gru: fix potential use-after-free when purging GRU tlbs

Fix potential SGI GRU bug that could cause a use-after-free.  If one
thread in a task is flushing the GRU and another thread destroys the GRU
context, there is the potential to access a table after it has been freed.

Copy the gms pointer to a local variable before unlocking the gts table.
Note that no refcnt is needed for the gms - the reference is held
indirectly by the task's mm_struct.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
index 1ad360cd3183..679e01778286 100644
--- a/drivers/misc/sgi-gru/grufault.c
+++ b/drivers/misc/sgi-gru/grufault.c
@@ -702,6 +702,7 @@ int gru_user_flush_tlb(unsigned long arg)
 {
         struct gru_thread_state *gts;
         struct gru_flush_tlb_req req;
+        struct gru_mm_struct *gms;
 
         STAT(user_flush_tlb);
         if (copy_from_user(&req, (void __user *)arg, sizeof(req)))
@@ -714,8 +715,9 @@ int gru_user_flush_tlb(unsigned long arg)
         if (!gts)
                 return -EINVAL;
 
-        gru_flush_tlb_range(gts->ts_gms, req.vaddr, req.len);
+        gms = gts->ts_gms;
         gru_unlock_gts(gts);
+        gru_flush_tlb_range(gms, req.vaddr, req.len);
 
         return 0;
 }