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arm64: bpf: Fix branch offset in JIT
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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+ raspberrypi#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 7c2e988 ("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: 2589726 ("bpf: introduce bounded loops")
Reported-by: Naresh Kamboju <[email protected]>
Reported-by: Jiri Olsa <[email protected]>
Co-developed-by: Jean-Philippe Brucker <[email protected]>
Co-developed-by: Yauheni Kaliuta <[email protected]>
Signed-off-by: Jean-Philippe Brucker <[email protected]>
Signed-off-by: Yauheni Kaliuta <[email protected]>
Signed-off-by: Ilias Apalodimas <[email protected]>
Acked-by: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>
  • Loading branch information
@apalos @ctmarinas
apalos authored and ctmarinas committed Sep 17, 2020
1 parent ed888cb commit 32f6865
Showing 1 changed file with 31 additions and 12 deletions.
43 changes: 31 additions & 12 deletions arch/arm64/net/bpf_jit_comp.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -143,14 +143,17 @@ static inline void emit_addr_mov_i64(const int reg, const u64 val,
}
}

static inline int bpf2a64_offset(int bpf_to, int bpf_from,
static inline int bpf2a64_offset(int bpf_insn, int off,
const struct jit_ctx *ctx)
{
int to = ctx->offset[bpf_to];
/* -1 to account for the Branch instruction */
int from = ctx->offset[bpf_from] - 1;

return to - from;
/* BPF JMP offset is relative to the next instruction */
bpf_insn++;
/*
* Whereas arm64 branch instructions encode the offset
* from the branch itself, so we must subtract 1 from the
* instruction offset.
*/
return ctx->offset[bpf_insn + off] - (ctx->offset[bpf_insn] - 1);
}

static void jit_fill_hole(void *area, unsigned int size)
Expand Down Expand Up @@ -642,7 +645,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,

/* JUMP off */
case BPF_JMP | BPF_JA:
jmp_offset = bpf2a64_offset(i + off, i, ctx);
jmp_offset = bpf2a64_offset(i, off, ctx);
check_imm26(jmp_offset);
emit(A64_B(jmp_offset), ctx);
break;
Expand All @@ -669,7 +672,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
case BPF_JMP32 | BPF_JSLE | BPF_X:
emit(A64_CMP(is64, dst, src), ctx);
emit_cond_jmp:
jmp_offset = bpf2a64_offset(i + off, i, ctx);
jmp_offset = bpf2a64_offset(i, off, ctx);
check_imm19(jmp_offset);
switch (BPF_OP(code)) {
case BPF_JEQ:
Expand Down Expand Up @@ -908,22 +911,38 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
const struct bpf_prog *prog = ctx->prog;
int i;

/*
* - offset[0] offset of the end of prologue,
* start of the 1st instruction.
* - offset[1] - offset of the end of 1st instruction,
* start of the 2nd instruction
* [....]
* - offset[3] - offset of the end of 3rd instruction,
* start of 4th instruction
*/
for (i = 0; i < prog->len; i++) {
const struct bpf_insn *insn = &prog->insnsi[i];
int ret;

if (ctx->image == NULL)
ctx->offset[i] = ctx->idx;
ret = build_insn(insn, ctx, extra_pass);
if (ret > 0) {
i++;
if (ctx->image == NULL)
ctx->offset[i] = ctx->idx;
continue;
}
if (ctx->image == NULL)
ctx->offset[i] = ctx->idx;
if (ret)
return ret;
}
/*
* offset is allocated with prog->len + 1 so fill in
* the last element with the offset after the last
* instruction (end of program)
*/
if (ctx->image == NULL)
ctx->offset[i] = ctx->idx;

return 0;
}
Expand Down Expand Up @@ -1002,7 +1021,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
memset(&ctx, 0, sizeof(ctx));
ctx.prog = prog;

ctx.offset = kcalloc(prog->len, sizeof(int), GFP_KERNEL);
ctx.offset = kcalloc(prog->len + 1, sizeof(int), GFP_KERNEL);
if (ctx.offset == NULL) {
prog = orig_prog;
goto out_off;
Expand Down Expand Up @@ -1089,7 +1108,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
prog->jited_len = prog_size;

if (!prog->is_func || extra_pass) {
bpf_prog_fill_jited_linfo(prog, ctx.offset);
bpf_prog_fill_jited_linfo(prog, ctx.offset + 1);
out_off:
kfree(ctx.offset);
kfree(jit_data);
Expand Down

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