[PATCH v2] bpf: btf: Fix a missing-check bug

[Wenwen Wang <wang6495@umn.edu>]

In btf_parse(), the header of the user-space btf data 'btf_data' is firstly
parsed and verified through btf_parse_hdr(). In btf_parse_hdr(), the header
is copied from user-space 'btf_data' to kernel-space 'btf->hdr' and then
verified. If no error happens during the verification process, the whole
data of 'btf_data', including the header, is then copied to 'data' in
btf_parse(). It is obvious that the header is copied twice here. More
importantly, no check is enforced after the second copy to make sure the
headers obtained in these two copies are same. Given that 'btf_data'
resides in the user space, a malicious user can race to modify the header
between these two copies. By doing so, the user can inject inconsistent
data, which can cause undefined behavior of the kernel and introduce
potential security risk.

To avoid the above issue, this patch copies the parsed header from
'btf->hdr' to 'data'. The remaining part in 'data' is still copied from the
user-space 'btf_data'.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
---
 kernel/bpf/btf.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 378cef7..b52a834a 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2152,6 +2152,7 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
 	struct btf_verifier_env *env = NULL;
 	struct bpf_verifier_log *log;
 	struct btf *btf = NULL;
+	u32 hdr_len;
 	u8 *data;
 	int err;
 
@@ -2200,7 +2201,15 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
 	btf->data_size = btf_data_size;
 	btf->nohdr_data = btf->data + btf->hdr.hdr_len;
 
-	if (copy_from_user(data, btf_data, btf_data_size)) {
+	/*
+	 * The header at btf_data could be modified by a malicious user
+	 * after it is parsed. So we copy the parsed header here. The
+	 * remaining part is still copied from btf_data.
+	 */
+	hdr_len = min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr));
+	memcpy(data, &btf->hdr, hdr_len);
+	if (copy_from_user(data + hdr_len, (u8 __user *)btf_data + hdr_len,
+				btf_data_size - hdr_len)) {
 		err = -EFAULT;
 		goto errout;
 	}
-- 
2.7.4