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01c66c48d4
kernel_optimize_test/kernel/bpf
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Yonghong Song 01c66c48d4 bpf: Fix an incorrect branch elimination by verifier 
Wenbo reported an issue in [1] where a checking of null
pointer is evaluated as always false. In this particular
case, the program type is tp_btf and the pointer to
compare is a PTR_TO_BTF_ID.

The current verifier considers PTR_TO_BTF_ID always
reprents a non-null pointer, hence all PTR_TO_BTF_ID compares
to 0 will be evaluated as always not-equal, which resulted
in the branch elimination.

For example,
 struct bpf_fentry_test_t {
     struct bpf_fentry_test_t *a;
 };
 int BPF_PROG(test7, struct bpf_fentry_test_t *arg)
 {
     if (arg == 0)
         test7_result = 1;
     return 0;
 }
 int BPF_PROG(test8, struct bpf_fentry_test_t *arg)
 {
     if (arg->a == 0)
         test8_result = 1;
     return 0;
 }

In above bpf programs, both branch arg == 0 and arg->a == 0
are removed. This may not be what developer expected.

The bug is introduced by Commit cac616db39 ("bpf: Verifier
track null pointer branch_taken with JNE and JEQ"),
where PTR_TO_BTF_ID is considered to be non-null when evaluting
pointer vs. scalar comparison. This may be added
considering we have PTR_TO_BTF_ID_OR_NULL in the verifier
as well.

PTR_TO_BTF_ID_OR_NULL is added to explicitly requires
a non-NULL testing in selective cases. The current generic
pointer tracing framework in verifier always
assigns PTR_TO_BTF_ID so users does not need to
check NULL pointer at every pointer level like a->b->c->d.

We may not want to assign every PTR_TO_BTF_ID as
PTR_TO_BTF_ID_OR_NULL as this will require a null test
before pointer dereference which may cause inconvenience
for developers. But we could avoid branch elimination
to preserve original code intention.

This patch simply removed PTR_TO_BTD_ID from reg_type_not_null()
in verifier, which prevented the above branches from being eliminated.

 [1]: https://lore.kernel.org/bpf/79dbb7c0-449d-83eb-5f4f-7af0cc269168@fb.com/T/

Fixes: cac616db39 ("bpf: Verifier track null pointer branch_taken with JNE and JEQ")
Reported-by: Wenbo Zhang <ethercflow@gmail.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200630171240.2523722-1-yhs@fb.com
2020-06-30 22:21:05 +02:00
..
arraymap.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2020-05-15 13:48:59 -07:00
bpf_iter.c bpf: Enable bpf_iter targets registering ctx argument types 2020-05-13 12:30:50 -07:00
bpf_lru_list.c
bpf_lru_list.h bpf: Fix a typo "inacitve" -> "inactive" 2020-04-06 21:54:10 +02:00
bpf_lsm.c bpf: Use tracing helpers for lsm programs 2020-06-01 15:08:04 -07:00
bpf_struct_ops_types.h
bpf_struct_ops.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
btf.c bpf: Do not allow btf_ctx_access with __int128 types 2020-06-25 16:17:05 +02:00
cgroup.c bpf: Don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE 2020-06-17 10:54:05 -07:00
core.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next 2020-06-03 16:27:18 -07:00
cpumap.c xdp: Rename convert_to_xdp_frame in xdp_convert_buff_to_frame 2020-06-01 15:02:53 -07:00
devmap.c devmap: Use bpf_map_area_alloc() for allocating hash buckets 2020-06-17 10:01:19 -07:00
disasm.c
disasm.h
dispatcher.c bpf: Remove bpf_image tree 2020-03-13 12:49:52 -07:00
hashtab.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
helpers.c bpf: Implement BPF ring buffer and verifier support for it 2020-06-01 14:38:22 -07:00
inode.c bpf: Create file bpf iterator 2020-05-09 17:05:26 -07:00
local_storage.c
lpm_trie.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
Makefile flow_dissector: Move out netns_bpf prog callbacks 2020-06-01 15:21:02 -07:00
map_in_map.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
map_in_map.h
map_iter.c bpf: Enable bpf_iter targets registering ctx argument types 2020-05-13 12:30:50 -07:00
net_namespace.c bpf, netns: Fix use-after-free in pernet pre_exit callback 2020-06-30 10:53:42 -07:00
offload.c bpf, offload: Replace bitwise AND by logical AND in bpf_prog_offload_info_fill 2020-02-17 16:53:49 +01:00
percpu_freelist.c bpf: Dont iterate over possible CPUs with interrupts disabled 2020-02-24 16:18:20 -08:00
percpu_freelist.h
queue_stack_maps.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
reuseport_array.c bpf: Implement CAP_BPF 2020-05-15 17:29:41 +02:00
ringbuf.c bpf: Enforce BPF ringbuf size to be the power of 2 2020-06-30 16:31:55 +02:00
stackmap.c mmap locking API: add mmap_read_trylock_non_owner() 2020-06-09 09:39:14 -07:00
syscall.c bpf: sockmap: Require attach_bpf_fd when detaching a program 2020-06-30 10:46:39 -07:00
sysfs_btf.c bpf: Support llvm-objcopy for vmlinux BTF 2020-03-19 12:32:38 +01:00
task_iter.c bpf: Fix bpf_iter's task iterator logic 2020-05-14 18:37:32 -07:00
tnum.c bpf: Verifier, do explicit ALU32 bounds tracking 2020-03-30 14:59:53 -07:00
trampoline.c bpf: lsm: Implement attach, detach and execution 2020-03-30 01:34:00 +02:00
verifier.c bpf: Fix an incorrect branch elimination by verifier 2020-06-30 22:21:05 +02:00