On Tue, 2020-03-03 at 23:57:08 UTC, 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: sta...@vger.kernel.org # v5.2+
> Signed-off-by: Michael Ellerman <m...@ellerman.id.au>
Applied to powerpc fixes.
https://git.kernel.org/powerpc/c/59bee45b9712c759ea4d3dcc4eff1752f3a66558
cheers
