[PATCH 4/6] s390: add system call to run tasks with modified branch prediction

Martin Schwidefsky
2018-01-17T09:48:37Z

Add a system call to switch a task from the standard branch prediction
to a modified, more secure but slower behaviour.

The user space wrapper to start a program with the modified branch
prediction:

int main(int argc, char *argv[], char *envp[])
{
int rc;

if (argc < 2) {
fprintf(stderr, "Usage: %s <file-to-exec> <arguments>\n",
argv[0]);
exit(EXIT_FAILURE);
}

rc = syscall(__NR_s390_modify_bp);
if (rc) {
perror("s390_modify_bp");
exit(EXIT_FAILURE);
}
execve(argv[1], argv + 1, envp);
perror("execve"); /* execve() returns only on error */
exit(EXIT_FAILURE);
}

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
arch/s390/include/asm/thread_info.h | 4 +++
arch/s390/include/uapi/asm/unistd.h | 3 ++-
arch/s390/kernel/entry.S | 51 +++++++++++++++++++++++++++++++++----
arch/s390/kernel/sys_s390.c | 8 ++++++
arch/s390/kernel/syscalls.S | 1 +
5 files changed, 61 insertions(+), 6 deletions(-)

diff --git a/arch/s390/include/asm/thread_info.h b/arch/s390/include/asm/thread_info.h
index 0880a37..ccf37c2 100644
--- a/arch/s390/include/asm/thread_info.h
+++ b/arch/s390/include/asm/thread_info.h
@@ -60,6 +60,8 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
#define TIF_GUARDED_STORAGE 4 /* load guarded storage control block */
#define TIF_PATCH_PENDING 5 /* pending live patching update */
#define TIF_PGSTE 6 /* New mm's will use 4K page tables */
+#define TIF_NOBP 8 /* Run process with BP off */
+#define TIF_NOBP_GUEST 9 /* Run KVM guests with BP off */

#define TIF_31BIT 16 /* 32bit process */
#define TIF_MEMDIE 17 /* is terminating due to OOM killer */
@@ -80,6 +82,8 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src);
#define _TIF_UPROBE _BITUL(TIF_UPROBE)
#define _TIF_GUARDED_STORAGE _BITUL(TIF_GUARDED_STORAGE)
#define _TIF_PATCH_PENDING _BITUL(TIF_PATCH_PENDING)
+#define _TIF_NOBP _BITUL(TIF_NOBP)
+#define _TIF_NOBP_GUEST _BITUL(TIF_NOBP_GUEST)

#define _TIF_31BIT _BITUL(TIF_31BIT)
#define _TIF_SINGLE_STEP _BITUL(TIF_SINGLE_STEP)
diff --git a/arch/s390/include/uapi/asm/unistd.h b/arch/s390/include/uapi/asm/unistd.h
index 7251209..8803723 100644
--- a/arch/s390/include/uapi/asm/unistd.h
+++ b/arch/s390/include/uapi/asm/unistd.h
@@ -317,7 +317,8 @@
#define __NR_s390_guarded_storage 378
#define __NR_statx 379
#define __NR_s390_sthyi 380
-#define NR_syscalls 381
+#define __NR_s390_modify_bp 381
+#define NR_syscalls 382

/*
* There are some system calls that are not present on 64 bit, some
diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S
index dab716b..2a22c03 100644
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -107,6 +107,7 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCALL_RESTART)
aghi %r15,-(STACK_FRAME_OVERHEAD + __PT_SIZE)
j 3f
1: UPDATE_VTIME %r14,%r15,\timer
+ BPENTER __TI_flags(%r12),_TIF_NOBP
2: lg %r15,__LC_ASYNC_STACK # load async stack
3: la %r11,STACK_FRAME_OVERHEAD(%r15)
.endm
@@ -187,6 +188,40 @@ _PIF_WORK = (_PIF_PER_TRAP | _PIF_SYSCALL_RESTART)
.popsection
.endm

+ .macro BPENTER tif_ptr,tif_mask
+ .pushsection .altinstr_replacement, "ax"
+662: .word 0xc004, 0x0000, 0x0000 # 6 byte nop
+ .word 0xc004, 0x0000, 0x0000 # 6 byte nop
+ .popsection
+664: TSTMSK \tif_ptr,\tif_mask
+ jz . + 8
+ .long 0xb2e8d000
+ .pushsection .altinstructions, "a"
+ .long 664b - .
+ .long 662b - .
+ .word 82
+ .byte 12
+ .byte 12
+ .popsection
+ .endm
+
+ .macro BPEXIT tif_ptr,tif_mask
+ TSTMSK \tif_ptr,\tif_mask
+ .pushsection .altinstr_replacement, "ax"
+662: jnz . + 8
+ .long 0xb2e8d000
+ .popsection
+664: jz . + 8
+ .long 0xb2e8c000
+ .pushsection .altinstructions, "a"
+ .long 664b - .
+ .long 662b - .
+ .word 82
+ .byte 8
+ .byte 8
+ .popsection
+ .endm
+
.section .kprobes.text, "ax"
.Ldummy:
/*
@@ -240,9 +275,11 @@ ENTRY(__switch_to)
*/
ENTRY(sie64a)
stmg %r6,%r14,__SF_GPRS(%r15) # save kernel registers
+ lg %r12,__LC_CURRENT
stg %r2,__SF_EMPTY(%r15) # save control block pointer
stg %r3,__SF_EMPTY+8(%r15) # save guest register save area
xc __SF_EMPTY+16(8,%r15),__SF_EMPTY+16(%r15) # reason code = 0
+ mvc __SF_EMPTY+24(8,%r15),__TI_flags(%r12) # copy thread flags
TSTMSK __LC_CPU_FLAGS,_CIF_FPU # load guest fp/vx registers ?
jno .Lsie_load_guest_gprs
brasl %r14,load_fpu_regs # load guest fp/vx regs
@@ -259,11 +296,12 @@ ENTRY(sie64a)
jnz .Lsie_skip
TSTMSK __LC_CPU_FLAGS,_CIF_FPU
jo .Lsie_skip # exit if fp/vx regs changed
- BPON
+ BPEXIT __SF_EMPTY+24(%r15),(_TIF_NOBP|_TIF_NOBP_GUEST)
.Lsie_entry:
sie 0(%r14)
.Lsie_exit:
BPOFF
+ BPENTER __SF_EMPTY+24(%r15),(_TIF_NOBP|_TIF_NOBP_GUEST)
.Lsie_skip:
ni __SIE_PROG0C+3(%r14),0xfe # no longer in SIE
lctlg %c1,%c1,__LC_USER_ASCE # load primary asce
@@ -318,6 +356,7 @@ ENTRY(system_call)
la %r11,STACK_FRAME_OVERHEAD(%r15) # pointer to pt_regs
.Lsysc_vtime:
UPDATE_VTIME %r8,%r9,__LC_SYNC_ENTER_TIMER
+ BPENTER __TI_flags(%r12),_TIF_NOBP
stmg %r0,%r7,__PT_R0(%r11)
mvc __PT_R8(64,%r11),__LC_SAVE_AREA_SYNC
mvc __PT_PSW(16,%r11),__LC_SVC_OLD_PSW
@@ -354,7 +393,7 @@ ENTRY(system_call)
jnz .Lsysc_work # check for work
TSTMSK __LC_CPU_FLAGS,_CIF_WORK
jnz .Lsysc_work
- BPON
+ BPEXIT __TI_flags(%r12),_TIF_NOBP
.Lsysc_restore:
lg %r14,__LC_VDSO_PER_CPU
lmg %r0,%r10,__PT_R0(%r11)
@@ -589,6 +628,7 @@ ENTRY(pgm_check_handler)
aghi %r15,-(STACK_FRAME_OVERHEAD + __PT_SIZE)
j 4f
2: UPDATE_VTIME %r14,%r15,__LC_SYNC_ENTER_TIMER
+ BPENTER __TI_flags(%r12),_TIF_NOBP
lg %r15,__LC_KERNEL_STACK
lgr %r14,%r12
aghi %r14,__TASK_thread # pointer to thread_struct
@@ -702,7 +742,7 @@ ENTRY(io_int_handler)
mvc __LC_RETURN_PSW(16),__PT_PSW(%r11)
tm __PT_PSW+1(%r11),0x01 # returning to user ?
jno .Lio_exit_kernel
- BPON
+ BPEXIT __TI_flags(%r12),_TIF_NOBP
.Lio_exit_timer:
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
@@ -1118,7 +1158,7 @@ ENTRY(mcck_int_handler)
mvc __LC_RETURN_MCCK_PSW(16),__PT_PSW(%r11) # move return PSW
tm __LC_RETURN_MCCK_PSW+1,0x01 # returning to user ?
jno 0f
- BPON
+ BPEXIT __TI_flags(%r12),_TIF_NOBP
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
0: lmg %r11,%r15,__PT_R11(%r11)
@@ -1245,7 +1285,8 @@ cleanup_critical:
clg %r9,BASED(.Lsie_crit_mcck_length)
jh 1f
oi __LC_CPU_FLAGS+7, _CIF_MCCK_GUEST
-1: lg %r9,__SF_EMPTY(%r15) # get control block pointer
+1: BPENTER __SF_EMPTY+24(%r15),(_TIF_NOBP|_TIF_NOBP_GUEST)
+ lg %r9,__SF_EMPTY(%r15) # get control block pointer
ni __SIE_PROG0C+3(%r9),0xfe # no longer in SIE
lctlg %c1,%c1,__LC_USER_ASCE # load primary asce
larl %r9,sie_exit # skip forward to sie_exit
diff --git a/arch/s390/kernel/sys_s390.c b/arch/s390/kernel/sys_s390.c
index 0090037..7579c97 100644
--- a/arch/s390/kernel/sys_s390.c
+++ b/arch/s390/kernel/sys_s390.c
@@ -90,3 +90,11 @@ SYSCALL_DEFINE1(s390_personality, unsigned int, personality)

return ret;
}
+
+SYSCALL_DEFINE0(s390_modify_bp)
+{
+ if (!test_facility(82))
+ return -EOPNOTSUPP;
+ set_thread_flag(TIF_NOBP);
+ return 0;
+}
diff --git a/arch/s390/kernel/syscalls.S b/arch/s390/kernel/syscalls.S
index f7fc633..0c6293b 100644
--- a/arch/s390/kernel/syscalls.S
+++ b/arch/s390/kernel/syscalls.S
@@ -390,3 +390,4 @@ SYSCALL(sys_pwritev2,compat_sys_pwritev2)
SYSCALL(sys_s390_guarded_storage,compat_sys_s390_guarded_storage) /* 378 */
SYSCALL(sys_statx,compat_sys_statx)
SYSCALL(sys_s390_sthyi,compat_sys_s390_sthyi)
+SYSCALL(sys_s390_modify_bp,sys_s390_modify_bp)
--
2.7.4