]> git.karo-electronics.de Git - linux-beck.git/commit
bpf: Detect identical PTR_TO_MAP_VALUE_OR_NULL registers
authorThomas Graf <tgraf@suug.ch>
Tue, 18 Oct 2016 17:51:19 +0000 (19:51 +0200)
committerDavid S. Miller <davem@davemloft.net>
Wed, 19 Oct 2016 15:09:28 +0000 (11:09 -0400)
commit57a09bf0a416700676e77102c28f9cfcb48267e0
treead40f415e0cc209db360cac500abcfb120031f17
parent3134e9bdbe9c81a943b2c027a78433d4b0cf50dc
bpf: Detect identical PTR_TO_MAP_VALUE_OR_NULL registers

A BPF program is required to check the return register of a
map_elem_lookup() call before accessing memory. The verifier keeps
track of this by converting the type of the result register from
PTR_TO_MAP_VALUE_OR_NULL to PTR_TO_MAP_VALUE after a conditional
jump ensures safety. This check is currently exclusively performed
for the result register 0.

In the event the compiler reorders instructions, BPF_MOV64_REG
instructions may be moved before the conditional jump which causes
them to keep their type PTR_TO_MAP_VALUE_OR_NULL to which the
verifier objects when the register is accessed:

0: (b7) r1 = 10
1: (7b) *(u64 *)(r10 -8) = r1
2: (bf) r2 = r10
3: (07) r2 += -8
4: (18) r1 = 0x59c00000
6: (85) call 1
7: (bf) r4 = r0
8: (15) if r0 == 0x0 goto pc+1
 R0=map_value(ks=8,vs=8) R4=map_value_or_null(ks=8,vs=8) R10=fp
9: (7a) *(u64 *)(r4 +0) = 0
R4 invalid mem access 'map_value_or_null'

This commit extends the verifier to keep track of all identical
PTR_TO_MAP_VALUE_OR_NULL registers after a map_elem_lookup() by
assigning them an ID and then marking them all when the conditional
jump is observed.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
include/linux/bpf_verifier.h
kernel/bpf/verifier.c
tools/testing/selftests/bpf/test_verifier.c