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

[guoren]

From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>

When use_asid_allocator is enabled, the userspace application would
crash by stale tlb entry. Because only using cpumask_clear_cpu without
local_flush_tlb_all couldn't guarantee CPU's tlb contains fresh mapping
entry. Then set_mm_asid would cause user space application 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/16 0x3fd6d92490
0x3fd6d92490:   0xd80ac8a8      0x0000003f
The core dump file show us the value of register s0 is wrong, but the
value in memory is right.

When task run on CPU0, the task loaded/speculative-loaded the value of
address-0x3fd6d92490, the first version tlb mapping enter in CPU0's tlb.
When the task switched from CPU0 to CPU1 without local_tlb_flush_all
(because of asid), the task happened to write a value on address:
0x3fd6d92490 that caused do_page_fault -> wp_page_copy ->
ptep_clear_flush -> ptep_get_and_clear & flush_tlb_page.
The flush_tlb_page would use mm_cpumask(mm) to determine which CPUs need
tlb flush, but CPU0 cleared the CPU0's mm_cpumask in previous switch_mm.
So we only flushed the CPU1's tlb entry, and setted second version mapping
of the pte. When the task switch from CPU1 to CPU0 again, it still used a
stale tlb entry on CPU0 which contained a wrong target physical address.
When the task happened to read that value, the bug would be raised.

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>
---
 arch/riscv/mm/context.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 7acbfbd14557..843e86b63532 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -317,7 +317,6 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	 */
 	cpu = smp_processor_id();
 
-	cpumask_clear_cpu(cpu, mm_cpumask(prev));
 	cpumask_set_cpu(cpu, mm_cpumask(next));
 
 	set_mm(next, cpu);
-- 
2.36.1