Re: [PATCH bpf] bpf: Fix a verifier bug due to incorrect branch offset comparison with cpu=v4

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On 11/29/23 5:51 PM, Daniel Borkmann wrote:
On 11/29/23 8:54 AM, Yonghong Song wrote:
Bpf cpu=v4 support is introduced in [1] and Commit 4cd58e9af8b9
("bpf: Support new 32bit offset jmp instruction") added support for new
32bit offset jmp instruction. Unfortunately, in function
bpf_adj_delta_to_off(), for new branch insn with 32bit offset, the offset
(plus/minor a small delta) compares to 16-bit offset bound
[S16_MIN, S16_MAX], which caused the following verification failure:
   $ ./test_progs-cpuv4 -t verif_scale_pyperf180
   ...
   insn 10 cannot be patched due to 16-bit range
   ...
   libbpf: failed to load object 'pyperf180.bpf.o'
   scale_test:FAIL:expect_success unexpected error: -12 (errno 12)
   #405     verif_scale_pyperf180:FAIL

Note that due to recent llvm18 development, the patch [2] (already applied
in bpf-next) needs to be applied to bpf tree for testing purpose.

The fix is rather simple. For 32bit offset branch insn, the adjusted
offset compares to [S32_MIN, S32_MAX] and then verification succeeded.

   [1] https://lore.kernel.org/all/20230728011143.3710005-1-yonghong.song@xxxxxxxxx    [2] https://lore.kernel.org/bpf/20231110193644.3130906-1-yonghong.song@xxxxxxxxx

Fixes: 4cd58e9af8b9 ("bpf: Support new 32bit offset jmp instruction")
Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx>
---
  kernel/bpf/core.c | 9 ++++++---
  1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index cd3afe57ece3..74f2fd48148c 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -371,14 +371,17 @@ static int bpf_adj_delta_to_imm(struct bpf_insn *insn, u32 pos, s32 end_old,   static int bpf_adj_delta_to_off(struct bpf_insn *insn, u32 pos, s32 end_old,
                  s32 end_new, s32 curr, const bool probe_pass)
  {
-    const s32 off_min = S16_MIN, off_max = S16_MAX;
+    s32 off_min = S16_MIN, off_max = S16_MAX;
      s32 delta = end_new - end_old;
      s32 off;

These should all be converted to s64, no? E.g. further below
the test will never trigger then for jmp32:

       if (off < off_min || off > off_max)
                return -ERANGE;


good point! Let us use s64 for potential overflows.
Will send v2 soon.


-    if (insn->code == (BPF_JMP32 | BPF_JA))
+    if (insn->code == (BPF_JMP32 | BPF_JA)) {
          off = insn->imm;
-    else
+        off_min = S32_MIN;
+        off_max = S32_MAX;
+    } else {
          off = insn->off;
+    }
        if (curr < pos && curr + off + 1 >= end_old)
          off += delta;






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