Re: [PATCH bpf] bpf: properly check TCP_CONGESTION optlen

On Sat, Feb 23, 2019 at 12:48:53PM -0800, Eric Dumazet wrote:

On 02/23/2019 12:38 PM, Alexei Starovoitov wrote:

quoted

On Sat, Feb 23, 2019 at 11:07:09AM -0800, Eric Dumazet wrote:

quoted

If caller of bpf_setsockopt() is silly passing a negative optlen
bad things happen.

Fixes: 91b5b21c7c16 ("bpf: Add support for changing congestion control")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Lawrence Brakmo <redacted>
---
 net/core/filter.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/core/filter.c b/net/core/filter.c
index f7d0004fc16096eb42ece3a6acf645540ee2326b..6a5d89464168f2f35f43986c1dbc0446c9390a3c 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c

@@ -4194,8 +4194,9 @@ BPF_CALL_5(bpf_setsockopt, struct bpf_sock_ops_kern *, bpf_sock,
 			char name[TCP_CA_NAME_MAX];
 			bool reinit = bpf_sock->op > BPF_SOCK_OPS_NEEDS_ECN;
 
-			strncpy(name, optval, min_t(long, optlen,
-						    TCP_CA_NAME_MAX-1));
+			if (optlen < 0)
+				return -EINVAL;
+			strncpy(name, optval, min(optlen, TCP_CA_NAME_MAX - 1));

Unnecessary.
The verifier guarantees that optlen > 0 because
static const struct bpf_func_proto bpf_setsockopt_proto = {
        .func           = bpf_setsockopt,
...
        .arg5_type      = ARG_CONST_SIZE,
};

Even on 32bit kernel ?

The suspect thing to me is the min_t(long, ....)

optlen is an integer, why do we need to promote to a long ?

I think the code is actually fine as-is.
I bet it was copy pasted from do_tcp_setsockopt
where similar min_t(long) is used to match strncpy_from_user() declaration.
Here min_t(long) or min_t(int) or min() doesn't matter.
I would keep it as-is to avoid noisy patches.