[PATCH] powerpc/fault: kernel can extend a user process's stack

[Michal Suchánek]

On Wed, Dec 11, 2019 at 12:43:37PM +1100, Daniel Axtens wrote:
> If a process page-faults trying to write beyond the end of its
> stack, we attempt to grow the stack.
> 
> However, if the kernel attempts to write beyond the end of a
> process's stack, it takes a bad fault. This can occur when the
> kernel is trying to set up a signal frame.
> 
> Permit the kernel to grow a process's stack. The same general
> limits as to how and when the stack can be grown apply: the kernel
> code still passes through expand_stack(), so anything that goes
> beyond e.g. the rlimit should still be blocked.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=205183
Fixes: 14cf11af6cf6 ("powerpc: Merge enough to start building in
arch/powerpc.")
> Reported-by: Tom Lane <tgl at sss.pgh.pa.us>
> Cc: Daniel Black <daniel at linux.ibm.com>
> Signed-off-by: Daniel Axtens <dja at axtens.net>
> ---
>  arch/powerpc/mm/fault.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index b5047f9b5dec..00183731ea22 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -287,7 +287,17 @@ static bool bad_stack_expansion(struct pt_regs *regs, unsigned long address,
>  			if (!res)
>  				return !store_updates_sp(inst);
>  			*must_retry = true;
> +		} else if ((flags & FAULT_FLAG_WRITE) &&
> +			   !(flags & FAULT_FLAG_USER)) {
> +			/*
> +			 * the kernel can also attempt to write beyond the end
> +			 * of a process's stack - for example setting up a
> +			 * signal frame. We assume this is valid, subject to
> +			 * the checks in expand_stack() later.
> +			 */
> +			return false;
>  		}
> +
>  		return true;
>  	}
>  	return false;
> -- 
> 2.20.1
>