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+ Signed-off-by: Sven Schnelle svens@linux.ibm.com ---
Changes in v2: - use cghi instead of clgfi in address compare - use b mnemonic instead of bc 15,x - Add Fixes: and Cc: stable
arch/s390/include/asm/lowcore.h | 4 +++- arch/s390/include/asm/setup.h | 7 +++++++ arch/s390/kernel/asm-offsets.c | 2 ++ arch/s390/kernel/entry.S | 30 ++++++++++++++++++------------ arch/s390/kernel/setup.c | 3 +++ arch/s390/kernel/smp.c | 2 ++ arch/s390/mm/vmem.c | 4 ++++ 7 files changed, 39 insertions(+), 13 deletions(-)
diff --git a/arch/s390/include/asm/lowcore.h b/arch/s390/include/asm/lowcore.h index 237ee0c4169f..612ed3c6d581 100644 --- 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 diff --git a/arch/s390/include/asm/setup.h b/arch/s390/include/asm/setup.h index 69289e99cabd..5017499601a3 100644 --- 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(void) 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) diff --git a/arch/s390/kernel/asm-offsets.c b/arch/s390/kernel/asm-offsets.c index ce33406cfe83..e80f0e6f5972 100644 --- 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); diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S index 270d1d145761..ead5efa07b0b 100644 --- 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:
# @@ -775,7 +778,7 @@ 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 +1217,7 @@ 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 +1274,8 @@ 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 +1429,7 @@ 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: diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c index d5fbd754f41a..c3890c05f71f 100644 --- 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(void) 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; diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c index a08bd2522dd9..f87d4e14269c 100644 --- a/arch/s390/kernel/smp.c +++ b/arch/s390/kernel/smp.c @@ -212,6 +212,8 @@ static int pcpu_alloc_lowcore(struct pcpu *pcpu, int cpu) 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)) diff --git a/arch/s390/mm/vmem.c b/arch/s390/mm/vmem.c index b403fa14847d..f810930aff42 100644 --- 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); }
On Fri, 24 Jan 2020 13:25:15 +0100 Sven Schnelle svens@linux.ibm.com wrote:
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+ Signed-off-by: Sven Schnelle svens@linux.ibm.com
Looks good, Reviewed-by: Gerald Schaefer gerald.schaefer@de.ibm.com
I think you can push to devel, but this should hang around a bit before sending upstream (@Vasily). Maybe at least wait until Heiko can also have a look.
Since the small extra window for critical section cleanup introduced by the lowcore lpswe is hit surprisingly easy and often, this will get some good testing on devel branch.
linux-stable-mirror@lists.linaro.org
-
Gerald Schaefer -
Sven Schnelle