linux-kernel - Re: [PATCH] x86/fault: Fix wrong signal when vsyscall fails with pkey

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Message-ID: <600fdad9d3955671b1a5af12d40a4e409bc7ba5f.camel@pku.edu.cn>
Date:   Thu, 29 Jul 2021 14:24:45 +0800
From:   Jiashuo Liang <liangjs@....edu.cn>
To:     dave.hansen@...el.com
Cc:     bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com,
        liangjs@....edu.cn, linux-kernel@...r.kernel.org, luto@...nel.org,
        mingo@...hat.com, peterz@...radead.org, tglx@...utronix.de,
        x86@...nel.org
Subject: Re: [PATCH] x86/fault: Fix wrong signal when vsyscall fails with
 pkey

On Wed, 2021-07-28 at 10:57 -0700, Dave Hansen wrote:
>> When emulating vsyscall, the kernel may fail to access user-given memory
>> pages that are protected by pkey. In such a case, the kernel should send a
>> SIGSEGV signal with si_code=SEGV_PKUERR and si_pkey=pkey.
> 
> This could use a bit more context.
> 
> First of all this is for user address space faults in the
> do_user_addr_fault() path.  Second, the buggy code is under a
> !user_mode() check, so this must be a kernel fault in the user address
> space.  Third, the only notice this problem when the page fault handler
> ends up delivering a signal as a result of the fault.  Most cases will
> simply return an error code to the faulting kernel code which will see
> -EFAULT come back from copy_to/from_user() and friends.
> 
> The *only* condition in which we generate that signal from the fault
> handler is when current->thread.sig_on_uaccess_err=1, and the only place
> that gets used is in emulate_vsyscall().
> 
> This makes me want to add some code that tickles vsyscall emulation in
> the pkey selftests, but I think I'll resist the urge for now. :)
> 
> Is that all correct?

Right.

>> So a new parameter "pkey" is added to kernelmode_fixup_or_oops to fix it.
> 
> Yeah, I think that's the right fix.  You also need this:
> 
> Fixes: 5042d40a264c ("x86/fault: Bypass no_context() for implicit kernel
> faults from usermode")
> 
> I believe that's where this issue originated.

Yeah, we need to add it.

> How did you find this, by the way?

I was learning about memory protection key. So I read the related code in
kernel and spotted this.

>>  arch/x86/mm/fault.c | 23 +++++++++++++++--------
>>  1 file changed, 15 insertions(+), 8 deletions(-)
>> 
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index b2eefdefc108..883294282e1e 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -710,7 +710,8 @@ page_fault_oops(struct pt_regs *regs, unsigned long error_code,
>>  
>>  static noinline void
>>  kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
>> -			 unsigned long address, int signal, int si_code)
>> +			 unsigned long address, int signal, int si_code,
>> +			 u32 pkey)
>>  {
>>  	WARN_ON_ONCE(user_mode(regs));
>>  
>> @@ -735,8 +736,12 @@ kernelmode_fixup_or_oops(struct pt_regs *regs, unsigned long error_code,
>>  
>>  			set_signal_archinfo(address, error_code);
>>  
>> -			/* XXX: hwpoison faults will set the wrong code. */
>> -			force_sig_fault(signal, si_code, (void __user *)address);
>> +			if (si_code == SEGV_PKUERR) {
>> +				force_sig_pkuerr((void __user *)address, pkey);
>> +			} else {
>> +				/* XXX: hwpoison faults will set the wrong code. */
>> +				force_sig_fault(signal, si_code, (void __user *)address);
>> +			}
>>  		}
>>  
>>  		/*
>> @@ -798,7 +803,8 @@ __bad_area_nosemaphore(struct pt_regs *regs, unsigned long error_code,
>>  	struct task_struct *tsk = current;
>>  
>>  	if (!user_mode(regs)) {
>> -		kernelmode_fixup_or_oops(regs, error_code, address, pkey, si_code);
>> +		kernelmode_fixup_or_oops(regs, error_code, address,
>> +					 SIGSEGV, si_code, pkey);
>>  		return;
>>  	}
>>  
>> @@ -930,7 +936,8 @@ do_sigbus(struct pt_regs *regs, unsigned long error_code, unsigned long address,
>>  {
>>  	/* Kernel mode? Handle exceptions or die: */
>>  	if (!user_mode(regs)) {
>> -		kernelmode_fixup_or_oops(regs, error_code, address, SIGBUS, BUS_ADRERR);
>> +		kernelmode_fixup_or_oops(regs, error_code, address,
>> +					 SIGBUS, BUS_ADRERR, 0);
>>  		return;
>>  	}
> 
> Could we please use ARCH_DEFAULT_PKEY instead of 0's in all these call
> sites?  I just detest seeing mystery functions with lots of 0's and 1's
> as parameters.

I agree that using ARCH_DEFAULT_PKEY is better. I think I am supposed to
send a patch v2 for the update?

By the way, the magic pkey number 0 also appears when bad_area_nosemaphore
calls __bad_area_nosemaphore and bad_area calls __bad_area. Do they need to
be changed to ARCH_DEFAULT_PKEY as well?

Thanks!

Jiashuo Liang