Fix racy use of anon_inode_getfd() in perf_event.c

once anon_inode_getfd() is called, you can't expect *anything* about
struct file that descriptor points to - another thread might be doing
whatever it likes with descriptor table at that point.

Cc: stable <stable@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 22 insertions, 18 deletions

diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index e099650cd249..bd7ce8ca5bb9 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -4999,8 +4999,8 @@ SYSCALL_DEFINE5(perf_event_open,
         struct perf_event_context *ctx;
         struct file *event_file = NULL;
         struct file *group_file = NULL;
+        int event_fd;
         int fput_needed = 0;
-        int fput_needed2 = 0;
         int err;
 
         /* for future expandability... */
@@ -5021,12 +5021,18 @@ SYSCALL_DEFINE5(perf_event_open,
                         return -EINVAL;
         }
 
+        event_fd = get_unused_fd_flags(O_RDWR);
+        if (event_fd < 0)
+                return event_fd;
+
         /*
          * Get the target context (task or percpu):
          */
         ctx = find_get_context(pid, cpu);
-        if (IS_ERR(ctx))
-                return PTR_ERR(ctx);
+        if (IS_ERR(ctx)) {
+                err = PTR_ERR(ctx);
+                goto err_fd;
+        }
 
         /*
          * Look up the group leader (we will attach this event to it):
@@ -5066,13 +5072,11 @@ SYSCALL_DEFINE5(perf_event_open,
         if (IS_ERR(event))
                 goto err_put_context;
 
-        err = anon_inode_getfd("[perf_event]", &perf_fops, event, O_RDWR);
-        if (err < 0)
-                goto err_free_put_context;
-
-        event_file = fget_light(err, &fput_needed2);
-        if (!event_file)
+        event_file = anon_inode_getfile("[perf_event]", &perf_fops, event, O_RDWR);
+        if (IS_ERR(event_file)) {
+                err = PTR_ERR(event_file);
                 goto err_free_put_context;
+        }
 
         if (flags & PERF_FLAG_FD_OUTPUT) {
                 err = perf_event_set_output(event, group_fd);
@@ -5093,19 +5097,19 @@ SYSCALL_DEFINE5(perf_event_open,
         list_add_tail(&event->owner_entry, &current->perf_event_list);
         mutex_unlock(&current->perf_event_mutex);
 
-err_fput_free_put_context:
-        fput_light(event_file, fput_needed2);
+        fput_light(group_file, fput_needed);
+        fd_install(event_fd, event_file);
+        return event_fd;
 
+err_fput_free_put_context:
+        fput(event_file);
 err_free_put_context:
-        if (err < 0)
-                free_event(event);
-
+        free_event(event);
 err_put_context:
-        if (err < 0)
-                put_ctx(ctx);
-
         fput_light(group_file, fput_needed);
-
+        put_ctx(ctx);
+err_fd:
+        put_unused_fd(event_fd);
         return err;
 }