On 4/15/15 8:59 AM, Hannes Frederic Sowa wrote:
On Di, 2015-04-14 at 15:57 -0700, Alexei Starovoitov wrote:
Due to missing bounds check the DAG pass of the BPF verifier can corrupt
the memory which can cause random crashes during program loading:
[8.449451] BUG: unable to handle kernel paging request at ffffffffffffffff
[8.451293] IP: [<ffffffff811de33d>] kmem_cache_alloc_trace+0x8d/0x2f0
[8.452329] Oops: 0000 [#1] SMP
[8.452329] Call Trace:
[8.452329] [<ffffffff8116cc82>] bpf_check+0x852/0x2000
[8.452329] [<ffffffff8116b7e4>] bpf_prog_load+0x1e4/0x310
[8.452329] [<ffffffff811b190f>] ? might_fault+0x5f/0xb0
[8.452329] [<ffffffff8116c206>] SyS_bpf+0x806/0xa30
Fixes: f1bca824dabb ("bpf: add search pruning optimization to verifier")
Signed-off-by: Alexei Starovoitov <a...@plumgrid.com>
---
Many things need to align for this crash to be seen, yet I managed to hit it.
In my case JA was last insn, 't' was 255 and explored_states array
had 256 elements. I've double checked other similar paths and all seems clean.
kernel/bpf/verifier.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index a28e09c7825d..36508e69e92a 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1380,7 +1380,8 @@ peek_stack:
/* tell verifier to check for equivalent states
* after every call and jump
*/
- env->explored_states[t + 1] = STATE_LIST_MARK;
+ if (t + 1 < insn_cnt)
+ env->explored_states[t + 1] = STATE_LIST_MARK;
} else {
/* conditional jump with two edges */
ret = push_insn(t, t + 1, FALLTHROUGH, env);
Quick review:
We have env->explored_states[t+1] access in the
} else {
/* conditional jump with two edges */
ret = push_insn(t, t + 1, FALLTHROUGH, env);
if (ret == 1)
goto peek_stack;
else if (ret < 0)
goto err_free;
ret = push_insn(t, t + insns[t].off + 1, BRANCH, env);
if (ret == 1)
goto peek_stack;
else if (ret < 0)
goto err_free;
}
} else {
push_insn call. At this point insn[t].off could be 0, no?
insn[t].off can be anything, but the first thing that push_insn()
checks is:
if (w < 0 || w >= env->prog->len)
only then it does:
env->explored_states[w] = STATE_LIST_MARK;
so we're good there.
Though thanks for triple checking :)
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