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bpf: Fix extable address check. 

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: 4c5de12759 ("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>
This commit is contained in:
Alexei Starovoitov 2021-12-14 19:25:13 -08:00 committed by Daniel Borkmann
parent 433956e912
commit 588a25e924
1 changed files with 42 additions and 7 deletions
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49
arch/x86/net/bpf_jit_comp.c
View File

@ -1252,19 +1252,54 @@ st: if (is_imm8(insn->off))
case BPF_LDX | BPF_MEM | BPF_DW: case BPF_LDX | BPF_MEM | BPF_DW:
case BPF_LDX | BPF_PROBE_MEM | BPF_DW: case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
if (BPF_MODE(insn->code) == BPF_PROBE_MEM) { if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
/* test src_reg, src_reg */ /* Though the verifier prevents negative insn->off in BPF_PROBE_MEM
maybe_emit_mod(&prog, src_reg, src_reg, true); /* always 1 byte */ * add abs(insn->off) to the limit to make sure that negative
EMIT2(0x85, add_2reg(0xC0, src_reg, src_reg)); * offset won't be an issue.
/* jne start_of_ldx */ * insn->off is s16, so it won't affect valid pointers.
EMIT2(X86_JNE, 0); */
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 */ /* xor dst_reg, dst_reg */
emit_mov_imm32(&prog, false, dst_reg, 0); emit_mov_imm32(&prog, false, dst_reg, 0);
/* jmp byte_after_ldx */ /* jmp byte_after_ldx */
EMIT2(0xEB, 0); EMIT2(0xEB, 0);
/* populate jmp_offset for JNE above */ /* populate jmp_offset for JB above to jump to xor dst_reg */
temp[4] = prog - temp - 5 /* sizeof(test + jne) */; 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; 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); emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
if (BPF_MODE(insn->code) == BPF_PROBE_MEM) { if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {