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authorDaniel Borkmann <daniel@iogearbox.net>2016-02-10 16:47:11 +0100
committerDavid S. Miller <davem@davemloft.net>2016-02-10 16:56:47 -0500
commita1b14d27ed0965838350f1377ff97c93ee383492 (patch)
treecdd4792d189fddd674a09c710882f9cb211088b8 /kernel/bpf
parent1902750b50757804775d56a1cfc9f827f734e755 (diff)
downloadlinux-sh-a1b14d27ed0965838350f1377ff97c93ee383492.tar.gz
bpf: fix branch offset adjustment on backjumps after patching ctx expansion
When ctx access is used, the kernel often needs to expand/rewrite instructions, so after that patching, branch offsets have to be adjusted for both forward and backward jumps in the new eBPF program, but for backward jumps it fails to account the delta. Meaning, for example, if the expansion happens exactly on the insn that sits at the jump target, it doesn't fix up the back jump offset. Analysis on what the check in adjust_branches() is currently doing: /* adjust offset of jmps if necessary */ if (i < pos && i + insn->off + 1 > pos) insn->off += delta; else if (i > pos && i + insn->off + 1 < pos) insn->off -= delta; First condition (forward jumps): Before: After: insns[0] insns[0] insns[1] <--- i/insn insns[1] <--- i/insn insns[2] <--- pos insns[P] <--- pos insns[3] insns[P] `------| delta insns[4] <--- target_X insns[P] `-----| insns[5] insns[3] insns[4] <--- target_X insns[5] First case is if we cross pos-boundary and the jump instruction was before pos. This is handeled correctly. I.e. if i == pos, then this would mean our jump that we currently check was the patchlet itself that we just injected. Since such patchlets are self-contained and have no awareness of any insns before or after the patched one, the delta is correctly not adjusted. Also, for the second condition in case of i + insn->off + 1 == pos, means we jump to that newly patched instruction, so no offset adjustment are needed. That part is correct. Second condition (backward jumps): Before: After: insns[0] insns[0] insns[1] <--- target_X insns[1] <--- target_X insns[2] <--- pos <-- target_Y insns[P] <--- pos <-- target_Y insns[3] insns[P] `------| delta insns[4] <--- i/insn insns[P] `-----| insns[5] insns[3] insns[4] <--- i/insn insns[5] Second interesting case is where we cross pos-boundary and the jump instruction was after pos. Backward jump with i == pos would be impossible and pose a bug somewhere in the patchlet, so the first condition checking i > pos is okay only by itself. However, i + insn->off + 1 < pos does not always work as intended to trigger the adjustment. It works when jump targets would be far off where the delta wouldn't matter. But, for example, where the fixed insn->off before pointed to pos (target_Y), it now points to pos + delta, so that additional room needs to be taken into account for the check. This means that i) both tests here need to be adjusted into pos + delta, and ii) for the second condition, the test needs to be <= as pos itself can be a target in the backjump, too. Fixes: 9bac3d6d548e ("bpf: allow extended BPF programs access skb fields") Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'kernel/bpf')
-rw-r--r--kernel/bpf/verifier.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d1d3e8f57de9..2e7f7ab739e4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -2082,7 +2082,7 @@ static void adjust_branches(struct bpf_prog *prog, int pos, int delta)
/* adjust offset of jmps if necessary */
if (i < pos && i + insn->off + 1 > pos)
insn->off += delta;
- else if (i > pos && i + insn->off + 1 < pos)
+ else if (i > pos + delta && i + insn->off + 1 <= pos + delta)
insn->off -= delta;
}
}