linux-kernel - Re: [PATCH 2/2] s390: mm: Fix secure storage access exception handling

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Date:   Tue, 19 Jan 2021 11:25:01 +0100
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>, linux-kernel@...r.kernel.org
Cc:     kvm@...r.kernel.org, thuth@...hat.com, david@...hat.com,
        imbrenda@...ux.ibm.com, cohuck@...hat.com,
        linux-s390@...r.kernel.org, gor@...ux.ibm.com,
        mihajlov@...ux.ibm.com
Subject: Re: [PATCH 2/2] s390: mm: Fix secure storage access exception
 handling



On 19.01.21 11:04, Janosch Frank wrote:
> Turns out that the bit 61 in the TEID is not always 1 and if that's
> the case the address space ID and the address are
> unpredictable. Without an address and it's address space ID we can't
> export memory and hence we can only send a SIGSEGV to the process or
> panic the kernel depending on who caused the exception.
> 
> Signed-off-by: Janosch Frank <frankja@...ux.ibm.com>
> Fixes: 084ea4d611a3d ("s390/mm: add (non)secure page access exceptions handlers")
> Cc: stable@...r.kernel.org

Reviewed-by: Christian Borntraeger <borntraeger@...ibm.com>

some small things to consider (or to reject)

> ---
>  arch/s390/mm/fault.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index e30c7c781172..5442937e5b4b 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -791,6 +791,20 @@ void do_secure_storage_access(struct pt_regs *regs)
>  	struct page *page;
>  	int rc;
>  
> +	/* There are cases where we don't have a TEID. */
> +	if (!(regs->int_parm_long & 0x4)) {
> +		/*
> +		 * Userspace could for example try to execute secure
> +		 * storage and trigger this. We should tell it that it
> +		 * shouldn't do that.

Maybe something like
		/*
		 * when this happens, userspace did something that it
		 * was not supposed to do, e.g. branching into secure
		 * secure memory. Trigger a segmentation fault.
> +		 */
> +		if (user_mode(regs)) {
> +			send_sig(SIGSEGV, current, 0);
> +			return;
> +		} else
> +			panic("Unexpected PGM 0x3d with TEID bit 61=0");

use BUG instead of panic? That would kill this process, but it allows
people to maybe save unaffected data.