Commit 1f415a74 authored Aug 02, 2016 by Jakub Kicinski Committed by David S. Miller Aug 03, 2016

bpf: fix method of PTR_TO_PACKET reg id generation



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.

Fixes: 969bf05e ("bpf: direct packet access")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent ea966cb6

kernel/bpf/verifier.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -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;
		};

		@@ -1301,7 +1302,7 @@ 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;

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