powerpc/ptrace: Expose HASHKEYR register to ptrace

The HASHKEYR register contains a secret per-process key to enable unique
hashes per process. In general it should not be exposed to userspace
at all and a regular process has no need to know its key.

However, checkpoint restore in userspace (CRIU) functionality requires
that a process be able to set the HASHKEYR of another process, otherwise
existing hashes on the stack would be invalidated by a new random key.

Exposing HASHKEYR in this way also makes it appear in core dumps, which
is a security concern. Multiple threads may share a key, for example
just after a fork() call, where the kernel cannot know if the child is
going to return back along the parent's stack. If such a thread is
coerced into making a core dump, then the HASHKEYR value will be
readable and able to be used against all other threads sharing that key,
effectively undoing any protection offered by hashst/hashchk.

Therefore we expose HASHKEYR to ptrace when CONFIG_CHECKPOINT_RESTORE is
enabled, providing a choice of increased security or migratable ROP
protected processes. This is similar to how ARM exposes its PAC keys.

Signed-off-by: Benjamin Gray <[e-mail]>
Reviewed-by: Russell Currey <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/include/uapi/asm/elf.h

···
 #define ELF_NPMU	5	/* includes siar, sdar, sier, mmcr2, mmcr0 */
 #define ELF_NPKEY	3	/* includes amr, iamr, uamor */
 #define ELF_NDEXCR	2	/* includes dexcr, hdexcr */
+#define ELF_NHASHKEYR	1	/* includes hashkeyr */
 
 typedef unsigned long elf_greg_t64;
 typedef elf_greg_t64 elf_gregset_t64[ELF_NGREG];

arch/powerpc/kernel/ptrace/ptrace-decl.h

···
 	REGSET_EBB,		/* EBB registers */
 	REGSET_PMR,		/* Performance Monitor Registers */
 	REGSET_DEXCR,		/* DEXCR registers */
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	REGSET_HASHKEYR,	/* HASHKEYR register */
+#endif
 #endif
 #ifdef CONFIG_PPC_MEM_KEYS
 	REGSET_PKEY,		/* AMR register */

arch/powerpc/kernel/ptrace/ptrace-view.c

···
 	return membuf_store(&to, (u64)lower_32_bits(mfspr(SPRN_HDEXCR_RO)));
 }
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+static int hashkeyr_active(struct task_struct *target, const struct user_regset *regset)
+{
+	if (!cpu_has_feature(CPU_FTR_ARCH_31))
+		return -ENODEV;
+
+	return regset->n;
+}
+
+static int hashkeyr_get(struct task_struct *target, const struct user_regset *regset,
+			struct membuf to)
+{
+	if (!cpu_has_feature(CPU_FTR_ARCH_31))
+		return -ENODEV;
+
+	return membuf_store(&to, target->thread.hashkeyr);
+}
+
+static int hashkeyr_set(struct task_struct *target, const struct user_regset *regset,
+			unsigned int pos, unsigned int count, const void *kbuf,
+			const void __user *ubuf)
+{
+	if (!cpu_has_feature(CPU_FTR_ARCH_31))
+		return -ENODEV;
+
+	return user_regset_copyin(&pos, &count, &kbuf, &ubuf, &target->thread.hashkeyr,
+				  0, sizeof(unsigned long));
+}
+#endif /* CONFIG_CHECKPOINT_RESTORE */
 #endif /* CONFIG_PPC_BOOK3S_64 */
 
 #ifdef CONFIG_PPC_MEM_KEYS
···
 		.size = sizeof(u64), .align = sizeof(u64),
 		.active = dexcr_active, .regset_get = dexcr_get
 	},
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	[REGSET_HASHKEYR] = {
+		.core_note_type = NT_PPC_HASHKEYR, .n = ELF_NHASHKEYR,
+		.size = sizeof(u64), .align = sizeof(u64),
+		.active = hashkeyr_active, .regset_get = hashkeyr_get, .set = hashkeyr_set
+	},
+#endif
 #endif
 #ifdef CONFIG_PPC_MEM_KEYS
 	[REGSET_PKEY] = {

include/uapi/linux/elf.h

···
 #define NT_PPC_TM_CDSCR	0x10f		/* TM checkpointed Data Stream Control Register */
 #define NT_PPC_PKEY	0x110		/* Memory Protection Keys registers */
 #define NT_PPC_DEXCR	0x111		/* PowerPC DEXCR registers */
+#define NT_PPC_HASHKEYR	0x112		/* PowerPC HASHKEYR register */
 #define NT_386_TLS	0x200		/* i386 TLS slots (struct user_desc) */
 #define NT_386_IOPERM	0x201		/* x86 io permission bitmap (1=deny) */
 #define NT_X86_XSTATE	0x202		/* x86 extended state using xsave */