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.
CPU0 CPU1
- switch 'task' in
- read addr (Fill stale mapping
entry into TLB)
- switch 'task' out (no tlb_flush)
- switch 'task' in (no tlb_flush)
- write addr cause pagefault
do_page_fault() (change to
new addr mapping)
wp_page_copy()
ptep_clear_flush()
ptep_get_and_clear()
& flush_tlb_page()
write new value into addr
- switch 'task' out (no tlb_flush)
- switch 'task' in (no tlb_flush)
- read addr again (Use stale
mapping entry in TLB)
get wrong value from old phyical
addr, BUG!
The solution is to keep all CPUs' footmarks of cpumask(mm) in switch_mm,
which could guarantee to invalidate all stale TLB entries during TLB
flush.
Fixes: 65d4b9c53017 ("RISC-V: Implement ASID allocator")
Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
Tested-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
Tested-by: Zong Li <zong.li@xxxxxxxxxx>
Tested-by: Sergey Matyukevich <sergey.matyukevich@xxxxxxxxxxxxx>
Cc: Anup Patel <apatel@xxxxxxxxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
arch/riscv/mm/context.c | 30 ++++++++++++++++++++----------
1 file changed, 20 insertions(+), 10 deletions(-)
diff --git a/arch/riscv/mm/context.c b/arch/riscv/mm/context.c
index 7acbfbd14557..0f784e3d307b 100644
--- a/arch/riscv/mm/context.c
+++ b/arch/riscv/mm/context.c
@@ -205,12 +205,24 @@ static void set_mm_noasid(struct mm_struct *mm)
local_flush_tlb_all();
}
-static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
+static inline void set_mm(struct mm_struct *prev,
+ struct mm_struct *next, unsigned int cpu)
{
- if (static_branch_unlikely(&use_asid_allocator))
- set_mm_asid(mm, cpu);
- else
- set_mm_noasid(mm);
+ /*
+ * The mm_cpumask indicates which harts' TLBs contain the virtual
+ * address mapping of the mm. Compared to noasid, using asid
+ * can't guarantee that stale TLB entries are invalidated because
+ * the asid mechanism wouldn't flush TLB for every switch_mm for
+ * performance. So when using asid, keep all CPUs footmarks in
+ * cpumask() until mm reset.
+ */
+ cpumask_set_cpu(cpu, mm_cpumask(next));
+ if (static_branch_unlikely(&use_asid_allocator)) {
+ set_mm_asid(next, cpu);
+ } else {
+ cpumask_clear_cpu(cpu, mm_cpumask(prev));
+ set_mm_noasid(next);
+ }
}
static int __init asids_init(void)
@@ -264,7 +276,8 @@ static int __init asids_init(void)
}
early_initcall(asids_init);
#else
-static inline void set_mm(struct mm_struct *mm, unsigned int cpu)
+static inline void set_mm(struct mm_struct *prev,
+ struct mm_struct *next, unsigned int cpu)
{
/* Nothing to do here when there is no MMU */
}
@@ -317,10 +330,7 @@ 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);
+ set_mm(prev, next, cpu);
flush_icache_deferred(next, cpu);
}
--
2.39.2
