linux-kernel - Re: [RFC v2 02/13] powerpc: mm: Fix kfence page fault reporting

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Message-ID: <65664ab8-4250-47c2-be50-d56c112a17fb@csgroup.eu>
Date: Thu, 19 Sep 2024 07:12:43 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: "Ritesh Harjani (IBM)" <ritesh.list@...il.com>,
 linuxppc-dev@...ts.ozlabs.org
Cc: Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin
 <npiggin@...il.com>, Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Hari Bathini <hbathini@...ux.ibm.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Donet Tom <donettom@...ux.vnet.ibm.com>,
 Pavithra Prakash <pavrampu@...ux.vnet.ibm.com>,
 Nirjhar Roy <nirjhar@...ux.ibm.com>, LKML <linux-kernel@...r.kernel.org>,
 kasan-dev@...glegroups.com, Disha Goel <disgoel@...ux.ibm.com>
Subject: Re: [RFC v2 02/13] powerpc: mm: Fix kfence page fault reporting

Le 19/09/2024 à 04:56, 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().

Searching the exception table is a heavy operation and all has been done 
in the past to minimise the number of times it is called, see for 
instance commit cbd7e6ca0210 ("powerpc/fault: Avoid heavy 
search_exception_tables() verification")

Also, by trying to hide false positives you also hide real ones. For 
instance if csum_partial_copy_generic() is using a kfence protected 
area, it will now go undetected.

IIUC, here your problem is limited to copy_from_kernel_nofault(). You 
should handle the root cause, not its effects. For that, you could 
perform additional verifications in copy_from_kernel_nofault_allowed().

Christophe