[PATCH 5.8 072/118] arm64: bpf: Fix branch offset in JIT

From: Greg Kroah-Hartman
Date: Mon Sep 21 2020 - 12:55:52 EST

From: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>

[ Upstream commit 32f6865c7aa3c422f710903baa6eb81abc6f559b ]

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>
Co-developed-by: Yauheni Kaliuta <yauheni.kaliuta@xxxxxxxxxx>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Signed-off-by: Yauheni Kaliuta <yauheni.kaliuta@xxxxxxxxxx>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@xxxxxxxxxx>
Acked-by: Will Deacon <will@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20200917084925.177348-1-ilias.apalodimas@xxxxxxxxxx
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
arch/arm64/net/bpf_jit_comp.c | 43 +++++++++++++++++++++++++----------
1 file changed, 31 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 3cb25b43b368e..1b2d82755e41f 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -141,14 +141,17 @@ 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)
@@ -578,7 +581,7 @@ emit_bswap_uxt:

/* 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;
@@ -605,7 +608,7 @@ emit_bswap_uxt:
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:
@@ -837,10 +840,21 @@ 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++;
@@ -848,11 +862,16 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
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;
}
@@ -928,7 +947,7 @@ 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;
@@ -1008,7 +1027,7 @@ skip_init_ctx:
prog->jited_len = image_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);
--
2.25.1