Subject: Re: [RFC RESEND v2 02/13] powerpc: mm: Fix kfence page fault reporting

From: Ritesh Harjani (IBM) <[email-masked]>
Date: Tue, 15 Oct 2024 13:49:46 +0530

Christophe Leroy <[email-masked]> writes:

> Le 15/10/2024 à 03:33, Ritesh Harjani (IBM) a écrit :
>> copy_from_kernel_nofault() can be called when doing read of /proc/kcore.
>> /proc/kcore can have some unmapped kfence objects which when read via
>> copy_from_kernel_nofault() can cause page faults. Since *_nofault()
>> functions define their own fixup table for handling fault, use that
>> instead of asking kfence to handle such faults.
>> 
>> Hence we search the exception tables for the nip which generated the
>> fault. If there is an entry then we let the fixup table handler handle the
>> page fault by returning an error from within ___do_page_fault().
>> 
>> This can be easily triggered if someone tries to do dd from /proc/kcore.
>> dd if=/proc/kcore of=/dev/null bs=1M
>> 
>> <some example false negatives>
>> ===============================
>> BUG: KFENCE: invalid read in copy_from_kernel_nofault+0xb0/0x1c8
>> Invalid read at 0x000000004f749d2e:
>>   copy_from_kernel_nofault+0xb0/0x1c8
>>   0xc0000000057f7950
>>   read_kcore_iter+0x41c/0x9ac
>>   proc_reg_read_iter+0xe4/0x16c
>>   vfs_read+0x2e4/0x3b0
>>   ksys_read+0x88/0x154
>>   system_call_exception+0x124/0x340
>>   system_call_common+0x160/0x2c4
>> 
>> BUG: KFENCE: use-after-free read in copy_from_kernel_nofault+0xb0/0x1c8
>> Use-after-free read at 0x000000008fbb08ad (in kfence-#0):
>>   copy_from_kernel_nofault+0xb0/0x1c8
>>   0xc0000000057f7950
>>   read_kcore_iter+0x41c/0x9ac
>>   proc_reg_read_iter+0xe4/0x16c
>>   vfs_read+0x2e4/0x3b0
>>   ksys_read+0x88/0x154
>>   system_call_exception+0x124/0x340
>>   system_call_common+0x160/0x2c4
>> 
>> Guessing the fix should go back to when we first got kfence on PPC32.
>> 
>> Fixes: 90cbac0e995d ("powerpc: Enable KFENCE for PPC32")
>> Reported-by: Disha Goel <[email-masked]>
>> Signed-off-by: Ritesh Harjani (IBM) <[email-masked]>
>> ---
>>   arch/powerpc/mm/fault.c | 10 +++++++++-
>>   1 file changed, 9 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 81c77ddce2e3..fa825198f29f 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -439,9 +439,17 @@ static int ___do_page_fault(struct pt_regs *regs, unsigned long address,
>>   	/*
>>   	 * The kernel should never take an execute fault nor should it
>>   	 * take a page fault to a kernel address or a page fault to a user
>> -	 * address outside of dedicated places
>> +	 * address outside of dedicated places.
>> +	 *
>> +	 * Rather than kfence reporting false negatives, let the fixup table
>> +	 * handler handle the page fault by returning SIGSEGV, if the fault
>> +	 * has come from functions like copy_from_kernel_nofault().
>>   	 */
>>   	if (unlikely(!is_user && bad_kernel_fault(regs, error_code, address, is_write))) {
>> +
>> +		if (search_exception_tables(instruction_pointer(regs)))
>> +			return SIGSEGV;
>
> This is a heavy operation. It should at least be done only when KFENCE 
> is built-in.
>
> kfence_handle_page_fault() bails out immediately when 
> is_kfence_address() returns false, and is_kfence_address() returns 
> always false when KFENCE is not built-in.
>
> So you could check that before calling the heavy weight 
> search_exception_tables().
>
> 		if (is_kfence_address(address) &&
> 		    !search_exception_tables(instruction_pointer(regs)) &&
> 		    kfence_handle_page_fault(address, is_write, regs))
> 			return 0;
>

Yes, thanks for the input. I agree with above. I will take that in v3.
I will wait for sometime for any review comments on other patches before
spinning a v3, though.

>
>
>  > +			return SIGSEGV;
>
>> +
>>   		if (kfence_handle_page_fault(address, is_write, regs))
>>   			return 0;
>>   

-ritesh