Re: [PATCH] powerpc/mm: Fix missing KUAP disable in flush_coherent_icache()

[Michael Ellerman <mpe@ellerman.id.au>]

Christophe Leroy <christophe.leroy@c-s.fr> writes:
> Le 03/03/2020 à 13:59, Michael Ellerman a écrit :
>> We received a report of strange kernel faults which turned out to be
>> due to a missing KUAP disable in flush_coherent_icache() called
>> from flush_icache_range().
>> 
>> The fault looks like:
>> 
>>    Kernel attempted to access user page (7fffc30d9c00) - exploit attempt? (uid: 1009)
>>    BUG: Unable to handle kernel data access on read at 0x7fffc30d9c00
>>    Faulting instruction address: 0xc00000000007232c
>>    Oops: Kernel access of bad area, sig: 11 [#1]
>>    LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
>>    CPU: 35 PID: 5886 Comm: sigtramp Not tainted 5.6.0-rc2-gcc-8.2.0-00003-gfc37a1632d40 #79
>>    NIP:  c00000000007232c LR: c00000000003b7fc CTR: 0000000000000000
>>    REGS: c000001e11093940 TRAP: 0300   Not tainted  (5.6.0-rc2-gcc-8.2.0-00003-gfc37a1632d40)
>>    MSR:  900000000280b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 28000884  XER: 00000000
>>    CFAR: c0000000000722fc DAR: 00007fffc30d9c00 DSISR: 08000000 IRQMASK: 0
>>    GPR00: c00000000003b7fc c000001e11093bd0 c0000000023ac200 00007fffc30d9c00
>>    GPR04: 00007fffc30d9c18 0000000000000000 c000001e11093bd4 0000000000000000
>>    GPR08: 0000000000000000 0000000000000001 0000000000000000 c000001e1104ed80
>>    GPR12: 0000000000000000 c000001fff6ab380 c0000000016be2d0 4000000000000000
>>    GPR16: c000000000000000 bfffffffffffffff 0000000000000000 0000000000000000
>>    GPR20: 00007fffc30d9c00 00007fffc30d8f58 00007fffc30d9c18 00007fffc30d9c20
>>    GPR24: 00007fffc30d9c18 0000000000000000 c000001e11093d90 c000001e1104ed80
>>    GPR28: c000001e11093e90 0000000000000000 c0000000023d9d18 00007fffc30d9c00
>>    NIP flush_icache_range+0x5c/0x80
>>    LR  handle_rt_signal64+0x95c/0xc2c
>>    Call Trace:
>>      0xc000001e11093d90 (unreliable)
>>      handle_rt_signal64+0x93c/0xc2c
>>      do_notify_resume+0x310/0x430
>>      ret_from_except_lite+0x70/0x74
>>    Instruction dump:
>>    409e002c 7c0802a6 3c62ff31 3863f6a0 f8010080 48195fed 60000000 48fe4c8d
>>    60000000 e8010080 7c0803a6 7c0004ac <7c00ffac> 7c0004ac 4c00012c 38210070
>> 
>> This path through handle_rt_signal64() to setup_trampoline() and
>> flush_icache_range() is only triggered by 64-bit processes that have
>> unmapped their VDSO, which is rare.
>> 
>> flush_icache_range() takes a range of addresses to flush. In
>> flush_coherent_icache() we implement an optimisation for CPUs where we
>> know we don't actually have to flush the whole range, we just need to
>> do a single icbi.
>> 
>> However we still execute the icbi on the user address of the start of
>> the range we're flushing. On CPUs that also implement KUAP (Power9)
>> that leads to the spurious fault above.
>> 
>> We should be able to pass any address, including a kernel address, to
>> the icbi on these CPUs, which would avoid any interaction with KUAP.
>> But I don't want to make that change in a bug fix, just in case it
>> surfaces some strange behaviour on some CPU.
>> 
>> So for now just disable KUAP around the icbi. Note the icbi is treated
>> as a load, so we allow read access, not write as you'd expect.
>> 
>> Fixes: 890274c2dc4c ("powerpc/64s: Implement KUAP for Radix MMU")
>> Cc: stable@vger.kernel.org # v5.2+
>> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
>> ---
>>   arch/powerpc/mm/mem.c | 2 ++
>>   1 file changed, 2 insertions(+)
>> 
>> diff --git a/arch/powerpc/mm/mem.c b/arch/powerpc/mm/mem.c
>> index ef7b1119b2e2..184850d9d000 100644
>> --- a/arch/powerpc/mm/mem.c
>> +++ b/arch/powerpc/mm/mem.c
>> @@ -373,7 +373,9 @@ static inline bool flush_coherent_icache(unsigned long addr)
>>   	 */
>>   	if (cpu_has_feature(CPU_FTR_COHERENT_ICACHE)) {
>>   		mb(); /* sync */
>> +		allow_read_from_user((void *)addr, 4);
>
> I know that's ignored on Radix, but shouldn't we use L1_CACHE_BYTES as a 
> length ?

Yes you're right, I actually meant to do that but forgot.

Will send a v2.

cheers