The potential issue of kprobe_write_ctx+freplace was mentioned in
"bpf: Disallow !kprobe_write_ctx progs tail-calling kprobe_write_ctx progs" [1].
It is true issue, that the test in patch #3 verifies that kprobe_write_ctx=false
kprobe progs can be abused to modify struct pt_regs via kprobe_write_ctx=true
freplace progs.
When struct pt_regs is modified, bpf_prog_test_run_opts() gets -EFAULT instead
of 0.
test_kprobe_write_ctx:FAIL:bpf_prog_test_run_opts unexpected error: -14 (errno
14)
We will disallow freplace on kprobe programs with different kprobe_write_ctx
values.
However, xdp_has_frags does not cause such severe issue.
The issue of xdp_has_frags+freplace is that it is able to break the backwards
compatibility of XDP.
At the commit f45d5b6ce2e8 ("bpf: generalise tail call map compatibility
check"),
the backwards compatibility was protected against tail calls, but it missed the
freplace case, which is also able to extend XDP progs. If xdp_has_frags=true
freplace progs are allowed to attach to xdp_has_frags=false XDP progs, the
backwards compatibility is broken.
We will disallow freplace on XDP programs with different xdp_has_frags values,
like the way in bpf_prog_map_compatible().
Links:
[1]
https://lore.kernel.org/bpf/cap01t74w4kvmn9bewpqxrk+bqcuxzb6vw1sq_qvny0a4ey-...@mail.gmail.com/
Leon Hwang (3):
bpf: Disallow freplace on XDP with mismatched xdp_has_frags values
bpf: Disallow freplace on kprobe with mismatched kprobe_write_ctx
values
selftests/bpf: Add tests to verify freplace compatibility
kernel/bpf/verifier.c | 12 ++
.../bpf/prog_tests/freplace_compatible.c | 124 ++++++++++++++++++
.../selftests/bpf/progs/freplace_compatible.c | 40 ++++++
3 files changed, 176 insertions(+)
create mode 100644 tools/testing/selftests/bpf/prog_tests/freplace_compatible.c
create mode 100644 tools/testing/selftests/bpf/progs/freplace_compatible.c
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2.53.0
