netdev - [PATCH net-next 1/2] bpf: support decreasing order in direct packet access

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 19 May 2016 18:17:13 -0700
From:	Alexei Starovoitov <ast@...com>
To:	"David S . Miller" <davem@...emloft.net>
CC:	Daniel Borkmann <daniel@...earbox.net>, <netdev@...r.kernel.org>,
	<kernel-team@...com>
Subject: [PATCH net-next 1/2] bpf: support decreasing order in direct packet access

when packet headers are accessed in 'decreasing' order (like TCP port
may be fetched before the program reads IP src) the llvm may generate
the following code:
[...]                // R7=pkt(id=0,off=22,r=70)
r2 = *(u32 *)(r7 +0) // good access
[...]
r7 += 40             // R7=pkt(id=0,off=62,r=70)
r8 = *(u32 *)(r7 +0) // good access
[...]
r1 = *(u32 *)(r7 -20) // this one will fail though it's within a safe range
                      // it's doing *(u32*)(skb->data + 42)
Fix verifier to recognize such code pattern

Alos turned out that 'off > range' condition is not a verifier bug.
It's a buggy program that may do something like:
if (ptr + 50 > data_end)
  return 0;
ptr += 60;
*(u32*)ptr;
in such case emit
"invalid access to packet, off=0 size=4, R1(id=0,off=60,r=50)" error message,
so all information is available for the program author to fix the program.

Fixes: 969bf05eb3ce ("bpf: direct packet access")
Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Daniel Borkmann <daniel@...earbox.net>
---
 kernel/bpf/verifier.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a08d66215245..d54e34874579 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -683,15 +683,11 @@ static int check_packet_access(struct verifier_env *env, u32 regno, int off,
 {
 	struct reg_state *regs = env->cur_state.regs;
 	struct reg_state *reg = &regs[regno];
-	int linear_size = (int) reg->range - (int) reg->off;
 
-	if (linear_size < 0 || linear_size >= MAX_PACKET_OFF) {
-		verbose("verifier bug\n");
-		return -EFAULT;
-	}
-	if (off < 0 || off + size > linear_size) {
-		verbose("invalid access to packet, off=%d size=%d, allowed=%d\n",
-			off, size, linear_size);
+	off += reg->off;
+	if (off < 0 || off + size > reg->range) {
+		verbose("invalid access to packet, off=%d size=%d, R%d(id=%d,off=%d,r=%d)\n",
+			off, size, regno, reg->id, reg->off, reg->range);
 		return -EACCES;
 	}
 	return 0;
-- 
2.8.0