linux-kernel - Re: [PATCH RESEND 1/1] um: oops on accessing a non-present page in the vmalloc area

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Date: Tue, 12 Mar 2024 16:07:45 +0100
From: Petr Tesarik <petrtesarik@...weicloud.com>
To: Richard Weinberger <richard@....at>,
 Anton Ivanov <anton.ivanov@...bridgegreys.com>,
 Johannes Berg <johannes@...solutions.net>,
 "open list:USER-MODE LINUX (UML)" <linux-um@...ts.infradead.org>,
 open list <linux-kernel@...r.kernel.org>
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>,
 Petr Tesařík <petr@...arici.cz>
Subject: Re: [PATCH RESEND 1/1] um: oops on accessing a non-present page in
 the vmalloc area

On 2/23/2024 3:04 PM, Petr Tesarik wrote:
> From: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> 
> If a segmentation fault is caused by accessing an address in the vmalloc
> area, check that the target page is present.
> 
> Currently, if the kernel hits a guard page in the vmalloc area, UML blindly
> assumes that the fault is caused by a stale mapping and will be fixed by
> flush_tlb_kernel_vm(). Unsurprisingly, if the fault is caused by accessing
> a guard page, no mapping is created, and when the faulting instruction is
> restarted, it will cause exactly the same fault again, effectively creating
> an infinite loop.

Ping. Any comment on this fix?

Petr T

> 
> Signed-off-by: Petr Tesarik <petr.tesarik1@...wei-partners.com>
> ---
>  arch/um/kernel/trap.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/um/kernel/trap.c b/arch/um/kernel/trap.c
> index 6d8ae86ae978..d5b85f1bfe33 100644
> --- a/arch/um/kernel/trap.c
> +++ b/arch/um/kernel/trap.c
> @@ -206,11 +206,15 @@ unsigned long segv(struct faultinfo fi, unsigned long ip, int is_user,
>  	int err;
>  	int is_write = FAULT_WRITE(fi);
>  	unsigned long address = FAULT_ADDRESS(fi);
> +	pte_t *pte;
>  
>  	if (!is_user && regs)
>  		current->thread.segv_regs = container_of(regs, struct pt_regs, regs);
>  
>  	if (!is_user && (address >= start_vm) && (address < end_vm)) {
> +		pte = virt_to_pte(&init_mm, address);
> +		if (!pte_present(*pte))
> +			page_fault_oops(regs, address, ip);
>  		flush_tlb_kernel_vm();
>  		goto out;
>  	}