On 11/23/22 3:15 PM, Hao Sun wrote:
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.
Thanks for looking into this! It's a bit unfortunate that this will need changes in every BPF JIT. Have you thought about a generic solution which would not require changes in JITs? Given this is for debugging and finding mischecking/bugs in the verifier, can't we reuse interpreter for this and only implement it there? I would be curious if we could achieve the same result from [3] with such approach.
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
Thanks, Daniel
