[PATCH bpf-next 0/3] bpf: Add LDX/STX/ST sanitize in jited BPF progs

Hao Sun
2022-11-23T14:17:13Z

The verifier sometimes makes mistakes[1][2] that may be exploited to
achieve arbitrary read/write. Currently, syzbot is continuously testing
bpf, and can find memory issues in bpf syscalls, but it can hardly find
mischecking/bugs in the verifier. We need runtime checks like KASAN in
BPF programs for this. This patch series implements address sanitize
in jited BPF progs for testing purpose, so that tools like syzbot can
find interesting bugs in the verifier automatically by, if possible,
generating and executing BPF programs that bypass the verifier but have
memory issues, then triggering this sanitizing.

The idea is to dispatch read/write addr of a BPF program to the kernel
functions that are instrumented by KASAN, to achieve indirect checking.
Indirect checking is adopted because this is much simple, instrument
direct checking like compilers makes the jit much more complex. The
main step is: back up R0&R1 and store addr in R1, and then insert the
checking function before load/store insns, during bpf_misc_fixup(), and
finally in the jit stage, backup R1~R5 to make sure the checking funcs
won't corrupt regs states. An extra Kconfig option is used to enable
this, so normal use case won't be impacted at all.

Also, not all ldx/stx/st are instrumented. Insns rewrote by other fixup
or conversion passes that use BPF_REG_AX are skipped, because that
conflicts with us; insns whose access addr is specified by R10 are also
skipped because they are trivial to verify.

Patch1 sanitizes st/stx insns, and Patch2 sanitizes ldx insns, Patch3 adds
selftests for instrumentation in each possible case, and all new/existing
selftests for the verifier can pass. Also, a BPF prog that also exploits
CVE-2022-23222 to achieve OOB read is provided[3], this can be perfertly
captured with this patch series.

I haven't found a better way to back up the regs before executing the
checking functions, and have to store them on the stack. Comments and
advice are surely welcome.

[1] http://bit.do/CVE-2021-3490
[2] http://bit.do/CVE-2022-23222
[3] OOB-read: https://pastebin.com/raw/Ee1Cw492

Hao Sun (3):
bpf: Sanitize STX/ST in jited BPF progs with KASAN
bpf: Sanitize LDX in jited BPF progs with KASAN
selftests/bpf: Add tests for LDX/STX/ST sanitize

arch/x86/net/bpf_jit_comp.c | 34 ++
include/linux/bpf.h | 14 +
kernel/bpf/Kconfig | 14 +
kernel/bpf/verifier.c | 190 +++++++++++
.../selftests/bpf/verifier/sanitize_st_ldx.c | 323 ++++++++++++++++++
5 files changed, 575 insertions(+)
create mode 100644 tools/testing/selftests/bpf/verifier/sanitize_st_ldx.c

base-commit: 8a2162a9227dda936a21fe72014a9931a3853a7b
--
2.38.1