 15:13:32 -0400 Xin Long wrote:

On Thu, May 27, 2021 at 3:12 PM Xin Long [off-list ref] wrote:

quoted

Currently, when calling UDP's getsockopt, it only makes sure 'len'
is not less than 0, then copys 'len' bytes back to usespace while
all data is 'int' type there.

If userspace sets 'len' with a value N (N < sizeof(int)), it will
only copy N bytes of the data to userspace with no error returned,
which doesn't seem right.

I'm not so sure of that. In cases where the value returned may grow
with newer kernel releases truncating the output to the size of buffer
user space provided is pretty normal. I think this code is working as
intended.

quoted

Like in Chen Yi's case where N is 0, it
called getsockopt and got an incorrect value but with no error
returned.

The patch is to fix the len check and make sure it's not less than
sizeof(int). Otherwise, '-EINVAL' is returned, as it does in other
protocols like SCTP/TIPC.

Reported-by: Chen Yi <redacted>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
---
 net/ipv4/udp.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 15f5504adf5b..90de2ac70ea9 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c

@@ -2762,11 +2762,11 @@ int udp_lib_getsockopt(struct sock *sk, int level, int optname,
        if (get_user(len, optlen))
                return -EFAULT;

-       len = min_t(unsigned int, len, sizeof(int));
-
-       if (len < 0)
+       if (len < sizeof(int))
                return -EINVAL;

+       len = sizeof(int);
+
        switch (optname) {
        case UDP_CORK:
                val = up->corkflag;

Note I'm not sure if this fix may break any APP, but the current
behavior definitely is not correct and doesn't match the man doc
of getsockopt, so please review.

Can you quote the part of man getsockopt you're referring to?