From: Sven Schnelle svens@linux.ibm.com
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@vger.kernel.org # v5.2+ Reviewed-by: Gerald Schaefer gerald.schaefer@de.ibm.com Signed-off-by: Sven Schnelle svens@linux.ibm.com Signed-off-by: Vasily Gorbik gor@linux.ibm.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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" @@ -155,6 +156,12 @@ 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 @@ -124,6 +124,8 @@ 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 @@ -106,6 +106,7 @@ 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"
/* @@ -450,6 +451,8 @@ 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); }