From 66e13b615a0ce76b785d780ecc9776ba71983629 Mon Sep 17 00:00:00 2001 From: Puranjay Mohan Date: Wed, 24 Apr 2024 10:02:08 +0000 Subject: bpf: verifier: prevent userspace memory access With BPF_PROBE_MEM, BPF allows de-referencing an untrusted pointer. To thwart invalid memory accesses, the JITs add an exception table entry for all such accesses. But in case the src_reg + offset is a userspace address, the BPF program might read that memory if the user has mapped it. Make the verifier add guard instructions around such memory accesses and skip the load if the address falls into the userspace region. The JITs need to implement bpf_arch_uaddress_limit() to define where the userspace addresses end for that architecture or TASK_SIZE is taken as default. The implementation is as follows: REG_AX = SRC_REG if(offset) REG_AX += offset; REG_AX >>= 32; if (REG_AX <= (uaddress_limit >> 32)) DST_REG = 0; else DST_REG = *(size *)(SRC_REG + offset); Comparing just the upper 32 bits of the load address with the upper 32 bits of uaddress_limit implies that the values are being aligned down to a 4GB boundary before comparison. The above means that all loads with address <= uaddress_limit + 4GB are skipped. This is acceptable because there is a large hole (much larger than 4GB) between userspace and kernel space memory, therefore a correctly functioning BPF program should not access this 4GB memory above the userspace. Let's analyze what this patch does to the following fentry program dereferencing an untrusted pointer: SEC("fentry/tcp_v4_connect") int BPF_PROG(fentry_tcp_v4_connect, struct sock *sk) { *(volatile long *)sk; return 0; } BPF Program before | BPF Program after ------------------ | ----------------- 0: (79) r1 = *(u64 *)(r1 +0) 0: (79) r1 = *(u64 *)(r1 +0) ----------------------------------------------------------------------- 1: (79) r1 = *(u64 *)(r1 +0) --\ 1: (bf) r11 = r1 ----------------------------\ \ 2: (77) r11 >>= 32 2: (b7) r0 = 0 \ \ 3: (b5) if r11 <= 0x8000 goto pc+2 3: (95) exit \ \-> 4: (79) r1 = *(u64 *)(r1 +0) \ 5: (05) goto pc+1 \ 6: (b7) r1 = 0 \-------------------------------------- 7: (b7) r0 = 0 8: (95) exit As you can see from above, in the best case (off=0), 5 extra instructions are emitted. Now, we analyze the same program after it has gone through the JITs of ARM64 and RISC-V architectures. We follow the single load instruction that has the untrusted pointer and see what instrumentation has been added around it. x86-64 JIT ========== JIT's Instrumentation (upstream) --------------------- 0: nopl 0x0(%rax,%rax,1) 5: xchg %ax,%ax 7: push %rbp 8: mov %rsp,%rbp b: mov 0x0(%rdi),%rdi --------------------------------- f: movabs $0x800000000000,%r11 19: cmp %r11,%rdi 1c: jb 0x000000000000002a 1e: mov %rdi,%r11 21: add $0x0,%r11 28: jae 0x000000000000002e 2a: xor %edi,%edi 2c: jmp 0x0000000000000032 2e: mov 0x0(%rdi),%rdi --------------------------------- 32: xor %eax,%eax 34: leave 35: ret The x86-64 JIT already emits some instructions to protect against user memory access. This patch doesn't make any changes for the x86-64 JIT. ARM64 JIT ========= No Intrumentation Verifier's Instrumentation (upstream) (This patch) ----------------- -------------------------- 0: add x9, x30, #0x0 0: add x9, x30, #0x0 4: nop 4: nop 8: paciasp 8: paciasp c: stp x29, x30, [sp, #-16]! c: stp x29, x30, [sp, #-16]! 10: mov x29, sp 10: mov x29, sp 14: stp x19, x20, [sp, #-16]! 14: stp x19, x20, [sp, #-16]! 18: stp x21, x22, [sp, #-16]! 18: stp x21, x22, [sp, #-16]! 1c: stp x25, x26, [sp, #-16]! 1c: stp x25, x26, [sp, #-16]! 20: stp x27, x28, [sp, #-16]! 20: stp x27, x28, [sp, #-16]! 24: mov x25, sp 24: mov x25, sp 28: mov x26, #0x0 28: mov x26, #0x0 2c: sub x27, x25, #0x0 2c: sub x27, x25, #0x0 30: sub sp, sp, #0x0 30: sub sp, sp, #0x0 34: ldr x0, [x0] 34: ldr x0, [x0] -------------------------------------------------------------------------------- 38: ldr x0, [x0] ----------\ 38: add x9, x0, #0x0 -----------------------------------\\ 3c: lsr x9, x9, #32 3c: mov x7, #0x0 \\ 40: cmp x9, #0x10, lsl #12 40: mov sp, sp \\ 44: b.ls 0x0000000000000050 44: ldp x27, x28, [sp], #16 \\--> 48: ldr x0, [x0] 48: ldp x25, x26, [sp], #16 \ 4c: b 0x0000000000000054 4c: ldp x21, x22, [sp], #16 \ 50: mov x0, #0x0 50: ldp x19, x20, [sp], #16 \--------------------------------------- 54: ldp x29, x30, [sp], #16 54: mov x7, #0x0 58: add x0, x7, #0x0 58: mov sp, sp 5c: autiasp 5c: ldp x27, x28, [sp], #16 60: ret 60: ldp x25, x26, [sp], #16 64: nop 64: ldp x21, x22, [sp], #16 68: ldr x10, 0x0000000000000070 68: ldp x19, x20, [sp], #16 6c: br x10 6c: ldp x29, x30, [sp], #16 70: add x0, x7, #0x0 74: autiasp 78: ret 7c: nop 80: ldr x10, 0x0000000000000088 84: br x10 There are 6 extra instructions added in ARM64 in the best case. This will become 7 in the worst case (off != 0). RISC-V JIT (RISCV_ISA_C Disabled) ========== No Intrumentation Verifier's Instrumentation (upstream) (This patch) ----------------- -------------------------- 0: nop 0: nop 4: nop 4: nop 8: li a6, 33 8: li a6, 33 c: addi sp, sp, -16 c: addi sp, sp, -16 10: sd s0, 8(sp) 10: sd s0, 8(sp) 14: addi s0, sp, 16 14: addi s0, sp, 16 18: ld a0, 0(a0) 18: ld a0, 0(a0) --------------------------------------------------------------- 1c: ld a0, 0(a0) --\ 1c: mv t0, a0 --------------------------\ \ 20: srli t0, t0, 32 20: li a5, 0 \ \ 24: lui t1, 4096 24: ld s0, 8(sp) \ \ 28: sext.w t1, t1 28: addi sp, sp, 16 \ \ 2c: bgeu t1, t0, 12 2c: sext.w a0, a5 \ \--> 30: ld a0, 0(a0) 30: ret \ 34: j 8 \ 38: li a0, 0 \------------------------------ 3c: li a5, 0 40: ld s0, 8(sp) 44: addi sp, sp, 16 48: sext.w a0, a5 4c: ret There are 7 extra instructions added in RISC-V. Fixes: 800834285361 ("bpf, arm64: Add BPF exception tables") Reported-by: Breno Leitao Suggested-by: Alexei Starovoitov Acked-by: Ilya Leoshkevich Signed-off-by: Puranjay Mohan Link: https://lore.kernel.org/r/20240424100210.11982-2-puranjay@kernel.org Signed-off-by: Alexei Starovoitov --- arch/x86/net/bpf_jit_comp.c | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'arch/x86/net') diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index df5fac428408..b520b66ad14b 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -3473,3 +3473,9 @@ bool bpf_jit_supports_ptr_xchg(void) { return true; } + +/* x86-64 JIT emits its own code to filter user addresses so return 0 here */ +u64 bpf_arch_uaddress_limit(void) +{ + return 0; +} -- cgit v1.2.3-70-g09d2 From b599d7d26d6ad1fc9975218574bc2ca6d0293cfd Mon Sep 17 00:00:00 2001 From: Puranjay Mohan Date: Wed, 24 Apr 2024 10:02:09 +0000 Subject: bpf, x86: Fix PROBE_MEM runtime load check When a load is marked PROBE_MEM - e.g. due to PTR_UNTRUSTED access - the address being loaded from is not necessarily valid. The BPF jit sets up exception handlers for each such load which catch page faults and 0 out the destination register. If the address for the load is outside kernel address space, the load will escape the exception handling and crash the kernel. To prevent this from happening, the emits some instruction to verify that addr is > end of userspace addresses. x86 has a legacy vsyscall ABI where a page at address 0xffffffffff600000 is mapped with user accessible permissions. The addresses in this page are considered userspace addresses by the fault handler. Therefore, a BPF program accessing this page will crash the kernel. This patch fixes the runtime checks to also check that the PROBE_MEM address is below VSYSCALL_ADDR. Example BPF program: SEC("fentry/tcp_v4_connect") int BPF_PROG(fentry_tcp_v4_connect, struct sock *sk) { *(volatile unsigned long *)&sk->sk_tsq_flags; return 0; } BPF Assembly: 0: (79) r1 = *(u64 *)(r1 +0) 1: (79) r1 = *(u64 *)(r1 +344) 2: (b7) r0 = 0 3: (95) exit x86-64 JIT ========== BEFORE AFTER ------ ----- 0: nopl 0x0(%rax,%rax,1) 0: nopl 0x0(%rax,%rax,1) 5: xchg %ax,%ax 5: xchg %ax,%ax 7: push %rbp 7: push %rbp 8: mov %rsp,%rbp 8: mov %rsp,%rbp b: mov 0x0(%rdi),%rdi b: mov 0x0(%rdi),%rdi ------------------------------------------------------------------------------- f: movabs $0x100000000000000,%r11 f: movabs $0xffffffffff600000,%r10 19: add $0x2a0,%rdi 19: mov %rdi,%r11 20: cmp %r11,%rdi 1c: add $0x2a0,%r11 23: jae 0x0000000000000029 23: sub %r10,%r11 25: xor %edi,%edi 26: movabs $0x100000000a00000,%r10 27: jmp 0x000000000000002d 30: cmp %r10,%r11 29: mov 0x0(%rdi),%rdi 33: ja 0x0000000000000039 --------------------------------\ 35: xor %edi,%edi 2d: xor %eax,%eax \ 37: jmp 0x0000000000000040 2f: leave \ 39: mov 0x2a0(%rdi),%rdi 30: ret \-------------------------------------------- 40: xor %eax,%eax 42: leave 43: ret Signed-off-by: Puranjay Mohan Link: https://lore.kernel.org/r/20240424100210.11982-3-puranjay@kernel.org Signed-off-by: Alexei Starovoitov --- arch/x86/net/bpf_jit_comp.c | 57 ++++++++++++++++++++------------------------- 1 file changed, 25 insertions(+), 32 deletions(-) (limited to 'arch/x86/net') diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c index b520b66ad14b..59cbc94b6e69 100644 --- a/arch/x86/net/bpf_jit_comp.c +++ b/arch/x86/net/bpf_jit_comp.c @@ -1807,36 +1807,41 @@ populate_extable: if (BPF_MODE(insn->code) == BPF_PROBE_MEM || BPF_MODE(insn->code) == BPF_PROBE_MEMSX) { /* Conservatively check that src_reg + insn->off is a kernel address: - * src_reg + insn->off >= TASK_SIZE_MAX + PAGE_SIZE - * src_reg is used as scratch for src_reg += insn->off and restored - * after emit_ldx if necessary + * src_reg + insn->off > TASK_SIZE_MAX + PAGE_SIZE + * and + * src_reg + insn->off < VSYSCALL_ADDR */ - u64 limit = TASK_SIZE_MAX + PAGE_SIZE; + u64 limit = TASK_SIZE_MAX + PAGE_SIZE - VSYSCALL_ADDR; u8 *end_of_jmp; - /* At end of these emitted checks, insn->off will have been added - * to src_reg, so no need to do relative load with insn->off offset - */ - insn_off = 0; + /* movabsq r10, VSYSCALL_ADDR */ + emit_mov_imm64(&prog, BPF_REG_AX, (long)VSYSCALL_ADDR >> 32, + (u32)(long)VSYSCALL_ADDR); - /* movabsq r11, limit */ - EMIT2(add_1mod(0x48, AUX_REG), add_1reg(0xB8, AUX_REG)); - EMIT((u32)limit, 4); - EMIT(limit >> 32, 4); + /* mov src_reg, r11 */ + EMIT_mov(AUX_REG, src_reg); if (insn->off) { - /* add src_reg, insn->off */ - maybe_emit_1mod(&prog, src_reg, true); - EMIT2_off32(0x81, add_1reg(0xC0, src_reg), insn->off); + /* add r11, insn->off */ + maybe_emit_1mod(&prog, AUX_REG, true); + EMIT2_off32(0x81, add_1reg(0xC0, AUX_REG), insn->off); } - /* cmp src_reg, r11 */ - maybe_emit_mod(&prog, src_reg, AUX_REG, true); - EMIT2(0x39, add_2reg(0xC0, src_reg, AUX_REG)); + /* sub r11, r10 */ + maybe_emit_mod(&prog, AUX_REG, BPF_REG_AX, true); + EMIT2(0x29, add_2reg(0xC0, AUX_REG, BPF_REG_AX)); + + /* movabsq r10, limit */ + emit_mov_imm64(&prog, BPF_REG_AX, (long)limit >> 32, + (u32)(long)limit); + + /* cmp r10, r11 */ + maybe_emit_mod(&prog, AUX_REG, BPF_REG_AX, true); + EMIT2(0x39, add_2reg(0xC0, AUX_REG, BPF_REG_AX)); - /* if unsigned '>=', goto load */ - EMIT2(X86_JAE, 0); + /* if unsigned '>', goto load */ + EMIT2(X86_JA, 0); end_of_jmp = prog; /* xor dst_reg, dst_reg */ @@ -1862,18 +1867,6 @@ populate_extable: /* populate jmp_offset for JMP above */ start_of_ldx[-1] = prog - start_of_ldx; - if (insn->off && src_reg != dst_reg) { - /* sub src_reg, insn->off - * Restore src_reg after "add src_reg, insn->off" in prev - * if statement. But if src_reg == dst_reg, emit_ldx - * above already clobbered src_reg, so no need to restore. - * If add src_reg, insn->off was unnecessary, no need to - * restore either. - */ - maybe_emit_1mod(&prog, src_reg, true); - EMIT2_off32(0x81, add_1reg(0xE8, src_reg), insn->off); - } - if (!bpf_prog->aux->extable) break; -- cgit v1.2.3-70-g09d2