[PATCH v4 7/7] ARM: implement support for vmap'ed stacks
Marek Szyprowski
m.szyprowski at samsung.com
Tue Dec 21 02:38:55 PST 2021
Hi,
On 22.11.2021 10:28, Ard Biesheuvel wrote:
> Wire up the generic support for managing task stack allocations via vmalloc,
> and implement the entry code that detects whether we faulted because of a
> stack overrun (or future stack overrun caused by pushing the pt_regs array)
>
> While this adds a fair amount of tricky entry asm code, it should be
> noted that it only adds a TST + branch to the svc_entry path. The code
> implementing the non-trivial handling of the overflow stack is emitted
> out-of-line into the .text section.
>
> Since on ARM, we rely on do_translation_fault() to keep PMD level page
> table entries that cover the vmalloc region up to date, we need to
> ensure that we don't hit such a stale PMD entry when accessing the
> stack. So we do a dummy read from the new stack while still running from
> the old one on the context switch path, and bump the vmalloc_seq counter
> when PMD level entries in the vmalloc range are modified, so that the MM
> switch fetches the latest version of the entries.
>
> Note that we need to increase the per-mode stack by 1 word, to gain some
> space to stash a GPR until we know it is safe to touch the stack.
> However, due to the cacheline alignment of the struct, this does not
> actually increase the memory footprint of the struct stack array at all.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> Tested-by: Keith Packard <keithpac at amazon.com>
This patch landed recently in linux-next 20211220 as commit a1c510d0adc6
("ARM: implement support for vmap'ed stacks"). Sadly it breaks
suspend/resume operation on all ARM 32bit Exynos SoCs. Probably the
suspend/resume related code must be updated somehow (it partially works
on physical addresses and disabled MMU), but I didn't analyze it yet. If
you have any hints, let me know.
> ---
> arch/arm/Kconfig | 1 +
> arch/arm/include/asm/page.h | 4 +
> arch/arm/include/asm/thread_info.h | 8 ++
> arch/arm/kernel/entry-armv.S | 97 +++++++++++++++++++-
> arch/arm/kernel/entry-header.S | 37 ++++++++
> arch/arm/kernel/irq.c | 9 +-
> arch/arm/kernel/setup.c | 8 +-
> arch/arm/kernel/sleep.S | 8 ++
> arch/arm/kernel/traps.c | 80 +++++++++++++++-
> arch/arm/kernel/unwind.c | 3 +-
> arch/arm/kernel/vmlinux.lds.S | 4 +-
> 11 files changed, 244 insertions(+), 15 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index b1eba1b4168c..7a0853bd298f 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -127,6 +127,7 @@ config ARM
> select RTC_LIB
> select SYS_SUPPORTS_APM_EMULATION
> select THREAD_INFO_IN_TASK if CURRENT_POINTER_IN_TPIDRURO
> + select HAVE_ARCH_VMAP_STACK if THREAD_INFO_IN_TASK && (!LD_IS_LLD || LLD_VERSION >= 140000)
> select TRACE_IRQFLAGS_SUPPORT if !CPU_V7M
> # Above selects are sorted alphabetically; please add new ones
> # according to that. Thanks.
> diff --git a/arch/arm/include/asm/page.h b/arch/arm/include/asm/page.h
> index 11b058a72a5b..7b871ed99ccf 100644
> --- a/arch/arm/include/asm/page.h
> +++ b/arch/arm/include/asm/page.h
> @@ -149,6 +149,10 @@ extern void copy_page(void *to, const void *from);
> #include <asm/pgtable-2level-types.h>
> #endif
>
> +#ifdef CONFIG_VMAP_STACK
> +#define ARCH_PAGE_TABLE_SYNC_MASK PGTBL_PMD_MODIFIED
> +#endif
> +
> #endif /* CONFIG_MMU */
>
> typedef struct page *pgtable_t;
> diff --git a/arch/arm/include/asm/thread_info.h b/arch/arm/include/asm/thread_info.h
> index 164e15f26485..004b89d86224 100644
> --- a/arch/arm/include/asm/thread_info.h
> +++ b/arch/arm/include/asm/thread_info.h
> @@ -25,6 +25,14 @@
> #define THREAD_SIZE (PAGE_SIZE << THREAD_SIZE_ORDER)
> #define THREAD_START_SP (THREAD_SIZE - 8)
>
> +#ifdef CONFIG_VMAP_STACK
> +#define THREAD_ALIGN (2 * THREAD_SIZE)
> +#else
> +#define THREAD_ALIGN THREAD_SIZE
> +#endif
> +
> +#define OVERFLOW_STACK_SIZE SZ_4K
> +
> #ifndef __ASSEMBLY__
>
> struct task_struct;
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index b447f7d0708c..54210dce80e1 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -57,6 +57,10 @@ UNWIND( .setfp fpreg, sp )
> @
> subs r2, sp, r0 @ SP above bottom of IRQ stack?
> rsbscs r2, r2, #THREAD_SIZE @ ... and below the top?
> +#ifdef CONFIG_VMAP_STACK
> + ldr_l r2, high_memory, cc @ End of the linear region
> + cmpcc r2, r0 @ Stack pointer was below it?
> +#endif
> movcs sp, r0 @ If so, revert to incoming SP
>
> #ifndef CONFIG_UNWINDER_ARM
> @@ -188,13 +192,18 @@ ENDPROC(__und_invalid)
> #define SPFIX(code...)
> #endif
>
> - .macro svc_entry, stack_hole=0, trace=1, uaccess=1
> + .macro svc_entry, stack_hole=0, trace=1, uaccess=1, overflow_check=1
> UNWIND(.fnstart )
> - UNWIND(.save {r0 - pc} )
> sub sp, sp, #(SVC_REGS_SIZE + \stack_hole)
> + THUMB( add sp, r1 ) @ get SP in a GPR without
> + THUMB( sub r1, sp, r1 ) @ using a temp register
> +
> + .if \overflow_check
> + UNWIND(.save {r0 - pc} )
> + do_overflow_check (SVC_REGS_SIZE + \stack_hole)
> + .endif
> +
> #ifdef CONFIG_THUMB2_KERNEL
> - add sp, r1 @ get SP in a GPR without
> - sub r1, sp, r1 @ using a temp register
> tst r1, #4 @ test stack pointer alignment
> sub r1, sp, r1 @ restore original R0
> sub sp, r1 @ restore original SP
> @@ -827,12 +836,20 @@ ENTRY(__switch_to)
> str r7, [r8]
> #endif
> mov r0, r5
> -#if !defined(CONFIG_THUMB2_KERNEL)
> +#if !defined(CONFIG_THUMB2_KERNEL) && !defined(CONFIG_VMAP_STACK)
> set_current r7
> ldmia r4, {r4 - sl, fp, sp, pc} @ Load all regs saved previously
> #else
> mov r1, r7
> ldmia r4, {r4 - sl, fp, ip, lr} @ Load all regs saved previously
> +#ifdef CONFIG_VMAP_STACK
> + @
> + @ Do a dummy read from the new stack while running from the old one so
> + @ that we can rely on do_translation_fault() to fix up any stale PMD
> + @ entries covering the vmalloc region.
> + @
> + ldr r2, [ip]
> +#endif
>
> @ When CONFIG_THREAD_INFO_IN_TASK=n, the update of SP itself is what
> @ effectuates the task switch, as that is what causes the observable
> @@ -849,6 +866,76 @@ ENTRY(__switch_to)
> UNWIND(.fnend )
> ENDPROC(__switch_to)
>
> +#ifdef CONFIG_VMAP_STACK
> + .text
> + .align 2
> +__bad_stack:
> + @
> + @ We've just detected an overflow. We need to load the address of this
> + @ CPU's overflow stack into the stack pointer register. We have only one
> + @ scratch register so let's use a sequence of ADDs including one
> + @ involving the PC, and decorate them with PC-relative group
> + @ relocations. As these are ARM only, switch to ARM mode first.
> + @
> + @ We enter here with IP clobbered and its value stashed on the mode
> + @ stack.
> + @
> +THUMB( bx pc )
> +THUMB( nop )
> +THUMB( .arm )
> + mrc p15, 0, ip, c13, c0, 4 @ Get per-CPU offset
> +
> + .globl overflow_stack_ptr
> + .reloc 0f, R_ARM_ALU_PC_G0_NC, overflow_stack_ptr
> + .reloc 1f, R_ARM_ALU_PC_G1_NC, overflow_stack_ptr
> + .reloc 2f, R_ARM_LDR_PC_G2, overflow_stack_ptr
> + add ip, ip, pc
> +0: add ip, ip, #-4
> +1: add ip, ip, #0
> +2: ldr ip, [ip, #4]
> +
> + str sp, [ip, #-4]! @ Preserve original SP value
> + mov sp, ip @ Switch to overflow stack
> + pop {ip} @ Original SP in IP
> +
> +#if defined(CONFIG_UNWINDER_FRAME_POINTER) && defined(CONFIG_CC_IS_GCC)
> + mov ip, ip @ mov expected by unwinder
> + push {fp, ip, lr, pc} @ GCC flavor frame record
> +#else
> + str ip, [sp, #-8]! @ store original SP
> + push {fpreg, lr} @ Clang flavor frame record
> +#endif
> +UNWIND( ldr ip, [r0, #4] ) @ load exception LR
> +UNWIND( str ip, [sp, #12] ) @ store in the frame record
> + ldr ip, [r0, #12] @ reload IP
> +
> + @ Store the original GPRs to the new stack.
> + svc_entry uaccess=0, overflow_check=0
> +
> +UNWIND( .save {sp, pc} )
> +UNWIND( .save {fpreg, lr} )
> +UNWIND( .setfp fpreg, sp )
> +
> + ldr fpreg, [sp, #S_SP] @ Add our frame record
> + @ to the linked list
> +#if defined(CONFIG_UNWINDER_FRAME_POINTER) && defined(CONFIG_CC_IS_GCC)
> + ldr r1, [fp, #4] @ reload SP at entry
> + add fp, fp, #12
> +#else
> + ldr r1, [fpreg, #8]
> +#endif
> + str r1, [sp, #S_SP] @ store in pt_regs
> +
> + @ Stash the regs for handle_bad_stack
> + mov r0, sp
> +
> + @ Time to die
> + bl handle_bad_stack
> + nop
> +UNWIND( .fnend )
> +ENDPROC(__bad_stack)
> +#endif
> +
> __INIT
>
> /*
> diff --git a/arch/arm/kernel/entry-header.S b/arch/arm/kernel/entry-header.S
> index ae24dd54e9ef..81df2a3561ca 100644
> --- a/arch/arm/kernel/entry-header.S
> +++ b/arch/arm/kernel/entry-header.S
> @@ -423,3 +423,40 @@ scno .req r7 @ syscall number
> tbl .req r8 @ syscall table pointer
> why .req r8 @ Linux syscall (!= 0)
> tsk .req r9 @ current thread_info
> +
> + .macro do_overflow_check, frame_size:req
> +#ifdef CONFIG_VMAP_STACK
> + @
> + @ Test whether the SP has overflowed. Task and IRQ stacks are aligned
> + @ so that SP & BIT(THREAD_SIZE_ORDER + PAGE_SHIFT) should always be
> + @ zero.
> + @
> +ARM( tst sp, #1 << (THREAD_SIZE_ORDER + PAGE_SHIFT) )
> +THUMB( tst r1, #1 << (THREAD_SIZE_ORDER + PAGE_SHIFT) )
> +THUMB( it ne )
> + bne .Lstack_overflow_check\@
> +
> + .pushsection .text
> +.Lstack_overflow_check\@:
> + @
> + @ The stack pointer is not pointing to a valid vmap'ed stack, but it
> + @ may be pointing into the linear map instead, which may happen if we
> + @ are already running from the overflow stack. We cannot detect overflow
> + @ in such cases so just carry on.
> + @
> + str ip, [r0, #12] @ Stash IP on the mode stack
> + ldr_l ip, high_memory @ Start of VMALLOC space
> +ARM( cmp sp, ip ) @ SP in vmalloc space?
> +THUMB( cmp r1, ip )
> +THUMB( itt lo )
> + ldrlo ip, [r0, #12] @ Restore IP
> + blo .Lout\@ @ Carry on
> +
> +THUMB( sub r1, sp, r1 ) @ Restore original R1
> +THUMB( sub sp, r1 ) @ Restore original SP
> + add sp, sp, #\frame_size @ Undo svc_entry's SP change
> + b __bad_stack @ Handle VMAP stack overflow
> + .popsection
> +.Lout\@:
> +#endif
> + .endm
> diff --git a/arch/arm/kernel/irq.c b/arch/arm/kernel/irq.c
> index e05219bca218..5deb40f39999 100644
> --- a/arch/arm/kernel/irq.c
> +++ b/arch/arm/kernel/irq.c
> @@ -56,7 +56,14 @@ static void __init init_irq_stacks(void)
> int cpu;
>
> for_each_possible_cpu(cpu) {
> - stack = (u8 *)__get_free_pages(GFP_KERNEL, THREAD_SIZE_ORDER);
> + if (!IS_ENABLED(CONFIG_VMAP_STACK))
> + stack = (u8 *)__get_free_pages(GFP_KERNEL,
> + THREAD_SIZE_ORDER);
> + else
> + stack = __vmalloc_node(THREAD_SIZE, THREAD_ALIGN,
> + THREADINFO_GFP, NUMA_NO_NODE,
> + __builtin_return_address(0));
> +
> if (WARN_ON(!stack))
> break;
> per_cpu(irq_stack_ptr, cpu) = &stack[THREAD_SIZE];
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index 284a80c0b6e1..039feb7cd590 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -141,10 +141,10 @@ EXPORT_SYMBOL(outer_cache);
> int __cpu_architecture __read_mostly = CPU_ARCH_UNKNOWN;
>
> struct stack {
> - u32 irq[3];
> - u32 abt[3];
> - u32 und[3];
> - u32 fiq[3];
> + u32 irq[4];
> + u32 abt[4];
> + u32 und[4];
> + u32 fiq[4];
> } ____cacheline_aligned;
>
> #ifndef CONFIG_CPU_V7M
> diff --git a/arch/arm/kernel/sleep.S b/arch/arm/kernel/sleep.S
> index 43077e11dafd..803b51e5cba0 100644
> --- a/arch/arm/kernel/sleep.S
> +++ b/arch/arm/kernel/sleep.S
> @@ -67,6 +67,14 @@ ENTRY(__cpu_suspend)
> ldr r4, =cpu_suspend_size
> #endif
> mov r5, sp @ current virtual SP
> +#ifdef CONFIG_VMAP_STACK
> + @ Run the suspend code from the overflow stack so we don't have to rely
> + @ on vmalloc-to-phys conversions anywhere in the arch suspend code.
> + @ The original SP value captured in R5 will be restored on the way out.
> + mov_l r6, overflow_stack_ptr @ Base pointer
> + mrc p15, 0, r7, c13, c0, 4 @ Get per-CPU offset
> + ldr sp, [r6, r7] @ Address of this CPU's overflow stack
> +#endif
> add r4, r4, #12 @ Space for pgd, virt sp, phys resume fn
> sub sp, sp, r4 @ allocate CPU state on stack
> ldr r3, =sleep_save_sp
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index b42c446cec9a..b28a705c49cb 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -121,7 +121,8 @@ void dump_backtrace_stm(u32 *stack, u32 instruction, const char *loglvl)
> static int verify_stack(unsigned long sp)
> {
> if (sp < PAGE_OFFSET ||
> - (sp > (unsigned long)high_memory && high_memory != NULL))
> + (!IS_ENABLED(CONFIG_VMAP_STACK) &&
> + sp > (unsigned long)high_memory && high_memory != NULL))
> return -EFAULT;
>
> return 0;
> @@ -291,7 +292,8 @@ static int __die(const char *str, int err, struct pt_regs *regs)
>
> if (!user_mode(regs) || in_interrupt()) {
> dump_mem(KERN_EMERG, "Stack: ", regs->ARM_sp,
> - ALIGN(regs->ARM_sp, THREAD_SIZE));
> + ALIGN(regs->ARM_sp - THREAD_SIZE, THREAD_ALIGN)
> + + THREAD_SIZE);
> dump_backtrace(regs, tsk, KERN_EMERG);
> dump_instr(KERN_EMERG, regs);
> }
> @@ -838,3 +840,77 @@ void __init early_trap_init(void *vectors_base)
> */
> #endif
> }
> +
> +#ifdef CONFIG_VMAP_STACK
> +
> +DECLARE_PER_CPU(u8 *, irq_stack_ptr);
> +
> +asmlinkage DEFINE_PER_CPU(u8 *, overflow_stack_ptr);
> +
> +static int __init allocate_overflow_stacks(void)
> +{
> + u8 *stack;
> + int cpu;
> +
> + for_each_possible_cpu(cpu) {
> + stack = (u8 *)__get_free_page(GFP_KERNEL);
> + if (WARN_ON(!stack))
> + return -ENOMEM;
> + per_cpu(overflow_stack_ptr, cpu) = &stack[OVERFLOW_STACK_SIZE];
> + }
> + return 0;
> +}
> +early_initcall(allocate_overflow_stacks);
> +
> +asmlinkage void handle_bad_stack(struct pt_regs *regs)
> +{
> + unsigned long tsk_stk = (unsigned long)current->stack;
> + unsigned long irq_stk = (unsigned long)this_cpu_read(irq_stack_ptr);
> + unsigned long ovf_stk = (unsigned long)this_cpu_read(overflow_stack_ptr);
> +
> + console_verbose();
> + pr_emerg("Insufficient stack space to handle exception!");
> +
> + pr_emerg("Task stack: [0x%08lx..0x%08lx]\n",
> + tsk_stk, tsk_stk + THREAD_SIZE);
> + pr_emerg("IRQ stack: [0x%08lx..0x%08lx]\n",
> + irq_stk - THREAD_SIZE, irq_stk);
> + pr_emerg("Overflow stack: [0x%08lx..0x%08lx]\n",
> + ovf_stk - OVERFLOW_STACK_SIZE, ovf_stk);
> +
> + die("kernel stack overflow", regs, 0);
> +}
> +
> +/*
> + * Normally, we rely on the logic in do_translation_fault() to update stale PMD
> + * entries covering the vmalloc space in a task's page tables when it first
> + * accesses the region in question. Unfortunately, this is not sufficient when
> + * the task stack resides in the vmalloc region, as do_translation_fault() is a
> + * C function that needs a stack to run.
> + *
> + * So we need to ensure that these PMD entries are up to date *before* the MM
> + * switch. As we already have some logic in the MM switch path that takes care
> + * of this, let's trigger it by bumping the counter every time the core vmalloc
> + * code modifies a PMD entry in the vmalloc region.
> + */
> +void arch_sync_kernel_mappings(unsigned long start, unsigned long end)
> +{
> + if (start > VMALLOC_END || end < VMALLOC_START)
> + return;
> +
> + /*
> + * This hooks into the core vmalloc code to receive notifications of
> + * any PMD level changes that have been made to the kernel page tables.
> + * This means it should only be triggered once for every MiB worth of
> + * vmalloc space, given that we don't support huge vmalloc/vmap on ARM,
> + * and that kernel PMD level table entries are rarely (if ever)
> + * updated.
> + *
> + * This means that the counter is going to max out at ~250 for the
> + * typical case. If it overflows, something entirely unexpected has
> + * occurred so let's throw a warning if that happens.
> + */
> + WARN_ON(++init_mm.context.vmalloc_seq == UINT_MAX);
> +}
> +
> +#endif
> diff --git a/arch/arm/kernel/unwind.c b/arch/arm/kernel/unwind.c
> index e8d729975f12..c5ea328c428d 100644
> --- a/arch/arm/kernel/unwind.c
> +++ b/arch/arm/kernel/unwind.c
> @@ -389,7 +389,8 @@ int unwind_frame(struct stackframe *frame)
>
> /* store the highest address on the stack to avoid crossing it*/
> ctrl.sp_low = frame->sp;
> - ctrl.sp_high = ALIGN(ctrl.sp_low, THREAD_SIZE);
> + ctrl.sp_high = ALIGN(ctrl.sp_low - THREAD_SIZE, THREAD_ALIGN)
> + + THREAD_SIZE;
>
> pr_debug("%s(pc = %08lx lr = %08lx sp = %08lx)\n", __func__,
> frame->pc, frame->lr, frame->sp);
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index f02d617e3359..aa12b65a7fd6 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -138,12 +138,12 @@ SECTIONS
> #ifdef CONFIG_STRICT_KERNEL_RWX
> . = ALIGN(1<<SECTION_SHIFT);
> #else
> - . = ALIGN(THREAD_SIZE);
> + . = ALIGN(THREAD_ALIGN);
> #endif
> __init_end = .;
>
> _sdata = .;
> - RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_SIZE)
> + RW_DATA(L1_CACHE_BYTES, PAGE_SIZE, THREAD_ALIGN)
> _edata = .;
>
> BSS_SECTION(0, 0, 0)
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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