powerpc/64s: flush L1D after user accesses

IBM Power9 processors can speculatively operate on data in the L1 cache
before it has been completely validated, via a way-prediction mechanism. It
is not possible for an attacker to determine the contents of impermissible
memory using this method, since these systems implement a combination of
hardware and software security measures to prevent scenarios where
protected data could be leaked.

However these measures don't address the scenario where an attacker induces
the operating system to speculatively execute instructions using data that
the attacker controls. This can be used for example to speculatively bypass
"kernel user access prevention" techniques, as discovered by Anthony
Steinhauser of Google's Safeside Project. This is not an attack by itself,
but there is a possibility it could be used in conjunction with
side-channels or other weaknesses in the privileged code to construct an
attack.

This issue can be mitigated by flushing the L1 cache between privilege
boundaries of concern. This patch flushes the L1 cache after user accesses.

This is part of the fix for CVE-2020-4788.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 50 insertions, 0 deletions

diff --git a/arch/powerpc/lib/feature-fixups.c b/arch/powerpc/lib/feature-fixups.c
index 70e83cfd74aa..321c12a9ef6b 100644
--- a/arch/powerpc/lib/feature-fixups.c
+++ b/arch/powerpc/lib/feature-fixups.c
@@ -234,6 +234,56 @@ void do_stf_barrier_fixups(enum stf_barrier_type types)
 	do_stf_exit_barrier_fixups(types);
 }
 
+void do_uaccess_flush_fixups(enum l1d_flush_type types)
+{
+	unsigned int instrs[4], *dest;
+	long *start, *end;
+	int i;
+
+	start = PTRRELOC(&__start___uaccess_flush_fixup);
+	end = PTRRELOC(&__stop___uaccess_flush_fixup);
+
+	instrs[0] = 0x60000000; /* nop */
+	instrs[1] = 0x60000000; /* nop */
+	instrs[2] = 0x60000000; /* nop */
+	instrs[3] = 0x4e800020; /* blr */
+
+	i = 0;
+	if (types == L1D_FLUSH_FALLBACK) {
+		instrs[3] = 0x60000000; /* nop */
+		/* fallthrough to fallback flush */
+	}
+
+	if (types & L1D_FLUSH_ORI) {
+		instrs[i++] = 0x63ff0000; /* ori 31,31,0 speculation barrier */
+		instrs[i++] = 0x63de0000; /* ori 30,30,0 L1d flush*/
+	}
+
+	if (types & L1D_FLUSH_MTTRIG)
+		instrs[i++] = 0x7c12dba6; /* mtspr TRIG2,r0 (SPR #882) */
+
+	for (i = 0; start < end; start++, i++) {
+		dest = (void *)start + *start;
+
+		pr_devel("patching dest %lx\n", (unsigned long)dest);
+
+		patch_instruction((struct ppc_inst *)dest, ppc_inst(instrs[0]));
+
+		patch_instruction((struct ppc_inst *)(dest + 1), ppc_inst(instrs[1]));
+		patch_instruction((struct ppc_inst *)(dest + 2), ppc_inst(instrs[2]));
+		patch_instruction((struct ppc_inst *)(dest + 3), ppc_inst(instrs[3]));
+	}
+
+	printk(KERN_DEBUG "uaccess-flush: patched %d locations (%s flush)\n", i,
+		(types == L1D_FLUSH_NONE)       ? "no" :
+		(types == L1D_FLUSH_FALLBACK)   ? "fallback displacement" :
+		(types &  L1D_FLUSH_ORI)        ? (types & L1D_FLUSH_MTTRIG)
+							? "ori+mttrig type"
+							: "ori type" :
+		(types &  L1D_FLUSH_MTTRIG)     ? "mttrig type"
+						: "unknown");
+}
+
 void do_entry_flush_fixups(enum l1d_flush_type types)
 {
 	unsigned int instrs[3], *dest;