[PATCH 5.15 158/177] bpf: Fix extable address check.

Greg Kroah-Hartman
2021-12-20T15:18:39Z

From: Alexei Starovoitov <ast@kernel.org>

commit 588a25e92458c6efeb7a261d5ca5726f5de89184 upstream.

The verifier checks that PTR_TO_BTF_ID pointer is either valid or NULL,
but it cannot distinguish IS_ERR pointer from valid one.

When offset is added to IS_ERR pointer it may become small positive
value which is a user address that is not handled by extable logic
and has to be checked for at the runtime.

Tighten BPF_PROBE_MEM pointer check code to prevent this case.

Fixes: 4c5de127598e ("bpf: Emit explicit NULL pointer checks for PROBE_LDX instructions.")
Reported-by: Lorenzo Fontana <lorenzo.fontana@elastic.co>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/net/bpf_jit_comp.c | 49 +++++++++++++++++++++++++++++++++++++-------
1 file changed, 42 insertions(+), 7 deletions(-)

--- a/arch/x86/net/bpf_jit_comp.c
+++ b/arch/x86/net/bpf_jit_comp.c
@@ -1280,19 +1280,54 @@ st: if (is_imm8(insn->off))
case BPF_LDX | BPF_MEM | BPF_DW:
case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
- /* test src_reg, src_reg */
- maybe_emit_mod(&prog, src_reg, src_reg, true); /* always 1 byte */
- EMIT2(0x85, add_2reg(0xC0, src_reg, src_reg));
- /* jne start_of_ldx */
- EMIT2(X86_JNE, 0);
+ /* Though the verifier prevents negative insn->off in BPF_PROBE_MEM
+ * add abs(insn->off) to the limit to make sure that negative
+ * offset won't be an issue.
+ * insn->off is s16, so it won't affect valid pointers.
+ */
+ u64 limit = TASK_SIZE_MAX + PAGE_SIZE + abs(insn->off);
+ u8 *end_of_jmp1, *end_of_jmp2;
+
+ /* Conservatively check that src_reg + insn->off is a kernel address:
+ * 1. src_reg + insn->off >= limit
+ * 2. src_reg + insn->off doesn't become small positive.
+ * Cannot do src_reg + insn->off >= limit in one branch,
+ * since it needs two spare registers, but JIT has only one.
+ */
+
+ /* movabsq r11, limit */
+ EMIT2(add_1mod(0x48, AUX_REG), add_1reg(0xB8, AUX_REG));
+ EMIT((u32)limit, 4);
+ EMIT(limit >> 32, 4);
+ /* cmp src_reg, r11 */
+ maybe_emit_mod(&prog, src_reg, AUX_REG, true);
+ EMIT2(0x39, add_2reg(0xC0, src_reg, AUX_REG));
+ /* if unsigned '<' goto end_of_jmp2 */
+ EMIT2(X86_JB, 0);
+ end_of_jmp1 = prog;
+
+ /* mov r11, src_reg */
+ emit_mov_reg(&prog, true, AUX_REG, src_reg);
+ /* add r11, insn->off */
+ maybe_emit_1mod(&prog, AUX_REG, true);
+ EMIT2_off32(0x81, add_1reg(0xC0, AUX_REG), insn->off);
+ /* jmp if not carry to start_of_ldx
+ * Otherwise ERR_PTR(-EINVAL) + 128 will be the user addr
+ * that has to be rejected.
+ */
+ EMIT2(0x73 /* JNC */, 0);
+ end_of_jmp2 = prog;
+
/* xor dst_reg, dst_reg */
emit_mov_imm32(&prog, false, dst_reg, 0);
/* jmp byte_after_ldx */
EMIT2(0xEB, 0);

- /* populate jmp_offset for JNE above */
- temp[4] = prog - temp - 5 /* sizeof(test + jne) */;
+ /* populate jmp_offset for JB above to jump to xor dst_reg */
+ end_of_jmp1[-1] = end_of_jmp2 - end_of_jmp1;
+ /* populate jmp_offset for JNC above to jump to start_of_ldx */
start_of_ldx = prog;
+ end_of_jmp2[-1] = start_of_ldx - end_of_jmp2;
}
emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {