netdev - [PATCH v3 6/8] bpf ppc64: Access only if addr is kernel address

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Message-Id: <20210921132943.489732-7-hbathini@linux.ibm.com>
Date:   Tue, 21 Sep 2021 18:59:41 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     naveen.n.rao@...ux.ibm.com, christophe.leroy@...roup.eu,
        mpe@...erman.id.au, ast@...nel.org, daniel@...earbox.net
Cc:     paulus@...ba.org, andrii@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, netdev@...r.kernel.org, bpf@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Hari Bathini <hbathini@...ux.ibm.com>
Subject: [PATCH v3 6/8] bpf ppc64: Access only if addr is kernel address

From: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>

On PPC64 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 is kernel address, 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@...nel.org>
Signed-off-by: Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Signed-off-by: Hari Bathini <hbathini@...ux.ibm.com>
---

Changes in v3:
* Used is_kernel_addr() logic instead of using TASK_SIZE_MAX check
  all the time.
* Addressed other comments from Christophe.


 arch/powerpc/net/bpf_jit_comp64.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index 506934c13ef7..06e1206a4266 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -734,6 +734,35 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 		case BPF_LDX | BPF_MEM | BPF_DW:
 			fallthrough;
 		case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
+			/*
+			 * As PTR_TO_BTF_ID that uses BPF_PROBE_MEM mode could either be a valid
+			 * kernel pointer or NULL but not a userspace address, execute BPF_PROBE_MEM
+			 * load only if addr is kernel address (see is_kernel_addr()), otherwise
+			 * set dst_reg=0 and move on.
+			 */
+			if (BPF_MODE(code) == BPF_PROBE_MEM) {
+				EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off));
+#ifdef CONFIG_PPC_BOOK3E_64
+				PPC_LI64(b2p[TMP_REG_2], 0x8000000000000000ul);
+#elif defined(CONFIG_PPC_BOOK3S_64)
+				PPC_LI64(b2p[TMP_REG_2], PAGE_OFFSET);
+#else
+				PPC_LI64(b2p[TMP_REG_2], TASK_SIZE);
+#endif
+				EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2]));
+				PPC_BCC(COND_GT, (ctx->idx + 4) * 4);
+				EMIT(PPC_RAW_LI(dst_reg, 0));
+				/*
+				 * Check if 'off' is word aligned because PPC_BPF_LL()
+				 * (BPF_DW case) generates two instructions if 'off' is not
+				 * word-aligned and one instruction otherwise.
+				 */
+				if (BPF_SIZE(code) == BPF_DW && (off & 3))
+					PPC_JMP((ctx->idx + 3) * 4);
+				else
+					PPC_JMP((ctx->idx + 2) * 4);
+			}
+
 			switch (size) {
 			case BPF_B:
 				EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
-- 
2.31.1