[PATCH V2] riscv: asid: Fixup stale TLB entry cause application crash

[guoren]

From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>

After use_asid_allocator is enabled, the userspace application will
crash by stale TLB entries. Because only using cpumask_clear_cpu without
local_flush_tlb_all couldn't guarantee CPU's TLB entries were fresh.
Then set_mm_asid would cause the user space application to get a stale
value by stale TLB entry, but set_mm_noasid is okay.

Here is the symptom of the bug:
unhandled signal 11 code 0x1 (coredump)
   0x0000003fd6d22524 <+4>:     auipc   s0,0x70
   0x0000003fd6d22528 <+8>:     ld      s0,-148(s0) # 0x3fd6d92490
=> 0x0000003fd6d2252c <+12>:    ld      a5,0(s0)
(gdb) i r s0
s0          0x8082ed1cc3198b21       0x8082ed1cc3198b21
(gdb) x /2x 0x3fd6d92490
0x3fd6d92490:   0xd80ac8a8      0x0000003f
The core dump file shows that register s0 is wrong, but the value in
memory is correct. Because 'ld s0, -148(s0)' used a stale mapping entry
in TLB and got a wrong result from an incorrect physical address.

When the task ran on CPU0, which loaded/speculative-loaded the value of
address(0x3fd6d92490), then the first version of the mapping entry was
PTWed into CPU0's TLB.
When the task switched from CPU0 to CPU1 (No local_tlb_flush_all here by
asid), it happened to write a value on the address (0x3fd6d92490). It
caused do_page_fault -> wp_page_copy -> ptep_clear_flush ->
ptep_get_and_clear & flush_tlb_page.
The flush_tlb_page used mm_cpumask(mm) to determine which CPUs need TLB
flush, but CPU0 had cleared the CPU0's mm_cpumask in the previous
switch_mm. So we only flushed the CPU1 TLB and set the second version
mapping of the PTE. When the task switched from CPU1 to CPU0 again, CPU0
still used a stale TLB mapping entry which contained a wrong target
physical address. It raised a bug when the task happened to read that
value.

The solution is to keep all CPUs' footmarks of cpumask(mm) in switch_mm,
which could prevent losing pieces of stuff during TLB flush.

Fixes: 65d4b9c53017 ("RISC-V: Implement ASID allocator")
Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
Cc: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
---
Changes in v2:
 - Fixup nommu compile problem (Thx Conor, Also Reported-by: kernel
   test robot <lkp@xxxxxxxxx>)
 - Keep cpumask_clear_cpu for noasid
---
 arch/riscv/mm/context.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 7acbfbd14557..f58e4b211595 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -317,7 +317,11 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	 */
 	cpu = smp_processor_id();
 
-	cpumask_clear_cpu(cpu, mm_cpumask(prev));
+#ifdef CONFIG_MMU
+	if (!static_branch_unlikely(&use_asid_allocator))
+#endif
+		cpumask_clear_cpu(cpu, mm_cpumask(prev));
+
 	cpumask_set_cpu(cpu, mm_cpumask(next));
 
 	set_mm(next, cpu);
-- 
2.36.1