From: Michael Ellerman mpe@ellerman.id.au
commit 59bee45b9712c759ea4d3dcc4eff1752f3a66558 upstream.
Stefan reported a strange kernel fault 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+ Reported-by: Stefan Berger stefanb@linux.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20200303235708.26004-1-mpe@ellerman.id.au Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/powerpc/mm/mem.c | 2 ++ 1 file changed, 2 insertions(+)
--- a/arch/powerpc/mm/mem.c +++ b/arch/powerpc/mm/mem.c @@ -369,7 +369,9 @@ static inline bool flush_coherent_icache */ if (cpu_has_feature(CPU_FTR_COHERENT_ICACHE)) { mb(); /* sync */ + allow_read_from_user((const void __user *)addr, L1_CACHE_BYTES); icbi((void *)addr); + prevent_read_from_user((const void __user *)addr, L1_CACHE_BYTES); mb(); /* sync */ isync(); return true;