Re: [PATCH v2] powerpc/mm: Fix missing KUAP disable in flush_coherent_icache()

[Russell Currey <ruscur@russell.cc>]

On Wed, 2020-03-04 at 10:57 +1100, Michael Ellerman wrote:
> 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>
> ---

Reviewed-by: Russell Currey <ruscur@russell.cc>