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

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Date: Mon, 18 Mar 2024 14:09:07 +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 3/12/2024 4:07 PM, Petr Tesarik wrote:
> 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?

I don't think I have seen a reply from you. If you did comment, then
your email has not reached me.

Please, can you confirm you have seen my patch?

Kind regards
Petr T

> 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;
>>  	}
>