Patch "bpf: fix method of PTR_TO_PACKET reg id generation" has been added to the 4.7-stable tree

This is a note to let you know that I've just added the patch titled

bpf: fix method of PTR_TO_PACKET reg id generation

to the 4.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
bpf-fix-method-of-ptr_to_packet-reg-id-generation.patch
and it can be found in the queue-4.7 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jakub Kicinski <[email protected]>
Date: Tue, 2 Aug 2016 16:12:14 +0100
Subject: bpf: fix method of PTR_TO_PACKET reg id generation

From: Jakub Kicinski <[email protected]>

[ Upstream commit 1f415a74b0ca64b5bfacbb12d71ed2ec050a8cfb ]

Using per-register incrementing ID can lead to find_good_pkt_pointers() confusing registers which have completely different values. Consider example:

0: (bf) r6 = r1
1: (61) r8 = *(u32 *)(r6 +76)
2: (61) r0 = *(u32 *)(r6 +80)
3: (bf) r7 = r8
4: (07) r8 += 32
5: (2d) if r8 > r0 goto pc+9
R0=pkt_end R1=ctx R6=ctx R7=pkt(id=0,off=0,r=32) R8=pkt(id=0,off=32,r=32) R10=fp
6: (bf) r8 = r7
7: (bf) r9 = r7
8: (71) r1 = *(u8 *)(r7 +0)
9: (0f) r8 += r1
10: (71) r1 = *(u8 *)(r7 +1)
11: (0f) r9 += r1
12: (07) r8 += 32
13: (2d) if r8 > r0 goto pc+1
R0=pkt_end R1=inv56 R6=ctx R7=pkt(id=0,off=0,r=32) R8=pkt(id=1,off=32,r=32) R9=pkt(id=1,off=0,r=32) R10=fp
14: (71) r1 = *(u8 *)(r9 +16)
15: (b7) r7 = 0
16: (bf) r0 = r7
17: (95) exit

We need to get a UNKNOWN_VALUE with imm to force id generation so lines 0-5 make r7 a valid packet pointer. We then read two different bytes from the packet and add them to copies of the constructed packet pointer. r8 (line 9) and r9 (line 11) will get the same id of 1, independently. When either of them is validated (line
13) - find_good_pkt_pointers() will also mark the other as safe. This leads to access on line 14 being mistakenly considered safe.

---
 kernel/bpf/verifier.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

@@ -194,6 +194,7 @@ struct verifier_env {
 	struct verifier_state_list **explored_states; /* search pruning optimization */
 	struct bpf_map *used_maps[MAX_USED_MAPS]; /* array of map's used by eBPF program */
 	u32 used_map_cnt;		/* number of used maps */
+	u32 id_gen;			/* used to generate unique reg IDs */
 	bool allow_ptr_leaks;
 };
 

@@ -1277,7 +1278,7 @@ add_imm:
 		/* dst_reg stays as pkt_ptr type and since some positive
 		 * integer value was added to the pointer, increment its 'id'
 		 */
-		dst_reg->id++;
+		dst_reg->id = ++env->id_gen;
 
 		/* something was added to pkt_ptr, set range and off to zero */
 		dst_reg->off = 0;

Patches currently in stable-queue which might be from [email protected] are

queue-4.7/bpf-fix-method-of-ptr_to_packet-reg-id-generation.patch