Hi Michael,
> We have powerpc specific logic in our page fault handling to decide if
> an access to an unmapped address below the stack pointer should expand
> the stack VMA.
>
> The logic aims to prevent userspace from doing bad accesses below the
> stack pointer. However as long as the stack is < 1MB in size, we allow
> all accesses without further checks. Adding some debug I see that I
> can do a full kernel build and LTP run, and not a single process has
> used more than 1MB of stack. So for the majority of processes the
> logic never even fires.
>
> We also recently found a nasty bug in this code which could cause
> userspace programs to be killed during signal delivery. It went
> unnoticed presumably because most processes use < 1MB of stack.
>
> The generic mm code has also grown support for stack guard pages since
> this code was originally written, so the most heinous case of the
> stack expanding into other mappings is now handled for us.
>
> Finally although some other arches have special logic in this path,
> from what I can tell none of x86, arm64, arm and s390 impose any extra
> checks other than those in expand_stack().
>
> So drop our complicated logic and like other architectures just let
> the stack expand as long as its within the rlimit.
>
I applied and tested this. While I wouldn't call my testing
comprehensive, I have not been able to reproduce the crash with this
patch applied.
Kind regards,
Daniel
> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> ---
> arch/powerpc/mm/fault.c | 106 ++--------------------------------------
> 1 file changed, 5 insertions(+), 101 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index ed01329dd12b..925a7231abb3 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -42,39 +42,7 @@
> #include <asm/kup.h>
> #include <asm/inst.h>
>
> -/*
> - * Check whether the instruction inst is a store using
> - * an update addressing form which will update r1.
> - */
> -static bool store_updates_sp(struct ppc_inst inst)
> -{
> - /* check for 1 in the rA field */
> - if (((ppc_inst_val(inst) >> 16) & 0x1f) != 1)
> - return false;
> - /* check major opcode */
> - switch (ppc_inst_primary_opcode(inst)) {
> - case OP_STWU:
> - case OP_STBU:
> - case OP_STHU:
> - case OP_STFSU:
> - case OP_STFDU:
> - return true;
> - case OP_STD: /* std or stdu */
> - return (ppc_inst_val(inst) & 3) == 1;
> - case OP_31:
> - /* check minor opcode */
> - switch ((ppc_inst_val(inst) >> 1) & 0x3ff) {
> - case OP_31_XOP_STDUX:
> - case OP_31_XOP_STWUX:
> - case OP_31_XOP_STBUX:
> - case OP_31_XOP_STHUX:
> - case OP_31_XOP_STFSUX:
> - case OP_31_XOP_STFDUX:
> - return true;
> - }
> - }
> - return false;
> -}
> +
> /*
> * do_page_fault error handling helpers
> */
> @@ -267,54 +235,6 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
> return false;
> }
>
> -static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
> - struct vm_area_struct *vma, unsigned int flags,
> - bool *must_retry)
> -{
> - /*
> - * N.B. The POWER/Open ABI allows programs to access up to
> - * 288 bytes below the stack pointer.
> - * The kernel signal delivery code writes up to 4KB
> - * below the stack pointer (r1) before decrementing it.
> - * The exec code can write slightly over 640kB to the stack
> - * before setting the user r1. Thus we allow the stack to
> - * expand to 1MB without further checks.
> - */
> - if (address + 0x100000 < vma->vm_end) {
> - struct ppc_inst __user *nip = (struct ppc_inst __user *)regs->nip;
> - /* get user regs even if this fault is in kernel mode */
> - struct pt_regs *uregs = current->thread.regs;
> - if (uregs == NULL)
> - return true;
> -
> - /*
> - * A user-mode access to an address a long way below
> - * the stack pointer is only valid if the instruction
> - * is one which would update the stack pointer to the
> - * address accessed if the instruction completed,
> - * i.e. either stwu rs,n(r1) or stwux rs,r1,rb
> - * (or the byte, halfword, float or double forms).
> - *
> - * If we don't check this then any write to the area
> - * between the last mapped region and the stack will
> - * expand the stack rather than segfaulting.
> - */
> - if (address + 4096 >= uregs->gpr[1])
> - return false;
> -
> - if ((flags & FAULT_FLAG_WRITE) && (flags & FAULT_FLAG_USER) &&
> - access_ok(nip, sizeof(*nip))) {
> - struct ppc_inst inst;
> -
> - if (!probe_user_read_inst(&inst, nip))
> - return !store_updates_sp(inst);
> - *must_retry = true;
> - }
> - return true;
> - }
> - return false;
> -}
> -
> #ifdef CONFIG_PPC_MEM_KEYS
> static bool access_pkey_error(bool is_write, bool is_exec, bool is_pkey,
> struct vm_area_struct *vma)
> @@ -480,7 +400,6 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
> int is_user = user_mode(regs);
> int is_write = page_fault_is_write(error_code);
> vm_fault_t fault, major = 0;
> - bool must_retry = false;
> bool kprobe_fault = kprobe_page_fault(regs, 11);
>
> if (unlikely(debugger_fault_handler(regs) || kprobe_fault))
> @@ -569,30 +488,15 @@ static int __do_page_fault(struct pt_regs *regs, unsigned long address,
> vma = find_vma(mm, address);
> if (unlikely(!vma))
> return bad_area(regs, address);
> - if (likely(vma->vm_start <= address))
> - goto good_area;
> - if (unlikely(!(vma->vm_flags & VM_GROWSDOWN)))
> - return bad_area(regs, address);
>
> - /* The stack is being expanded, check if it's valid */
> - if (unlikely(bad_stack_expansion(regs, address, vma, flags,
> - &must_retry))) {
> - if (!must_retry)
> + if (unlikely(vma->vm_start > address)) {
> + if (unlikely(!(vma->vm_flags & VM_GROWSDOWN)))
> return bad_area(regs, address);
>
> - mmap_read_unlock(mm);
> - if (fault_in_pages_readable((const char __user *)regs->nip,
> - sizeof(unsigned int)))
> - return bad_area_nosemaphore(regs, address);
> - goto retry;
> + if (unlikely(expand_stack(vma, address)))
> + return bad_area(regs, address);
> }
>
> - /* Try to expand it */
> - if (unlikely(expand_stack(vma, address)))
> - return bad_area(regs, address);
> -
> -good_area:
> -
> #ifdef CONFIG_PPC_MEM_KEYS
> if (unlikely(access_pkey_error(is_write, is_exec,
> (error_code & DSISR_KEYFAULT), vma)))
> --
> 2.25.1
