[PATCH] 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 rewrites the header after the second
copy, using 'btf->hdr', which is obtained in the first copy.

Signed-off-by: Wenwen Wang <wang6495@umn.edu>
---
 kernel/bpf/btf.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 138f030..2a85f91 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2202,6 +2202,9 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
 		goto errout;
 	}
 
+	memcpy(data, &btf->hdr,
+		min_t(u32, btf->hdr.hdr_len, sizeof(btf->hdr)));
+
 	err = btf_parse_str_sec(env);
 	if (err)
 		goto errout;
-- 
2.7.4