Re: [PATCH 4/4] bpf powerpc: Add addr > TASK_SIZE_MAX explicit check
From: Christophe Leroy
Date: Tue Jul 06 2021 - 06:01:02 EST
Le 06/07/2021 à 09:32, Ravi Bangoria a écrit :
On PowerPC with KUAP enabled, any kernel code which wants to
access userspace needs to be surrounded by disable-enable KUAP.
But that is not happening for BPF_PROBE_MEM load instruction.
So, when BPF program tries to access invalid userspace address,
page-fault handler considers it as bad KUAP fault:
Kernel attempted to read user page (d0000000) - exploit attempt? (uid: 0)
Considering the fact that PTR_TO_BTF_ID (which uses BPF_PROBE_MEM
mode) could either be a valid kernel pointer or NULL but should
never be a pointer to userspace address, execute BPF_PROBE_MEM load
only if addr > TASK_SIZE_MAX, otherwise set dst_reg=0 and move on.
This will catch NULL, valid or invalid userspace pointers. Only bad
kernel pointer will be handled by BPF exception table.
[Alexei suggested for x86]
Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
---
arch/powerpc/net/bpf_jit_comp64.c | 38 +++++++++++++++++++++++++++++++
1 file changed, 38 insertions(+)
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 1884c6dca89a..46becae76210 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -753,6 +753,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
/* dst = *(u8 *)(ul) (src + off) */
case BPF_LDX | BPF_MEM | BPF_B:
case BPF_LDX | BPF_PROBE_MEM | BPF_B:
+ if (BPF_MODE(code) == BPF_PROBE_MEM) {
+ EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
+ PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
+ EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
+ PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
+ EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
Prefered way to clear a register is to do 'li reg, 0'
+ PPC_JMP((ctx->idx + 2) * 4);
+ }
EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
if (insn_is_zext(&insn[i + 1]))
addrs[++i] = ctx->idx * 4;
@@ -763,6 +771,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
/* dst = *(u16 *)(ul) (src + off) */
case BPF_LDX | BPF_MEM | BPF_H:
case BPF_LDX | BPF_PROBE_MEM | BPF_H:
+ if (BPF_MODE(code) == BPF_PROBE_MEM) {
+ EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
+ PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
+ EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
+ PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
+ EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
+ PPC_JMP((ctx->idx + 2) * 4);
+ }
That code seems strictly identical to the previous one and the next one.
Can you refactor in a function ?
EMIT(PPC_RAW_LHZ(dst_reg, src_reg, off));
if (insn_is_zext(&insn[i + 1]))
addrs[++i] = ctx->idx * 4;
@@ -773,6 +789,14 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
/* dst = *(u32 *)(ul) (src + off) */
case BPF_LDX | BPF_MEM | BPF_W:
case BPF_LDX | BPF_PROBE_MEM | BPF_W:
+ if (BPF_MODE(code) == BPF_PROBE_MEM) {
+ EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
+ PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
+ EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
+ PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
+ EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
+ PPC_JMP((ctx->idx + 2) * 4);
+ }
EMIT(PPC_RAW_LWZ(dst_reg, src_reg, off));
if (insn_is_zext(&insn[i + 1]))
addrs[++i] = ctx->idx * 4;
@@ -783,6 +807,20 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
/* dst = *(u64 *)(ul) (src + off) */
case BPF_LDX | BPF_MEM | BPF_DW:
case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
+ if (BPF_MODE(code) == BPF_PROBE_MEM) {
+ EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
+ PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX);
+ EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
+ if (off % 4)
That test is worth a comment.
And I'd prefer
if (off & 3) {
PPC_BCC(COND_GT, (ctx->idx + 5) * 4);
EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
PPC_JMP((ctx->idx + 3) * 4);
} else {
PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
PPC_JMP((ctx->idx + 2) * 4);
}
+ PPC_BCC(COND_GT, (ctx->idx + 5) * 4);
+ else
+ PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
+ EMIT(PPC_RAW_XOR(dst_reg, dst_reg, dst_reg));
Use PPC_RAW_LI(dst_reg, 0);
+ if (off % 4)
+ PPC_JMP((ctx->idx + 3) * 4);
+ else
+ PPC_JMP((ctx->idx + 2) * 4);
+ }
PPC_BPF_LL(dst_reg, src_reg, off);
ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
if (ret)