The patch below does not apply to the 5.11-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From b61b1595124a1694501105e5dd488de0c0c6bc2a Mon Sep 17 00:00:00 2001

From: Sven Schnelle svens@linux.ibm.com Date: Wed, 3 Feb 2021 09:02:51 +0100 Subject: [PATCH] s390: add stack for machine check handler

The previous code used the normal kernel stack for machine checks. This is problematic when a machine check interrupts a system call or interrupt handler right at the beginning where registers are set up.

Assume system_call is interrupted at the first instruction and a machine check is triggered. The machine check handler is called, checks the PSW to see whether it is coming from user space, notices that it is already in kernel mode but %r15 still contains the user space stack. This would lead to a kernel crash.

There are basically two ways of fixing that: Either using the 'critical cleanup' approach which compares the address in the PSW to see whether it is already at a point where the stack has been set up, or use an extra stack for the machine check handler.

For simplicity, we will go with the second approach and allocate an extra stack. This adds some memory overhead for large systems, but usually large system have plenty of memory so this isn't really a concern. But it keeps the mchk stack setup simple and less error prone.

Fixes: 0b0ed657fe00 ("s390: remove critical section cleanup from entry.S") Signed-off-by: Sven Schnelle svens@linux.ibm.com Cc: stable@kernel.org # v5.8+ Reviewed-by: Heiko Carstens hca@linux.ibm.com Signed-off-by: Vasily Gorbik gor@linux.ibm.com

diff --git a/arch/s390/include/asm/lowcore.h b/arch/s390/include/asm/lowcore.h index 4d65c8e4e6d0..22bceeeba4bc 100644 --- a/arch/s390/include/asm/lowcore.h +++ b/arch/s390/include/asm/lowcore.h @@ -107,16 +107,15 @@ struct lowcore { __u64 async_stack; /* 0x0350 */ __u64 nodat_stack; /* 0x0358 */ __u64 restart_stack; /* 0x0360 */ - + __u64 mcck_stack; /* 0x0368 */ /* Restart function and parameter. */ - __u64 restart_fn; /* 0x0368 */ - __u64 restart_data; /* 0x0370 */ - __u64 restart_source; /* 0x0378 */ + __u64 restart_fn; /* 0x0370 */ + __u64 restart_data; /* 0x0378 */ + __u64 restart_source; /* 0x0380 */

/* Address space pointer. */ - __u64 kernel_asce; /* 0x0380 */ - __u64 user_asce; /* 0x0388 */ - __u8 pad_0x0390[0x0398-0x0390]; /* 0x0390 */ + __u64 kernel_asce; /* 0x0388 */ + __u64 user_asce; /* 0x0390 */

/* * The lpp and current_pid fields form a diff --git a/arch/s390/kernel/asm-offsets.c b/arch/s390/kernel/asm-offsets.c index d22bb28ef50c..15e637728a4b 100644 --- a/arch/s390/kernel/asm-offsets.c +++ b/arch/s390/kernel/asm-offsets.c @@ -118,6 +118,7 @@ int main(void) OFFSET(__LC_ASYNC_STACK, lowcore, async_stack); OFFSET(__LC_NODAT_STACK, lowcore, nodat_stack); OFFSET(__LC_RESTART_STACK, lowcore, restart_stack); + OFFSET(__LC_MCCK_STACK, lowcore, mcck_stack); OFFSET(__LC_RESTART_FN, lowcore, restart_fn); OFFSET(__LC_RESTART_DATA, lowcore, restart_data); OFFSET(__LC_RESTART_SOURCE, lowcore, restart_source); diff --git a/arch/s390/kernel/entry.S b/arch/s390/kernel/entry.S index ed5acf95235f..f7953bb17558 100644 --- a/arch/s390/kernel/entry.S +++ b/arch/s390/kernel/entry.S @@ -70,6 +70,8 @@ _LPP_OFFSET = __LC_LPP je \oklabel clg %r14,__LC_ASYNC_STACK je \oklabel + clg %r14,__LC_MCCK_STACK + je \oklabel clg %r14,__LC_NODAT_STACK je \oklabel clg %r14,__LC_RESTART_STACK @@ -548,20 +550,16 @@ ENTRY(mcck_int_handler) jhe .Lmcck_stack lghi %r11,__LC_GPREGS_SAVE_AREA+64 # inside critical section, do cleanup brasl %r14,.Lcleanup_sie -.Lmcck_stack: #endif - CHECK_STACK __LC_GPREGS_SAVE_AREA+64 - lgr %r11,%r15 - aghi %r15,-(STACK_FRAME_OVERHEAD + __PT_SIZE) - stg %r11,__SF_BACKCHAIN(%r15) - j 5f + j .Lmcck_stack .Lmcck_user: BPENTER __TI_flags(%r12),_TIF_ISOLATE_BP +.Lmcck_stack: + lg %r15,__LC_MCCK_STACK +.Lmcck_skip: + la %r11,STACK_FRAME_OVERHEAD(%r15) lctlg %c1,%c1,__LC_KERNEL_ASCE - lg %r15,__LC_KERNEL_STACK xc __SF_BACKCHAIN(8,%r15),__SF_BACKCHAIN(%r15) -5: la %r11,STACK_FRAME_OVERHEAD(%r15) -.Lmcck_skip: lghi %r14,__LC_GPREGS_SAVE_AREA+64 stmg %r0,%r7,__PT_R0(%r11) # clear user controlled registers to prevent speculative use @@ -602,7 +600,6 @@ ENTRY(mcck_int_handler)

.Lmcck_panic: lg %r15,__LC_NODAT_STACK - la %r11,STACK_FRAME_OVERHEAD(%r15) j .Lmcck_skip ENDPROC(mcck_int_handler)

diff --git a/arch/s390/kernel/setup.c b/arch/s390/kernel/setup.c index 6b004940c4dc..60da976eee6f 100644 --- a/arch/s390/kernel/setup.c +++ b/arch/s390/kernel/setup.c @@ -338,7 +338,7 @@ int __init arch_early_irq_init(void) return 0; }

-static int __init async_stack_realloc(void) +static int __init stack_realloc(void) { unsigned long old, new;

@@ -348,9 +348,16 @@ static int __init async_stack_realloc(void) panic("Couldn't allocate async stack"); WRITE_ONCE(S390_lowcore.async_stack, new + STACK_INIT_OFFSET); free_pages(old, THREAD_SIZE_ORDER); + + old = S390_lowcore.mcck_stack - STACK_INIT_OFFSET; + new = stack_alloc(); + if (!new) + panic("Couldn't allocate machine check stack"); + WRITE_ONCE(S390_lowcore.mcck_stack, new + STACK_INIT_OFFSET); + memblock_free(old, THREAD_SIZE); return 0; } -early_initcall(async_stack_realloc); +early_initcall(stack_realloc);

void __init arch_call_rest_init(void) { @@ -372,6 +379,7 @@ void __init arch_call_rest_init(void) static void __init setup_lowcore_dat_off(void) { unsigned long int_psw_mask = PSW_KERNEL_BITS; + unsigned long mcck_stack; struct lowcore *lc;

if (IS_ENABLED(CONFIG_KASAN)) @@ -439,6 +447,12 @@ static void __init setup_lowcore_dat_off(void) lc->restart_data = 0; lc->restart_source = -1UL;

+ mcck_stack = (unsigned long)memblock_alloc(THREAD_SIZE, THREAD_SIZE); + if (!mcck_stack) + panic("%s: Failed to allocate %lu bytes align=0x%lx\n", + __func__, THREAD_SIZE, THREAD_SIZE); + lc->mcck_stack = mcck_stack + STACK_INIT_OFFSET; + /* Setup absolute zero lowcore */ mem_assign_absolute(S390_lowcore.restart_stack, lc->restart_stack); mem_assign_absolute(S390_lowcore.restart_fn, lc->restart_fn); diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c index c5abbb94ac6e..e299892440b6 100644 --- a/arch/s390/kernel/smp.c +++ b/arch/s390/kernel/smp.c @@ -189,7 +189,7 @@ static void pcpu_ec_call(struct pcpu *pcpu, int ec_bit)

static int pcpu_alloc_lowcore(struct pcpu *pcpu, int cpu) { - unsigned long async_stack, nodat_stack; + unsigned long async_stack, nodat_stack, mcck_stack; struct lowcore *lc;

if (pcpu != &pcpu_devices[0]) { @@ -202,13 +202,15 @@ static int pcpu_alloc_lowcore(struct pcpu *pcpu, int cpu) nodat_stack = pcpu->lowcore->nodat_stack - STACK_INIT_OFFSET; } async_stack = stack_alloc(); - if (!async_stack) - goto out; + mcck_stack = stack_alloc(); + if (!async_stack || !mcck_stack) + goto out_stack; lc = pcpu->lowcore; memcpy(lc, &S390_lowcore, 512); memset((char *) lc + 512, 0, sizeof(*lc) - 512); lc->async_stack = async_stack + STACK_INIT_OFFSET; lc->nodat_stack = nodat_stack + STACK_INIT_OFFSET; + lc->mcck_stack = mcck_stack + STACK_INIT_OFFSET; lc->cpu_nr = cpu; lc->spinlock_lockval = arch_spin_lockval(cpu); lc->spinlock_index = 0; @@ -216,12 +218,13 @@ static int pcpu_alloc_lowcore(struct pcpu *pcpu, int cpu) 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; + goto out_stack; lowcore_ptr[cpu] = lc; pcpu_sigp_retry(pcpu, SIGP_SET_PREFIX, (u32)(unsigned long) lc); return 0;

-out_async: +out_stack: + stack_free(mcck_stack); stack_free(async_stack); out: if (pcpu != &pcpu_devices[0]) { @@ -233,16 +236,18 @@ static int pcpu_alloc_lowcore(struct pcpu *pcpu, int cpu)

static void pcpu_free_lowcore(struct pcpu *pcpu) { - unsigned long async_stack, nodat_stack, lowcore; + unsigned long async_stack, nodat_stack, mcck_stack, lowcore;

nodat_stack = pcpu->lowcore->nodat_stack - STACK_INIT_OFFSET; async_stack = pcpu->lowcore->async_stack - STACK_INIT_OFFSET; + mcck_stack = pcpu->lowcore->mcck_stack - STACK_INIT_OFFSET; lowcore = (unsigned long) pcpu->lowcore;

pcpu_sigp_retry(pcpu, SIGP_SET_PREFIX, 0); lowcore_ptr[pcpu - pcpu_devices] = NULL; nmi_free_per_cpu(pcpu->lowcore); stack_free(async_stack); + stack_free(mcck_stack); if (pcpu == &pcpu_devices[0]) return; free_pages(nodat_stack, THREAD_SIZE_ORDER);