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

[Martin Lau <kafai@fb.com>]

On Wed, Oct 24, 2018 at 06:22:46PM +0000, Martin Lau wrote:
> On Wed, Oct 24, 2018 at 05:26:23PM +0000, Martin Lau wrote:
> > On Wed, Oct 24, 2018 at 08:00:19AM -0500, Wenwen Wang wrote:
> > > 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.
> btw, I am working on a patch that copies the btf_data before parsing/verifying
> the header.  That should avoid this from happening but that will
> require a bit more code churns for the bpf branch.
> 
It is what I have in mind:


It is not a good idea to check the BTF header before copying the
user btf_data.  The verified header may not be the one actually
copied to btf->data (e.g. userspace may modify the passed in
btf_data in between).  Like the one fixed in
commit 8af03d1ae2e1 ("bpf: btf: Fix a missing check bug").

This patch copies the user btf_data before parsing/verifying
the BTF header.

Fixes: 69b693f0aefa ("bpf: btf: Introduce BPF Type Format (BTF)")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
 kernel/bpf/btf.c | 58 +++++++++++++++++++++---------------------------
 1 file changed, 25 insertions(+), 33 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 378cef70341c..ee4c82667d65 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -2067,56 +2067,47 @@ static int btf_check_sec_info(struct btf_verifier_env *env,
 	return 0;
 }
 
-static int btf_parse_hdr(struct btf_verifier_env *env, void __user *btf_data,
-			 u32 btf_data_size)
+static int btf_parse_hdr(struct btf_verifier_env *env)
 {
+	u32 hdr_len, hdr_copy, btf_data_size;
 	const struct btf_header *hdr;
-	u32 hdr_len, hdr_copy;
-	/*
-	 * Minimal part of the "struct btf_header" that
-	 * contains the hdr_len.
-	 */
-	struct btf_min_header {
-		u16	magic;
-		u8	version;
-		u8	flags;
-		u32	hdr_len;
-	} __user *min_hdr;
 	struct btf *btf;
 	int err;
 
 	btf = env->btf;
-	min_hdr = btf_data;
+	btf_data_size = btf->data_size;
 
-	if (btf_data_size < sizeof(*min_hdr)) {
+	if (btf_data_size <
+	    offsetof(struct btf_header, hdr_len) + sizeof(hdr->hdr_len)) {
 		btf_verifier_log(env, "hdr_len not found");
 		return -EINVAL;
 	}
 
-	if (get_user(hdr_len, &min_hdr->hdr_len))
-		return -EFAULT;
-
+	hdr = btf->data;
+	hdr_len = hdr->hdr_len;
 	if (btf_data_size < hdr_len) {
 		btf_verifier_log(env, "btf_header not found");
 		return -EINVAL;
 	}
 
-	err = bpf_check_uarg_tail_zero(btf_data, sizeof(btf->hdr), hdr_len);
-	if (err) {
-		if (err == -E2BIG)
-			btf_verifier_log(env, "Unsupported btf_header");
-		return err;
+	/* Ensure the unsupported header fields are zero */
+	if (hdr_len > sizeof(btf->hdr)) {
+		u8 *expected_zero = btf->data + sizeof(btf->hdr);
+		u8 *end = btf->data + hdr_len;
+
+		for (; expected_zero < end; expected_zero++) {
+			if (*expected_zero) {
+				btf_verifier_log(env, "Unsupported btf_header");
+				return -E2BIG;
+			}
+		}
 	}
 
 	hdr_copy = min_t(u32, hdr_len, sizeof(btf->hdr));
-	if (copy_from_user(&btf->hdr, btf_data, hdr_copy))
-		return -EFAULT;
+	memcpy(&btf->hdr, btf->data, hdr_copy);
 
 	hdr = &btf->hdr;
 
-	if (hdr->hdr_len != hdr_len)
-		return -EINVAL;
-
 	btf_verifier_log_hdr(env, btf_data_size);
 
 	if (hdr->magic != BTF_MAGIC) {
@@ -2186,10 +2177,6 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
 	}
 	env->btf = btf;
 
-	err = btf_parse_hdr(env, btf_data, btf_data_size);
-	if (err)
-		goto errout;
-
 	data = kvmalloc(btf_data_size, GFP_KERNEL | __GFP_NOWARN);
 	if (!data) {
 		err = -ENOMEM;
@@ -2198,13 +2185,18 @@ static struct btf *btf_parse(void __user *btf_data, u32 btf_data_size,
 
 	btf->data = data;
 	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)) {
 		err = -EFAULT;
 		goto errout;
 	}
 
+	err = btf_parse_hdr(env);
+	if (err)
+		goto errout;
+
+	btf->nohdr_data = btf->data + btf->hdr.hdr_len;
+
 	err = btf_parse_str_sec(env);
 	if (err)
 		goto errout;
-- 
2.17.1