Re: [PATCH v3] arm64: bpf: Fix branch offset in JIT

[Will Deacon]

On Thu, Sep 17, 2020 at 11:49:25AM +0300, Ilias Apalodimas wrote:
> Running the eBPF test_verifier leads to random errors looking like this:
> 
> [ 6525.735488] Unexpected kernel BRK exception at EL1
> [ 6525.735502] Internal error: ptrace BRK handler: f2000100 [#1] SMP
> [ 6525.741609] Modules linked in: nls_utf8 cifs libdes libarc4 dns_resolver fscache binfmt_misc nls_ascii nls_cp437 vfat fat aes_ce_blk crypto_simd cryptd aes_ce_cipher ghash_ce gf128mul efi_pstore sha2_ce sha256_arm64 sha1_ce evdev efivars efivarfs ip_tables x_tables autofs4 btrfs blake2b_generic xor xor_neon zstd_compress raid6_pq libcrc32c crc32c_generic ahci xhci_pci libahci xhci_hcd igb libata i2c_algo_bit nvme realtek usbcore nvme_core scsi_mod t10_pi netsec mdio_devres of_mdio gpio_keys fixed_phy libphy gpio_mb86s7x
> [ 6525.787760] CPU: 3 PID: 7881 Comm: test_verifier Tainted: G        W         5.9.0-rc1+ #47
> [ 6525.796111] Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #1 Jun  6 2020
> [ 6525.804812] pstate: 20000005 (nzCv daif -PAN -UAO BTYPE=--)
> [ 6525.810390] pc : bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
> [ 6525.815613] lr : bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
> [ 6525.820832] sp : ffff8000130cbb80
> [ 6525.824141] x29: ffff8000130cbbb0 x28: 0000000000000000
> [ 6525.829451] x27: 000005ef6fcbf39b x26: 0000000000000000
> [ 6525.834759] x25: ffff8000130cbb80 x24: ffff800011dc7038
> [ 6525.840067] x23: ffff8000130cbd00 x22: ffff0008f624d080
> [ 6525.845375] x21: 0000000000000001 x20: ffff800011dc7000
> [ 6525.850682] x19: 0000000000000000 x18: 0000000000000000
> [ 6525.855990] x17: 0000000000000000 x16: 0000000000000000
> [ 6525.861298] x15: 0000000000000000 x14: 0000000000000000
> [ 6525.866606] x13: 0000000000000000 x12: 0000000000000000
> [ 6525.871913] x11: 0000000000000001 x10: ffff8000000a660c
> [ 6525.877220] x9 : ffff800010951810 x8 : ffff8000130cbc38
> [ 6525.882528] x7 : 0000000000000000 x6 : 0000009864cfa881
> [ 6525.887836] x5 : 00ffffffffffffff x4 : 002880ba1a0b3e9f
> [ 6525.893144] x3 : 0000000000000018 x2 : ffff8000000a4374
> [ 6525.898452] x1 : 000000000000000a x0 : 0000000000000009
> [ 6525.903760] Call trace:
> [ 6525.906202]  bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
> [ 6525.911076]  bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
> [ 6525.915957]  bpf_dispatcher_xdp_func+0x14/0x20
> [ 6525.920398]  bpf_test_run+0x70/0x1b0
> [ 6525.923969]  bpf_prog_test_run_xdp+0xec/0x190
> [ 6525.928326]  __do_sys_bpf+0xc88/0x1b28
> [ 6525.932072]  __arm64_sys_bpf+0x24/0x30
> [ 6525.935820]  el0_svc_common.constprop.0+0x70/0x168
> [ 6525.940607]  do_el0_svc+0x28/0x88
> [ 6525.943920]  el0_sync_handler+0x88/0x190
> [ 6525.947838]  el0_sync+0x140/0x180
> [ 6525.951154] Code: d4202000 d4202000 d4202000 d4202000 (d4202000)
> [ 6525.957249] ---[ end trace cecc3f93b14927e2 ]---
> 
> The reason is the offset[] creation and later usage, while building
> the eBPF body. The code currently omits the first instruction, since
> build_insn() will increase our ctx->idx before saving it.
> That was fine up until bounded eBPF loops were introduced. After that
> introduction, offset[0] must be the offset of the end of prologue which
> is the start of the 1st insn while, offset[n] holds the
> offset of the end of n-th insn.
> 
> When "taken loop with back jump to 1st insn" test runs, it will
> eventually call bpf2a64_offset(-1, 2, ctx). Since negative indexing is
> permitted, the current outcome depends on the value stored in
> ctx->offset[-1], which has nothing to do with our array.
> If the value happens to be 0 the tests will work. If not this error
> triggers.
> 
> commit 7c2e988f400e ("bpf: fix x64 JIT code generation for jmp to 1st insn")
> fixed an indentical bug on x86 when eBPF bounded loops were introduced.
> 
> So let's fix it by creating the ctx->offset[] differently. Track the
> beginning of instruction and account for the extra instruction while
> calculating the arm instruction offsets.
> 
> Fixes: 2589726d12a1 ("bpf: introduce bounded loops")
> Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> Reported-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> Co-developed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
> Co-developed-by: Yauheni Kaliuta <yauheni.kaliuta@xxxxxxxxxx>
> Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@xxxxxxxxxx>
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>

Acked-by: Will Deacon <will@xxxxxxxxxx>

Catalin -- do you want to take this as a fix?

Will