[PATCH bpf] bpf: don't leak memory in bpf getsockopt when optlen == 0

optlen == 0 indicates that the kernel should ignore BPF buffer
and use the original one from the user. We, however, forget
to free the temporary buffer that we've allocated for BPF.

Reported-by: Martin KaFai Lau <redacted>
Fixes: d8fe449a9c51 ("bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE")
Signed-off-by: Stanislav Fomichev <redacted>
---
 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 6ec088a96302..09179ab72c03 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c

@@ -1395,7 +1395,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 	}
 
 out:
-	if (ret)
+	if (*kernel_optval == NULL)
 		sockopt_free_buf(&ctx);
 	return ret;
 }

-- 
2.30.0.284.gd98b1dd5eaa7-goog

On Mon, Jan 11, 2021 at 11:47:38AM -0800, Stanislav Fomichev wrote:

It seems fragile to depend on the caller to init *kernel_optval to NULL.

How about something like:

diff --git i/kernel/bpf/cgroup.c w/kernel/bpf/cgroup.c
index 6ec088a96302..8d94c004e781 100644
--- i/kernel/bpf/cgroup.c
+++ w/kernel/bpf/cgroup.c

@@ -1358,7 +1358,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 
 	if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) != 0) {
 		ret = -EFAULT;
-		goto out;
+		goto err_out;
 	}
 
 	lock_sock(sk);

@@ -1368,7 +1368,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 
 	if (!ret) {
 		ret = -EPERM;
-		goto out;
+		goto err_out;
 	}
 
 	if (ctx.optlen == -1) {

@@ -1379,7 +1379,6 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 		ret = -EFAULT;
 	} else {
 		/* optlen within bounds, run kernel handler */
-		ret = 0;
 
 		/* export any potential modifications */
 		*level = ctx.level;

@@ -1391,12 +1390,15 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
 		if (ctx.optlen != 0) {
 			*optlen = ctx.optlen;
 			*kernel_optval = ctx.optval;
+		} else {
+			sockopt_free_buf(&ctx);
 		}
+
+		return 0;
 	}
 
-out:
-	if (ret)
-		sockopt_free_buf(&ctx);
+err_out:
+	sockopt_free_buf(&ctx);
 	return ret;
 }

On Mon, Jan 11, 2021 at 2:32 PM Martin KaFai Lau [off-list ref] wrote:

On Mon, Jan 11, 2021 at 11:47:38AM -0800, Stanislav Fomichev wrote:

quoted

optlen == 0 indicates that the kernel should ignore BPF buffer
and use the original one from the user. We, however, forget
to free the temporary buffer that we've allocated for BPF.

Reported-by: Martin KaFai Lau <redacted>
Fixes: d8fe449a9c51 ("bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE")
Signed-off-by: Stanislav Fomichev <redacted>
---
 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 6ec088a96302..09179ab72c03 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c

@@ -1395,7 +1395,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
      }

 out:
-     if (ret)
+     if (*kernel_optval == NULL)

It seems fragile to depend on the caller to init *kernel_optval to NULL.

We can manually reset it to NULL when we enter
__cgroup_bpf_run_filter_setsockopt,
I didn't bother since there is only one existing caller.

But you patch also LGTM, I don't really have a preference.

On Mon, Jan 11, 2021 at 02:38:02PM -0800, Stanislav Fomichev wrote:

On Mon, Jan 11, 2021 at 2:32 PM Martin KaFai Lau [off-list ref] wrote:

quoted

On Mon, Jan 11, 2021 at 11:47:38AM -0800, Stanislav Fomichev wrote:

quoted

optlen == 0 indicates that the kernel should ignore BPF buffer
and use the original one from the user. We, however, forget
to free the temporary buffer that we've allocated for BPF.

Reported-by: Martin KaFai Lau <redacted>
Fixes: d8fe449a9c51 ("bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE")
Signed-off-by: Stanislav Fomichev <redacted>
---
 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 6ec088a96302..09179ab72c03 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c

@@ -1395,7 +1395,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
      }

 out:
-     if (ret)
+     if (*kernel_optval == NULL)

It seems fragile to depend on the caller to init *kernel_optval to NULL.

We can manually reset it to NULL when we enter
__cgroup_bpf_run_filter_setsockopt,

It feels weird to reset the caller value at the beginning while this is not
intended to be an _init() like function, so I avoided it.

but yeah, I am fine on this way also and won't oppose it strongly ;)

I didn't bother since there is only one existing caller.

But you patch also LGTM, I don't really have a preference.

quoted

How about something like:

diff --git i/kernel/bpf/cgroup.c w/kernel/bpf/cgroup.c
index 6ec088a96302..8d94c004e781 100644
--- i/kernel/bpf/cgroup.c
+++ w/kernel/bpf/cgroup.c

@@ -1358,7 +1358,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,

        if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) != 0) {
                ret = -EFAULT;
-               goto out;
+               goto err_out;
        }

        lock_sock(sk);

@@ -1368,7 +1368,7 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,

        if (!ret) {
                ret = -EPERM;
-               goto out;
+               goto err_out;
        }

        if (ctx.optlen == -1) {

@@ -1379,7 +1379,6 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
                ret = -EFAULT;
        } else {
                /* optlen within bounds, run kernel handler */
-               ret = 0;

                /* export any potential modifications */
                *level = ctx.level;

@@ -1391,12 +1390,15 @@ int __cgroup_bpf_run_filter_setsockopt(struct sock *sk, int *level,
                if (ctx.optlen != 0) {
                        *optlen = ctx.optlen;
                        *kernel_optval = ctx.optval;
+               } else {
+                       sockopt_free_buf(&ctx);
                }
+
+               return 0;
        }

-out:
-       if (ret)
-               sockopt_free_buf(&ctx);
+err_out:
+       sockopt_free_buf(&ctx);
        return ret;
 }