Le 21/09/2021 à 15:29, Hari Bathini a écrit :
From: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
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@xxxxxxxxxx>
Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
Signed-off-by: Hari Bathini <hbathini@xxxxxxxxxxxxx>
---
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
It is better to use IS_ENABLED() whenever possible,
if (IS_ENABLED((CONFIG_PPC_BOOK3E_64))
PPC_LI64(b2p[TMP_REG_2], 0x8000000000000000ul);
else
PPC_LI64(b2p[TMP_REG_2], PAGE_OFFSET);
+ 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
PPC64 is either CONFIG_PPC_BOOK3S_64 or CONFIG_PPC_BOOK3E_64. The else
is PPC32.
+ 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));
