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SubjectRe: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification
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Le 08/12/2020 à 15:52, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@csgroup.eu> writes:
>
>> search_exception_tables() is an heavy operation, we have to avoid it.
>> When KUAP is selected, we'll know the fault has been blocked by KUAP.
>> Otherwise, it behaves just as if the address was already in the TLBs
>> and no fault was generated.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
>> ---
>> v3: rebased
>> v2: Squashed with the preceeding patch which was re-ordering tests that get removed in this patch.
>> ---
>> arch/powerpc/mm/fault.c | 23 +++++++----------------
>> 1 file changed, 7 insertions(+), 16 deletions(-)
>>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 3fcd34c28e10..1770b41e4730 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -210,28 +210,19 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
>> return true;
>> }
>>
>> - if (!is_exec && address < TASK_SIZE && (error_code & (DSISR_PROTFAULT | DSISR_KEYFAULT)) &&
>> - !search_exception_tables(regs->nip)) {
>> - pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",
>> - address,
>> - from_kuid(&init_user_ns, current_uid()));
>> - }
>> -
>> // Kernel fault on kernel address is bad
>> if (address >= TASK_SIZE)
>> return true;
>>
>> - // Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
>> - if (!search_exception_tables(regs->nip))
>> - return true;
>> -
>> - // Read/write fault in a valid region (the exception table search passed
>> - // above), but blocked by KUAP is bad, it can never succeed.
>> - if (bad_kuap_fault(regs, address, is_write))
>> + // Read/write fault blocked by KUAP is bad, it can never succeed.
>> + if (bad_kuap_fault(regs, address, is_write)) {
>> + pr_crit_ratelimited("Kernel attempted to %s user page (%lx) - exploit attempt? (uid: %d)\n",
>> + is_write ? "write" : "read", address,
>> + from_kuid(&init_user_ns, current_uid()));
>> return true;
>> + }
>
>
> With this I am wondering whether the WARN() in bad_kuap_fault() is
> needed. A direct access of userspace address will trigger this, whereas
> previously we used bad_kuap_fault() only to identify incorrect restore
> of AMR register (ie, to identify kernel bugs). Hence a WARN() there was
> useful. We loose that differentiation now?

Yes, I wanted to remove the WARN(), see
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/cc9129bdda1dbc2f0a09cf45fece7d0b0e690784.1605541983.git.christophe.leroy@csgroup.eu/
but I understood from Michael that maybe it was not a good idea, so I left it aside for now when
rebasing to v3.

Yes previously we were able to differentiate between a direct access of userspace and a valid access
triggering a KUAP fault, but at the cost of the heavy search_exception_tables().
The issue was reported by Nick through https://github.com/linuxppc/issues/issues/317

Should be perform the search_exception_tables() once we have hit the KUAP fault and WARN() only in
that case ?

I was wondering also if we should keep the WARN() only when CONFIG_PPC_KUAP_DEBUG is set ?

>
>
>>
>> - // What's left? Kernel fault on user in well defined regions (extable
>> - // matched), and allowed by KUAP in the faulting context.
>> + // What's left? Kernel fault on user and allowed by KUAP in the faulting context.
>> return false;
>> }
>>
>> --
>> 2.25.0

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