[PATCH 5.4 15/41] s390: prevent leaking kernel address in BEAR

From: Greg Kroah-Hartman
Date: Sat Apr 11 2020 - 08:25:01 EST

From: Sven Schnelle <svens@xxxxxxxxxxxxx>

commit 0b38b5e1d0e2f361e418e05c179db05bb688bbd6 upstream.

When userspace executes a syscall or gets interrupted,
BEAR contains a kernel address when returning to userspace.
This make it pretty easy to figure out where the kernel is
mapped even with KASLR enabled. To fix this, add lpswe to
lowcore and always execute it there, so userspace sees only
the lowcore address of lpswe. For this we have to extend
both critical_cleanup and the SWITCH_ASYNC macro to also check
for lpswe addresses in lowcore.

Fixes: b2d24b97b2a9 ("s390/kernel: add support for kernel address space layout randomization (KASLR)")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.2+
Reviewed-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxx>
Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
Signed-off-by: Vasily Gorbik <gor@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
arch/s390/include/asm/lowcore.h | 4 +-
arch/s390/include/asm/processor.h | 1
arch/s390/include/asm/setup.h | 7 ++++
arch/s390/kernel/asm-offsets.c | 2 +
arch/s390/kernel/entry.S | 65 ++++++++++++++++++++++----------------
arch/s390/kernel/process.c | 1
arch/s390/kernel/setup.c | 3 +
arch/s390/kernel/smp.c | 2 +
arch/s390/mm/vmem.c | 4 ++
9 files changed, 62 insertions(+), 27 deletions(-)

--- a/arch/s390/include/asm/lowcore.h
+++ b/arch/s390/include/asm/lowcore.h
@@ -141,7 +141,9 @@ struct lowcore {

/* br %r1 trampoline */
__u16 br_r1_trampoline; /* 0x0400 */
- __u8 pad_0x0402[0x0e00-0x0402]; /* 0x0402 */
+ __u32 return_lpswe; /* 0x0402 */
+ __u32 return_mcck_lpswe; /* 0x0406 */
+ __u8 pad_0x040a[0x0e00-0x040a]; /* 0x040a */

/*
* 0xe00 contains the address of the IPL Parameter Information
--- a/arch/s390/include/asm/processor.h
+++ b/arch/s390/include/asm/processor.h
@@ -162,6 +162,7 @@ typedef struct thread_struct thread_stru
#define INIT_THREAD { \
.ksp = sizeof(init_stack) + (unsigned long) &init_stack, \
.fpu.regs = (void *) init_task.thread.fpu.fprs, \
+ .last_break = 1, \
}

/*
--- a/arch/s390/include/asm/setup.h
+++ b/arch/s390/include/asm/setup.h
@@ -8,6 +8,7 @@

#include <linux/bits.h>
#include <uapi/asm/setup.h>
+#include <linux/build_bug.h>

#define EP_OFFSET 0x10008
#define EP_STRING "S390EP"
@@ -157,6 +158,12 @@ static inline unsigned long kaslr_offset
return __kaslr_offset;
}

+static inline u32 gen_lpswe(unsigned long addr)
+{
+ BUILD_BUG_ON(addr > 0xfff);
+ return 0xb2b20000 | addr;
+}
+
#else /* __ASSEMBLY__ */

#define IPL_DEVICE (IPL_DEVICE_OFFSET)
--- a/arch/s390/kernel/asm-offsets.c
+++ b/arch/s390/kernel/asm-offsets.c
@@ -125,6 +125,8 @@ int main(void)
OFFSET(__LC_EXT_DAMAGE_CODE, lowcore, external_damage_code);
OFFSET(__LC_MCCK_FAIL_STOR_ADDR, lowcore, failing_storage_address);
OFFSET(__LC_LAST_BREAK, lowcore, breaking_event_addr);
+ OFFSET(__LC_RETURN_LPSWE, lowcore, return_lpswe);
+ OFFSET(__LC_RETURN_MCCK_LPSWE, lowcore, return_mcck_lpswe);
OFFSET(__LC_RST_OLD_PSW, lowcore, restart_old_psw);
OFFSET(__LC_EXT_OLD_PSW, lowcore, external_old_psw);
OFFSET(__LC_SVC_OLD_PSW, lowcore, svc_old_psw);
--- a/arch/s390/kernel/entry.S
+++ b/arch/s390/kernel/entry.S
@@ -115,26 +115,29 @@ _LPP_OFFSET = __LC_LPP

.macro SWITCH_ASYNC savearea,timer
tmhh %r8,0x0001 # interrupting from user ?
- jnz 1f
+ jnz 2f
lgr %r14,%r9
+ cghi %r14,__LC_RETURN_LPSWE
+ je 0f
slg %r14,BASED(.Lcritical_start)
clg %r14,BASED(.Lcritical_length)
- jhe 0f
+ jhe 1f
+0:
lghi %r11,\savearea # inside critical section, do cleanup
brasl %r14,cleanup_critical
tmhh %r8,0x0001 # retest problem state after cleanup
- jnz 1f
-0: lg %r14,__LC_ASYNC_STACK # are we already on the target stack?
+ jnz 2f
+1: lg %r14,__LC_ASYNC_STACK # are we already on the target stack?
slgr %r14,%r15
srag %r14,%r14,STACK_SHIFT
- jnz 2f
+ jnz 3f
CHECK_STACK \savearea
aghi %r15,-(STACK_FRAME_OVERHEAD + __PT_SIZE)
- j 3f
-1: UPDATE_VTIME %r14,%r15,\timer
+ j 4f
+2: UPDATE_VTIME %r14,%r15,\timer
BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP
-2: lg %r15,__LC_ASYNC_STACK # load async stack
-3: la %r11,STACK_FRAME_OVERHEAD(%r15)
+3: lg %r15,__LC_ASYNC_STACK # load async stack
+4: la %r11,STACK_FRAME_OVERHEAD(%r15)
.endm

.macro UPDATE_VTIME w1,w2,enter_timer
@@ -401,7 +404,7 @@ ENTRY(system_call)
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
lmg %r11,%r15,__PT_R11(%r11)
- lpswe __LC_RETURN_PSW
+ b __LC_RETURN_LPSWE(%r0)
.Lsysc_done:

#
@@ -608,43 +611,50 @@ ENTRY(pgm_check_handler)
BPOFF
stmg %r8,%r15,__LC_SAVE_AREA_SYNC
lg %r10,__LC_LAST_BREAK
- lg %r12,__LC_CURRENT
+ srag %r11,%r10,12
+ jnz 0f
+ /* if __LC_LAST_BREAK is < 4096, it contains one of
+ * the lpswe addresses in lowcore. Set it to 1 (initial state)
+ * to prevent leaking that address to userspace.
+ */
+ lghi %r10,1
+0: lg %r12,__LC_CURRENT
lghi %r11,0
larl %r13,cleanup_critical
lmg %r8,%r9,__LC_PGM_OLD_PSW
tmhh %r8,0x0001 # test problem state bit
- jnz 2f # -> fault in user space
+ jnz 3f # -> fault in user space
#if IS_ENABLED(CONFIG_KVM)
# cleanup critical section for program checks in sie64a
lgr %r14,%r9
slg %r14,BASED(.Lsie_critical_start)
clg %r14,BASED(.Lsie_critical_length)
- jhe 0f
+ jhe 1f
lg %r14,__SF_SIE_CONTROL(%r15) # get control block pointer
ni __SIE_PROG0C+3(%r14),0xfe # no longer in SIE
lctlg %c1,%c1,__LC_USER_ASCE # load primary asce
larl %r9,sie_exit # skip forward to sie_exit
lghi %r11,_PIF_GUEST_FAULT
#endif
-0: tmhh %r8,0x4000 # PER bit set in old PSW ?
- jnz 1f # -> enabled, can't be a double fault
+1: tmhh %r8,0x4000 # PER bit set in old PSW ?
+ jnz 2f # -> enabled, can't be a double fault
tm __LC_PGM_ILC+3,0x80 # check for per exception
jnz .Lpgm_svcper # -> single stepped svc
-1: CHECK_STACK __LC_SAVE_AREA_SYNC
+2: CHECK_STACK __LC_SAVE_AREA_SYNC
aghi %r15,-(STACK_FRAME_OVERHEAD + __PT_SIZE)
- # CHECK_VMAP_STACK branches to stack_overflow or 4f
- CHECK_VMAP_STACK __LC_SAVE_AREA_SYNC,4f
-2: UPDATE_VTIME %r14,%r15,__LC_SYNC_ENTER_TIMER
+ # CHECK_VMAP_STACK branches to stack_overflow or 5f
+ CHECK_VMAP_STACK __LC_SAVE_AREA_SYNC,5f
+3: UPDATE_VTIME %r14,%r15,__LC_SYNC_ENTER_TIMER
BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP
lg %r15,__LC_KERNEL_STACK
lgr %r14,%r12
aghi %r14,__TASK_thread # pointer to thread_struct
lghi %r13,__LC_PGM_TDB
tm __LC_PGM_ILC+2,0x02 # check for transaction abort
- jz 3f
+ jz 4f
mvc __THREAD_trap_tdb(256,%r14),0(%r13)
-3: stg %r10,__THREAD_last_break(%r14)
-4: lgr %r13,%r11
+4: stg %r10,__THREAD_last_break(%r14)
+5: lgr %r13,%r11
la %r11,STACK_FRAME_OVERHEAD(%r15)
stmg %r0,%r7,__PT_R0(%r11)
# clear user controlled registers to prevent speculative use
@@ -663,14 +673,14 @@ ENTRY(pgm_check_handler)
stg %r13,__PT_FLAGS(%r11)
stg %r10,__PT_ARGS(%r11)
tm __LC_PGM_ILC+3,0x80 # check for per exception
- jz 5f
+ jz 6f
tmhh %r8,0x0001 # kernel per event ?
jz .Lpgm_kprobe
oi __PT_FLAGS+7(%r11),_PIF_PER_TRAP
mvc __THREAD_per_address(8,%r14),__LC_PER_ADDRESS
mvc __THREAD_per_cause(2,%r14),__LC_PER_CODE
mvc __THREAD_per_paid(1,%r14),__LC_PER_ACCESS_ID
-5: REENABLE_IRQS
+6: REENABLE_IRQS
xc __SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15)
larl %r1,pgm_check_table
llgh %r10,__PT_INT_CODE+2(%r11)
@@ -775,7 +785,7 @@ ENTRY(io_int_handler)
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
.Lio_exit_kernel:
lmg %r11,%r15,__PT_R11(%r11)
- lpswe __LC_RETURN_PSW
+ b __LC_RETURN_LPSWE(%r0)
.Lio_done:

#
@@ -1214,7 +1224,7 @@ ENTRY(mcck_int_handler)
stpt __LC_EXIT_TIMER
mvc __VDSO_ECTG_BASE(16,%r14),__LC_EXIT_TIMER
0: lmg %r11,%r15,__PT_R11(%r11)
- lpswe __LC_RETURN_MCCK_PSW
+ b __LC_RETURN_MCCK_LPSWE

.Lmcck_panic:
lg %r15,__LC_NODAT_STACK
@@ -1271,6 +1281,8 @@ ENDPROC(stack_overflow)
#endif

ENTRY(cleanup_critical)
+ cghi %r9,__LC_RETURN_LPSWE
+ je .Lcleanup_lpswe
#if IS_ENABLED(CONFIG_KVM)
clg %r9,BASED(.Lcleanup_table_sie) # .Lsie_gmap
jl 0f
@@ -1424,6 +1436,7 @@ ENDPROC(cleanup_critical)
mvc __LC_RETURN_PSW(16),__PT_PSW(%r9)
mvc 0(64,%r11),__PT_R8(%r9)
lmg %r0,%r7,__PT_R0(%r9)
+.Lcleanup_lpswe:
1: lmg %r8,%r9,__LC_RETURN_PSW
BR_EX %r14,%r11
.Lcleanup_sysc_restore_insn:
--- a/arch/s390/kernel/process.c
+++ b/arch/s390/kernel/process.c
@@ -105,6 +105,7 @@ int copy_thread_tls(unsigned long clone_
p->thread.system_timer = 0;
p->thread.hardirq_timer = 0;
p->thread.softirq_timer = 0;
+ p->thread.last_break = 1;

frame->sf.back_chain = 0;
/* new return point is ret_from_fork */
--- a/arch/s390/kernel/setup.c
+++ b/arch/s390/kernel/setup.c
@@ -73,6 +73,7 @@
#include <asm/nospec-branch.h>
#include <asm/mem_detect.h>
#include <asm/uv.h>
+#include <asm/asm-offsets.h>
#include "entry.h"

/*
@@ -457,6 +458,8 @@ static void __init setup_lowcore_dat_off
lc->spinlock_index = 0;
arch_spin_lock_setup(0);
lc->br_r1_trampoline = 0x07f1; /* br %r1 */
+ lc->return_lpswe = gen_lpswe(__LC_RETURN_PSW);
+ lc->return_mcck_lpswe = gen_lpswe(__LC_RETURN_MCCK_PSW);

set_prefix((u32)(unsigned long) lc);
lowcore_ptr[0] = lc;
--- a/arch/s390/kernel/smp.c
+++ b/arch/s390/kernel/smp.c
@@ -212,6 +212,8 @@ static int pcpu_alloc_lowcore(struct pcp
lc->spinlock_lockval = arch_spin_lockval(cpu);
lc->spinlock_index = 0;
lc->br_r1_trampoline = 0x07f1; /* br %r1 */
+ lc->return_lpswe = gen_lpswe(__LC_RETURN_PSW);
+ lc->return_mcck_lpswe = gen_lpswe(__LC_RETURN_MCCK_PSW);
if (nmi_alloc_per_cpu(lc))
goto out_async;
if (vdso_alloc_per_cpu(lc))
--- a/arch/s390/mm/vmem.c
+++ b/arch/s390/mm/vmem.c
@@ -415,6 +415,10 @@ void __init vmem_map_init(void)
SET_MEMORY_RO | SET_MEMORY_X);
__set_memory(__stext_dma, (__etext_dma - __stext_dma) >> PAGE_SHIFT,
SET_MEMORY_RO | SET_MEMORY_X);
+
+ /* we need lowcore executable for our LPSWE instructions */
+ set_memory_x(0, 1);
+
pr_info("Write protected kernel read-only data: %luk\n",
(unsigned long)(__end_rodata - _stext) >> 10);
}