[RFC PATCH 4/5] powerpc/mm: Remove custom stack expansion checking

[Christophe Leroy]

Le 03/07/2020 à 16:13, Michael Ellerman a écrit :
> 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 agree that's probably not worth a so complicated logic that is nowhere 
documented.

This patch looks good to me, minor comments below.

> 
> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
> ---
>   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 we need this additional blank line ?

>   /*
>    * 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)))

We are already in an unlikely() branch, I don't think it is worth having 
a second level of unlikely(), better let gcc decide what's most efficient.

>   			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)))

Same.

> +			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)))
> 

Christophe